Home-immediately access 800+ free online publications. Download CD3WD (680 Megabytes) and distribute it to the 3rd World. CD3WD is a 3rd World Development private-sector initiative, mastered by Software Developer Alex Weir and hosted by GNUveau_Networks (From globally distributed organizations, to supercomputers, to a small home server, if it's Linux, we know it.)ar.cn.de.en.es.fr.id.it.ph.po.ru.sw

CLOSE THIS BOOKIrrigation Reference Manual (Peace Corps, 1994, 485 p.)
Chapter 4 - Estimating irrigation requirements
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENT4.1 Introduction
VIEW THE DOCUMENT4.3 FAO crop coefficients
VIEW THE DOCUMENT4.4 Dependable precipitation
5.1 Control of irrigation water
VIEW THE DOCUMENT5.1.1 Components of farm irrigation systems
VIEW THE DOCUMENT5.1.2 Open channel systems
VIEW THE DOCUMENT5.1.3 Control structures
VIEW THE DOCUMENT5.2.1 Continuity equation
VIEW THE DOCUMENT5.2.2 Pressure, head, and friction losses
VIEW THE DOCUMENT5.2.3 Factors influencing head loss
VIEW THE DOCUMENT5.2.4 Pine design
VIEW THE DOCUMENT5.2.5 The hydraulic gradient line (HGL)
VIEW THE DOCUMENT5.2.6 Pipeline design sample problems
VIEW THE DOCUMENT5.2.7 Pipes and pipeworking
VIEW THE DOCUMENT5.2.8 Working with pipes
VIEW THE DOCUMENT5.2.9 Water hammer
VIEW THE DOCUMENT5.2.10 Air relief. Vacuum relief, and pressure relief
VIEW THE DOCUMENT5.2.11 Other pipeline structures and accessories
VIEW THE DOCUMENT5.2.12 Pipeline materials
VIEW THE DOCUMENT5.4.1 Characteristics of irrigation systems
VIEW THE DOCUMENT5.5.1 Criteria for design and operation
VIEW THE DOCUMENT5.5.2 Description of different surface irrigation methods
VIEW THE DOCUMENT5.5.3 Contour ditch
VIEW THE DOCUMENT5.5.4 Contour levee
VIEW THE DOCUMENT5.5.5 Furrow irrigation
VIEW THE DOCUMENT5.5.6 Corrugation irrigation
VIEW THE DOCUMENT5.5.7 Operation and maintenance of farm surface irrigation systems
5.6 Sprinkler irrigation systems
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENT5.6.1 Principal components
VIEW THE DOCUMENT5.6.2 Pine specifications
VIEW THE DOCUMENT5.6.3 Sprinkler heads and nozzles
VIEW THE DOCUMENT5.6.4 Sprinkler system design
VIEW THE DOCUMENT5.6.5 Lateral design
VIEW THE DOCUMENT5.6.6 Sprinkler system installation
VIEW THE DOCUMENT5.6.7 System operation and maintenance
VIEW THE DOCUMENT5.7.1 Characteristics
VIEW THE DOCUMENT5.7.2 Operation and maintenance
6.1 Farm water management
VIEW THE DOCUMENT6.1.1 General concepts
VIEW THE DOCUMENT6.2.1 Factors affecting irrigation scheduling
VIEW THE DOCUMENT6.2.2 The practice of irrigation scheduling
VIEW THE DOCUMENT6.2.3 Techniques for preparing irrigation schedules
VIEW THE DOCUMENT6.2.4 Useful relationships in irrigation scheduling
VIEW THE DOCUMENT6.2.5 The soil water budget approach
VIEW THE DOCUMENT6.2.6 The feel and appearance method
VIEW THE DOCUMENT6.2.7 Summary of scheduling techniques
VIEW THE DOCUMENT6.2.8 A comparison of scheduling criteria for surface, sprinkler, and drip irrigation
VIEW THE DOCUMENT6.2.9 Rice irrigation scheduling
VIEW THE DOCUMENT6.2.11 Delivery system schedules
VIEW THE DOCUMENT6.2.12 Project scheduling a summary
VIEW THE DOCUMENT6.3.1 Strategies for farm management
VIEW THE DOCUMENT6.3.2 Rapid on-site evaluations
VIEW THE DOCUMENT6.3.3 Evaluation of multiple farm irrigation systems
7.1 Basic concepts in waterlogging and salinity
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENT7.1.1 Waterlogging and high ground water tables
VIEW THE DOCUMENT7.1.2 Soil and water salinity
VIEW THE DOCUMENT7.1.3 Classification of salt affected soils
VIEW THE DOCUMENT7.1.4 Evaluating waterlogging and salinity problems
VIEW THE DOCUMENT7.2.1 Surface and subsurface drains
VIEW THE DOCUMENT7.2.2 Reclamation of salt affected soils
VIEW THE DOCUMENT7.2.3 Correcting sodium problems with amendments
VIEW THE DOCUMENT7.2.4 Management of saline and sodic soils
VIEW THE DOCUMENTA.1 Conversion factors
VIEW THE DOCUMENTA.3 Trigonometric table
VIEW THE DOCUMENTA.4 List of common tools
VIEW THE DOCUMENTB.1 Community situation analysis/needs assessment

Irrigation Reference Manual (Peace Corps, 1994, 485 p.)

Chapter 4 - Estimating irrigation requirements

Reference:

Primary:

(11), (12), (22),(44)

4.1 Introduction

Proper irrigation design and management requires that net and gross irrigation requirements be accurately estimated. This information is necessary for determining the timing and amounts of irrigation (irrigation schedules) and the design capacities of the water storage and distribution systems.

Crop water use and requirements, rainfall, stored soil water and contributions of ground water to the crop needs are generally expressed as equivalent depths of water over the crop growing area (e.g., cm or mm). In determining delivery requirements, the crop irrigation requirements are then usually expressed in terms of volume per unit time or flow rate (e.g., liters/sec).

The net irrigation requirement for a crop maintained without water stress for any time period can be determined through the following relationship:

where:

Irn is the net irrigation requirement for a given crop.
ETc is the crop evapotranspiration or crop water use under no stress conditions.
Pe is the effective precipitation.
Gw is the ground water contribution.
Wb is the available stored soil water at the beginning of the period.

Evapotranspiration is the process by which water is transferred from the plant and soil into the atmosphere. It includes evaporation of water from the plant and soil surfaces, as well as transpiration of water through the plant tissue.

Evapotranspiration rates or amounts are determined by climatic, plant, and soil conditions. Solar radiation, temperatures, wind, and humidity are the primary climatic influences. Crop ground cover, physiology, and metabolism are some of the plant factors. Soil moisture, composition, and salinity also affect evapotranspiration rates.

Irrigation requirements are usually estimated with the assumption that the crop(s) will be kept at or near optimum growth conditions. The crop evapotranspiration (ETc) can be estimated by multiplying a reference crop evapotranspiration (ETo) by corresponding crop coefficients (Kc). The relationship is usually expressed as:

<<TOC3>> 4.2 Reference crop evapotranspiration (ETo)

A method known as the Hargreaves Temperature Method, which requires only maximum and minimum temperature data, has been used in many developing countries for determining ETo (mm). The equation is:

Ra is the evaporation equivalent of extraterrestrial solar radiation (mm per day), which is a function of latitude and time of year. It can be obtained from Table 4.1.

Td is the difference between average daily maximum and minimum temperatures for the period in °C.

T°C is the average temperature in °C for the period, i.e., T°C = (Tmx + Tmn)/2.

The relationship between crop ET and the reference crop evapotranspiration is affected by climatic conditions, soil profile moisture, soil surface moisture, crop variety, canopy, stage of growth, and other factors.

Maximum and minimum temperature data are the most common and reliable data obtained in field meteorologic stations worldwide. The irrigation Volunteer should be able to access this data, along with precipitation data, from the local or national weather service.

Example: In the Azua region of the Dominican Republic, the Ciaza station (latitude 18° north) has weather records from 1981 to 1988 that include the maximum and minimum temperature data. The average maximum and minimum temperatures and precipitation from the station and the extraterrestrial radiation (Ra) from Table 4.1 (for a latitude of 18°) are as follows:

Month

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Tmax (°C)

30.5

30.2

31.8

30.8

30.6

30.5

32.4

32.1

31.9

32.0

31.3

31.3

Tmin (°C)

20.1

19.0

19.9

20.8

22.2

22.4

22.6

21.6

23.3

21.7

22.8

19.7

Precip1

10.0

13.0

15.0

38.0

121

67.0

51.0

77.0

124

138

55.0

16.0

Ra

11.6

13.0

14.6

15.6

16.1

16.1

16.1

15.8

14.9

13.6

12.0

11.1

1(mm/m)

ETo for January:

ETo

=.0023 (11.6) (30.5-20.1)½ [(30.5+20.1)/2 + 17.8]
=.0023 (11.6) (10.4)½ (25.3 + 17.8)
=.0023 (11.6) (3.22) (43) = 3.7 mm/day

Table 4.1 Extraterrestrial Radiation (Ra) Expressed in Equivalent Evaporation in mm/day (Ref. 12)

Northern Hemisphere


Southern Hemisphere

Jan

Feb

Mar

Apr

May

June

July

Aug

Sept

Oct

Nov

Dec

Lat

Jan

Feb

Mar

Apr

May

June

July

Aug

Sept

Oct

Nov

Dec

3.8

6.1

9.4

12.7

15.8

17.1

16.4

14.1

10.9

7.4

4.5

3.2

50°

17.5

14.7

10.9

7.0

4.2

3.1

35

55

8.9

12.9

16.5

18.2

4.3

6.6

9.8

13.0

15.9

17.2

16.5

14.3

11.2

7.8

5.0

3.7

48

17.6

14.9

11.2

7.5

4.7

3.5

4.0

6.0

9.3

13.2

16.6

18.2

4.9

6.6

9.8

13.0

15.9

17.2

16.5

14.5

11.5

8.3

5.5

4.3

46

17.7

15.1

11.5

7.9

5.2

4.0

4.4

65

9.7

13.4

16.7

18.3

5.3

7.5

10.6

13.7

16.1

17.2

16.6

14.7

11.9

8.7

6.0

4.7

44

17.8

15.5

12.2

8.8

6.1

4.9

5.4

7.4

10.6

14.0

16.8

18.3

5.9

8.1

11.0

14.0

16.2

17.3

16.7

15.0

12.2

9.1

6.5

5.2

42

17.8

1.55

12.2

8.8

6.1

4.9

5.4

7.4

10.6

14.0

16.8

18.3


























6.4

8.6

11.4

14.3

16.4

17.3

16.7

15.2

12.5

9.6

7.0

5.7

40

17.9

15.7

12.5

9.2

6.6

5.3

5.9

7.9

11.0

14.2

16.9

18.3

6.9

9.0

11.8

14.5

16.4

17.2

16.7

15.3

12.8

10.0

7.5

6.1

38

17.9

15.8

12.8

9.6

7.1

5.8

6.3

8.3

11.4

14.4

17.0

18.3

7.4

9.4

12.1

14.7

16.4

17.2

16.7

15.4

13.1

10.6

8.0

6.6

36

17.9

16.0

13.2

10.1

7.5

6.3

6.8

8.8

11.7

14.6

17.0

18.2

7.9

9.8

12.4

14.8

16.5

17.1

16.8

15.5

13.4

10.8

8.5

7.2

34

17.8

16.1

13.5

10.5

8.0

6.8

7.2

9.2

12.0

14.9

17.1

18.2

8.3

10.2

12.8

15.0

16.5

17.0

16.8

15.6

13.6

11.2

9.0

7.8

32

17.8

16.2

13.8

10.9

85

7.3

7.7

9.6

12.4

15.1

17.2

18.1


























8.8

10.7

13.1

15.2

16.5

17.0

16.8*

15.7

13.9

11.6

9.5

8.3

30

17.8

16.4

14.0

113

8.9

7.8

8.1

10.1

12.7

15.3

17.3

18.1

9.3

11.1

13.4

15.3

16.5

16.8

16.7

15.7

14.1

12.0

9.9

8.8

28

17.7

16.4

14.3

11.6

9.3

8.2

8.6

10.4

13.0

15.4

17.2

17.9

9.8

11.5

13.7

15.3

16.4

16.7

16.6

15.7

14.3

12.3

10.3

9.3

26

17.6

16.4

14.4

12.0

9.7

8.7

9.1

10.9

13.2

15.5

17.2

17.8

10.2

11.9

13.9

15.4

16.4

16.6

16.5

15.8

14.5

12.6

10.7

9.7

24

17.5

165

14.6

12.3

10.2

9.1

95

11.2

13.4

15.6

17.1

17.7

10.7

12.3

14.2

15.5

163

16.4

16.4

15.8

14.6

13.0

11.1

10.2

22

17.4

165

14.8

12.6

10.6

9.6

10.0

11.6

13.7

15.7

17.0

17.5


























11.2

12.7

14.4

15.6

16.3

16.4

16.3

15.9

14.8

13.3

11.6

10.7

20

173

16.5

15.0

13.0

11.0

10.0

10.4

12.0

13.9

15.8

17.0

17.4

11.6

13.0

14.6

15.6

16.1

16.1

16.1

15.8

14.9

13.6

12.0

11.1

18

17.1

16.5

15.1

13.2

11.4

10.4

10.8

12.3

14.1

15.8

16.8

17.1

12.0

13.3

14.7

15.6

16.0

15.9

15.9

15.7

15.0

139

12.4

11.6

16

16.9

16.4

15.2

13.5

11.7

10.8

11.2

12.6

14.3

15.8

16.7

16.8

12.4

13.6

14.9

15.7

15.8

15.7

15.7

15.7

15.1

14.1

12.8

12.0

14

16.7

16.4

15.3

13.7

12.1

11.2

11.6

12.9

14.5

15.8

16.5

16.6

12.8

13.9

15.1

15.9

15.7

15.5

15.5

15.6

15.2

14.4

13.3

12.5

12

16.6

16.3

15.4

14.0

12.5

11.6

12.0

13.2

14.7

15.8

16.4

16.5


























13.2

14.2

15.3

15.7

15.5

15.3

15.3

15.5

15.3

14.7

13.6

12.9

10

16.4

163

15.5

14.2

12.8

12.0

12.4

13.5

14.8

15.9

16.2

16.2

13.6

14.5

15.3

15.6

15.3

15.0

15.1

15.4

15.3

14.8

13.9

13.3

8

16.1

16.1

15.5

14.4

13.1

12.4

12.7

13.7

14.9

15.8

16.0

16.0

13.9

14.8

15.4

15.4

15.1

14.7

14.9

15.2

15.3

15.0

14.2

13.7

6

15.8

16.0

15.6

14.7

13.4

12.8

13.1

14.0

15.0

15.7

15.8

15.7

14.3

15.0

15.5

15.5

14.9

14.4

14.6

15.1

15.3

15.1

14.5

14.1

4

15.5

15.8

15.6

14.9

13.8

13.2

13.4

14.3

15.1

15.6

15.5

15.4

14.7

15.3

15.6

15.3

14.6

14.2

14.3

14.9

15.3

15.3

14.8

14.4

2

15.3

15.7

15.7

15.1

14.1

13.5

13.7

14.5

15.2

15.5

15.3

15.1

15.0

15.5

15.7

15.3

14.4

13.9

14.1

14.8

15.3

15.4

15.1

14.8

0

15.0

15.5

15.7

15.3

14.4

13.9

14.1

14.8

15.3

15.4

15.1

14.8

ETo for February is:

ETo =.0023 (13.0) (30.2-19)½ [(30.2+19)/2 + 17.8] = 4.2

Likewise, for all months we have:


Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

ETo

3.7

4.2

5.1

4.9

4.7

4.7

5.3

5.3

4.6

4.5

3.6

3.7

In very hot, dry climates with long days, ETo can often reach 10 mm per day. In cool, humid climates, ETo may be as low as 3 mm per day.

Locations that have long, hot, sunny days (14 hours or more) in summer may exceed an average of 8 mm per day of ETo. Individual days may exceed 10 mm per day. Locations nearer the equator will typically have 5 to 6 mm of water use per day in the hotter months and around 4 mm per day during the cooler, cloudier months. ETo decreases with elevation, as temperatures decrease with elevation.

A designer in a hot, dry climate might need to design irrigation systems using 8 mm per day as a value for ETo while, in a more moderate climate, a value of 5 mm might be more acceptable. The previous table, however, clearly shows the need for using local data to establish ETo. Table 4.2 provides an index of general levels of ETo as a function of climatic zones. The following example provides an indicator of how ETo can vary with factors such as latitude and elevation.

Location

J

F

M

A

M

J

J

A

S

O

N

D

Indonesia1

3.7

4.1

4.1

4.2

4.0

3.8

4.0

4.5

4.7

4.5

4.2

3.9

Egypt2

2.0

2.9

3.8

4.9

6.2

6.8

6.8

6.3

5.3

4.1

2.7

2.0

Phillipines3

3.1

3.2

4.6

5.6

5.1

4.7

3.7

3.8

3.7

3.8

3.6

3.5

Ecuador4

3.6

3.6

3.4

3.2

3.0

2.8

2.9

3.1

3.2

3.0

3.0

3.0

Ecuador5

5.3

5.4

5.5

5.4

4.6

3.9

4.3

4.2

4.2

4.7

4.8

4.9

Bolivia6

5.6

5.7

5.2

5.1

3.5

3.9

3.9

4.5

5.0

5.3

6.4

5.8

U.S7

2.1

3.0

4.3

5.9

7.4

8.4

7.9

7.5

6.1

4.2

2.6

1.9

1 Jakarta, Lat. = 6°S, Elev. = 5 m
2 Cairo, Lat. = 3°N, Elev. = 74 m
3 Manila, Lat. = 15°N, Elev. = 15 m
4 Quito, Lat. = 0°S, Elev. = 2818 m
5 Talara, Lat. = 5°S, Elev. = 90 m
6 Tarija, Lat. = 22°S, Elev. = 1905 m
7 Yuma, Arizona, Lat. = 33 N. Elev. = 63 m

TABLE 4.2 Reference Crop Evapotranspiration (ETo in mm/day) for Various Agroclimatic Zones (Ref. 11)

Mean Daily Temperature (C)
<10----------------20----------------->30

Region

Cold

Moderate

Hot

TROPICAL

Humid

3-4

4-5

5-6

Sub-humid

3-5

5-6

7-8

Semi-arid

4-5

6-7

8-9

Arid

4-5

7-8

9-10

SUBTROPICAL

Summer Rainy Period

Humid

3-4

4-5

5-6

Sub-humid

3-5

5-6

6-7

Semi-arid

4-5

6-7

7-8

Arid

4-5

7-8

10-11

Winter Rainy Period

Humid, sub-humid

2-3

4-5

5-6

Semi-arid

3-4

5-6

7-8

Arid

3-4

6-7

10-11

TEMPERATE

Humid, sub-humid

2-3

3-4

5-7

Semi-arid, arid

3-4

5-6

8-9

4.3 FAO crop coefficients

Table 4.3 presents a set of crop coefficients, using grass as the reference crop, for various stages of crop growth. In this case, the crop evapotranspiration (ETc) under optimum condition is:

ETc = Kc ETo

where:

Kc is the coefficient for crops growing under conditions of optimum fertility and soil moisture and achieving full production potential.

During germination and initial crop development, the majority of water loss from the plant and soil surface is evaporation. A soil surface that is kept continually wet will have very high evaporation; thus, ETc can be almost equal to ETo. With infrequent wetting of the soil surface, ETc will be much lower than ETo during initial development stages, and Kc is generally less than 0.5. When the crop has developed a full canopy and shading of the soil is almost complete, the majority of water loss is through transpiration. Evapotranspiration is generally at or near maximum, and the crop coefficient is usually between 1.0 and 1.2. As the crop begins to mature, its physiological ability to use water is decreased, and the crop coefficient rapidly decreases.

Until most of the ground is shaded, Kc is dependent on the stage of crop development, frequency of irrigation or significant rainfall, and the evaporative potential (as indicated by ETo). After effective cover, the coefficient is primarily dependent on stage of growth and climatic conditions of wind and humidity.

In developing the crop coefficients for the growing season, different stages of crop development are considered:

1. Initial state: from planting through germination and plant emergence, and until about 10% ground cover is achieved. Water loss is practically all evaporation.

2. Crop development stage: from 10% of ground cover to effective full ground cover. This occurs at about 70% or 80% ground cover.

3. Mid-season stage: from effective cover to the start of maturity. The crop is physiologically capable of the highest water use during this time. The crop coefficient is highest.

Table 4.3 Crop Coefficients (Kc) (Ref. 11)

Crop Coefficients (kc)

CROP

Crop Development Stages

Total


Initial

Crop development

Mid- season

Late season

At harvest

growing period

Banana


tropical

0.4-0.5

0.7-0.85

1.0-1.1

0.9-1.0

0.75-0.85

0.7-0.8


subtropical

0.5-0.65

0.8-0.9

1.0-1.2

1.0-1.15

1.0-1.15

0.85-0.95

Bean


green

0.3-0.4

0.65-0.75

0.95-1.05

0.9-0.95

0.85-0.95

0.85-0.9


dry

0.4

0.7-0.8

1.05-12

0.65-0.75

0.25-0.3

0.7-0.8

Cabbage

0.4-0.5

0.7-0.8

0.95-1.1

0.9-1.0

0.8-0.95

0.7-0.8

Cotton

0.4-0.4

0.7-0.8

1.05-125

0.8-0.9

0.65-0.7

0.8-0.9

Grape

0.35-0.55

0.6-0.8

0.7-0.9

0.6-0.8

0.55-0.7

0.55-0.75

Groundout

0.4-0.5

0.7-0.8

0.95-1.1

0.75-0.85

0.55-0.6

0.75-0.8

Maize


sweet

0.3-0.5

0.7-0.9

1.05-1.2

1.0-1.15

0.95-1.1

0.8-0.95


grain

0.3-0.5

0.7-0.85

1.05-1.2

0.8-0.95

0.55-0.6

0.75-0.9

Onion


dry

0.4-0.6

0.7-0.8

0.95-1.1

0.85-0.9

0.75-0.85

0.8-0.9


green

0.4-0.6

0.6-0.75

0.95-1.05

0.95-1.05

0.95-1.05

0.65-0.8

Pea, fresh

0.4-0.5

0.7-0.85

1.05-1.2

1.0-1.15

0.95-1.1

0.8-0.95

Pepper, fresh

0.3-0.4

0.6-0.75

0.95-1.1

0.85-1.0

0.8-0.9

0.7-0.8

Potato

0.4-0.5

0.7-0.8

1.05-1.2

0.85-0.95

0.7-0.75

0.75-0.9

Rice

1.1-1.15

1.1-1.5

1.1-1.3

0.95-1.05

0.95-1.05

1.05-12

Safflower

0.3-0.4

0.7-0.8

0.7-0.8

0.65-0.7

0.2-0.25

0.65-0.7

Sorghum

0.3-0.4

0.7-0.8

1.0-1.15

0.75-0.8

0.5-0.55

0.75-0.85

Soybean

0.3-0.4

0.7-0.8

1.0-1.15

0.7-0.8

0.4-0.5

0.75-0.9

Sugarbeet

0.4-0.5

0.75-0.85

1.05-1.2

0.9-1.0

0.6-0.7

0.8-0.9

Sugarcane

0.4-0.5

0.7-1.0

0.7-1.0

0.75-0.8

0.5-0.6

0.85-1.05

Sunflower

0.3-0.4

0.7-0.8

1.05-1.2

0.7-0.8

0.7-0.8

0.75-0.85

Tobacco

0.3-0.4

0.7-0.8

1.0-1.2

0.9-1.0

0.75-0.85

0.85-0.95

Tomato

0.4-0.5

0.7-0.8

1.05-1.25

0.8-0.95

0.6-0.65

0.75-0.9

Waterrnelon

0.4-0.5

0.7-0.8

0.95-1.05

0.8-0.9

0.65-0.75

0.75-0.85

Wheat

0.3-0.4

0.7-0.8

1.05-1.2

0.65-0.75

0.2-0.25

0.8-0.9

Alfalfa

0.3-0.4




1.05-1.2

0.85-1.05

Citrus


clean weeding






0.65-0.75


no weed control






0.85-0.9

Olive






0.4-0.6

First figure: Under high humidity (RHmin>70%) ant low wind (U<5m/sec).
Second Figure: Under low humidity (RHmin<20%) and strong wind (U>5m/sec).

4. Late season stage: from the start of maturity until full maturity or harvest.

The procedures for establishing the crop coefficient are as follows:

1. Establish planting dates and length of growing season for local crop varieties under irrigation. This may vary significantly from dryland to irrigated conditions, from well-fertilized to non-fertilized crops, and even for plantings at different times of the year (e.g., if temperature and radiation conditions vary significantly through the season).

2. Establish the length of the crop development stages. Local research and extension agencies, interviews with farmers and agricultural technicians, or crop data from similar climatic zones can be used to establish these. Since the dates when crops reach these stages, or the length of these stages, are not typically recorded by research or extension personnel or farmers, it is often necessary to correlate these dates with more identifiable characteristics. For grain crops, 10% ground cover is usually reached from 10 to 15 days after emergence. Effective cover for annual crops occurs approximately at the time of flowering. The start of maturity for many crops is indicated by discoloring or dropping of leaves.

3. From Table 4.3, determine the Kc values for Stage 1 (initial state), Stage 2 (development), Stage 3 (midseason), and Stage 4 (the maturing phase). The lower values should be used for low advective conditions (low wind and high humidity) and the higher values for higher advection (high wind and low humidity).

Example: Corn is an important crop in the Azua region of the Dominican Republic. It is a 4-month crop, often planted in March and harvested at the end of June. Determine the average water use for each stage of growth during the growing season. Azua is fairly humid, as it is near the ocean, and winds are generally calm, so use the lower values in the table, or use the ETo values from the previous example. Each growth stage is about 1 month in length.

Solution:

Month

March

April

May

June

Crop stage

1

2

3

4

ETo

5.1

4.9

4.70

4.70

Kc

0.3

0.7

1.05

0.55

ETc

1.5

3.4

4.94

2.59

Table 4.4 provides a range of seasonal ET values for a number of common crops.

TABLE 4.4 Seasonal ET Requirements for Maximum Yields of Crops (Ref. 11)

Crop

Seasonal ET (mm)

Alfalfa

800-1600

Banana

1200-2200

Beans

300- 500

Cabbage

380- 500

Citrus

900-1200

Cotton

700-1300

Grapes

500-1200

Peanuts

500- 700

Maize

500- 800

Olives

600- 800

Onions

350- 550

Peppers

600- 900

Pineapple

700-1000

Potatoes

500- 700

Rice

350- 700

Sorghum

450- 650

Sugar beets

550- 750

Sugar cane

1500-2500

Sunflower

600-1000

Tobacco

400- 600

Tomato

400- 600

Watermelon

400- 600

Wheat

450- 650

4.4 Dependable precipitation

In areas where rainfall provides any significant portion of the crop water supply, it is essential that water availability from precipitation be evaluated in planning water delivery requirements, adjusting irrigation schedules, and even in developing possible crops and cropping patterns.

A probability analysis of rainfall based on historical precipitation is generally a part of this evaluation for large projects. Average rainfall can usually be obtained by the Peace Corps Volunteer from local weather stations, government agencies, or from a climatic atlas. The use of average precipitation for the planning and design of a small project will result in the design of a project that will ensure the farmer against crop failure, and crop yields will generally be near to their potential.

<<TOC3>> 4.5 Effective precipitation

Many definitions of effective precipitation are found in the literature. One of the more popular and useful definitions is that effective rainfall is that portion of the rainfall that contributes to the evapotranspiration requirements of a crop. Thus, that portion of rainfall that is not lost from the farm, either as surface runoff or as deep percolation to subsurface drainage, may be considered effective.

Even small amounts of water retained on the plant surface may be considered effective, as they help to satisfy evapotranspiration demand. It is common to discount small amounts of up to 4 or 5 mm, however, when ground cover is incomplete and evaporation from the soil surface would be very rapid.

High intensity rains producing a large amount of runoff and soils that have little capacity to store moisture may determine what little of the precipitation is effective. Rainfall that is lost during the non-cropped season as evaporation is ineffective.

Since effective rainfall is difficult to determine, it may be necessary to evaluate soil moisture after a rainfall event to evaluate the effectiveness of the rainfall in replenishing the soil profile so that irrigation can be adjusted accordingly. Generally, low intensity rainfall that does not exceed the soil water deficit is highly effective after significant canopy has been established.

Since effective rainfall is determined by rainfall intensity, soil infiltration characteristics, soil moisture deficit, surface storage, and evaporative conditions, it is difficult to estimate precisely. For planning purposes, it is generally adequate to assume that on deep rooted crops, flat ground, or sandy soils, the effective precipitation will usually be around 80%, if rainfall is generally not of great intensity and if moisture conservation practices are used. On steep terrains, heavy soils, and in areas of high intensity rainfall, the effective rainfall may be less than 50%.

<<TOC3>> 4.6 Ground water contributions to crop requirements

The rate of upward capillary movement from the ground water depends on the depth of water table below the root zone, soil moisture content and gradient, soil texture, structure and capillary properties, and on evaporative conditions. Generally, in coarse textured soils, rapid movement can occur over short distances with large moisture gradients. Water can move greater distances in fine textured soils, but movement is slower.

Because upward movement of the ground water is so greatly influenced by texture, structure, and other conditions, it is difficult to determine ground water contributions without detailed studies.

Although a significant portion of the total water requirement may be supplied from ground water at shallow depths, it is important to consider the detrimental effects of shallow water or waterlogged soils on crops. Shallow water tables may prevent adequate root development, which will result in low moisture storage capacity in the root zone if water tables drop. Some sugar cane varieties, potatoes, and broad beans can do well with water tables at 50 cm, while corn will be affected by water at 1 meter.

Upward movement of water may also result in salinization unless excess water and adequate drainage for leaching fraction can be provided at some time of year.

For these reasons, it may sometimes be necessary to plan for the elimination of shallow ground water, rather than to consider it as a contributor to crop requirements. Generally, water tables lower than 1 meter below the surface will not contribute significantly to most crops.

<<TOC3>> 4.7 Gross irrigation requirements

Not all water available at the head of a canal is available to fulfill the net irrigation requirements (Irn). Losses to deep percolation, evaporation, and surface runoff, as well as leaching requirements, must be accounted for in the conveyance systems and in the farm application system. The gross irrigation water requirement (Irg) can be determined if field application and canal distribution system efficiencies are known or can be estimated.

The basic equation for determining gross irrigation requirements is:

where:

Irn

is the net irrigation requirement per day (depth).

Ea

is the farm application efficiency (fraction), or the ratio between the water that enters and stays in the root zone to meet crop needs and that which is delivered to the field.

Ec

is the canal conveyance efficiency (fraction), or the ratio of the water delivered to the field and that which enters the irrigation canal.

Primary factors affecting conveyance losses are management aspects that cause fluctuations or require adjustments in the supply, as well as physical factors such as seepage losses through canal banks and canal outlets. Typically, canal losses are very high when a large number of canals serve many small farms, and where organizational control of water is not strict and orderly. Highly permeable soils and poor maintenance of canals are other primary causes.

Primary factors affecting or resulting in low application efficiencies are improper irrigation system design, construction, and maintenance, as well as inadequate farmer knowledge of crop water requirements and irrigation scheduling criteria, irrigation system evaluation and monitoring criteria, and delivery system behavior. Many times, the delivery of water to the farm may be untimely, in improper amounts, and with excessive variation in the available discharge. These factors, which are beyond the farmer's control, may make efficient irrigation an impossibility.

Table 4.5 indicates some typical efficiencies for both the delivery system and the irrigation system. Irrigation scheduling, proper maintenance, and other techniques can significantly improve efficiencies above those given in the table.

Table 4.5 Irrigation System Efficiency

IRRIGATION SYSTEM

EXPECTED APPLICATION EFFICIENCY
(fraction)

Surface methods



light soils

0.55


medium soils

0.70


heavy soils

0.60

Sprinkler



hot, dry climate

0.60


moderate climate

0.70


humid and cool

0.80

Flooded rice

0.30

Drip or Trickle Irrigation

0.80

Expected conveyance efficiencies in small, short canals (<2 km) are about 80% (0.8) for heavy soils. 70% (0.7) for medium soils, and 60% (0.6) for sandy soils. Canals with lower efficiencies should be evaluated to diminish the losses. Pipeline conveyance systems should always have close to 100% efficiencies.

Gross requirements can be expressed as a depth required per day, or as a flow rate required per unit area per day. The following equations help us to determine the maximum flow rate that we will require on a daily basis for a farm.

where:

Q

is the flow rate required (liters/sec).

A

is the area to be irrigated (hectares).

Irg

is the gross daily water requirement (mm).

t

is the number of hours per day that the farmer expects to irrigate during the peak season.

Example: For the Azua region of the Dominican Republic, we have determined crop water use of corn on a daily basis during the months of March through June. We have obtained the mean precipitation from local sources, and we know that in this dry,

flat region with a deep rooted crop such as corn, the rainfall will be about 80% effective. Water tables are lower than 1 meter, and there will be no ground water contribution. Soils are medium textured. Farmers use surface irrigation methods, and they will generally irrigate 12 hours per day. Canals are well maintained and generally less than 2 km long. Neglect soil moisture storage in your analysis, and estimate what the flow rate required will be for each hectare.

Solution:

Month

March

April

May

June

ETc

1.5

3.4

4.9

2.6

P mean

0.5

1.3

3.9

2.2

Pe

0.4

1.0

3.1

1.8

Irn

1.1

2.4

1.8

0.8

Irg

2.2

4.9

3.7

1.6

P mean monthly precipitation divided by days in month.
Pe effective precipitation = mean precipitation 0.8 (in this case).
Irn is ETc - Pe - Gw - Wb = ETc - Pe - 0 - 0.

The month with the highest gross water requirement is April, with 4.9 mm/day of gross requirement. On a flow rate basis, this is:

In summary, the crop during April will use 3.4 mm/day. Considering effective precipitation, our average daily water need would be 2.4. Due to efficiency losses, however, the gross amount of water that will be required will be 4.9 mm/day on average. If this amount is to be applied on a 12-hour per day schedule, we will need to be able to supply 1.1 liters per second for each hectare during the month of April. If we will irrigate only once per week for 12 hours, we will need seven times the flow rate.


<<TOC2>> Chapter 5 - Farm water delivery systems

References

Primary:

(21), (41)

5.1 Control of irrigation water

5.1.1 Components of farm irrigation systems

Some of the components of a water supply and distribution system are shown in Figure 5.1. The primary components are canals or pipelines, control structures in canals, and the field application systems, which are generally surface, sprinkler, or drip irrigation systems. This section covers all of these components.

Good water control is an essential requirement for an efficient irrigation system, both at the project level and at the farm level. Conveyance systems should be designed and maintained to minimize seepage losses, provide for adequate control by the operator, and allow for efficient irrigation. Generally, pipelines or lined ditches that provide for greater seepage control with low maintenance will have higher initial costs.

5.1.2 Open channel systems

(Adapted from Ref. 41, with appropriate modifications)

Unlined ditches are commonly used because of their low cost and ease of construction. They may be bare, temporary earth ditches or protected with some type of vegetation, commonly sod. Special precautions are required in erodible soils, and seepage losses are likely to be high in non-cohesive, coarse textured soils.

Ditches are usually designed for a capacity equal to the crop water requirement during peak demand, plus irrigation and operational losses.

Delivery Channels and Ditches (Extracted from Ref. 41)

Channel Design

In order to determine the channel size required, the maximum discharge, the shape of the planned section, and an estimate of the channel roughness, must be known. The Manning Equation is the most commonly used relationship for determining channel discharge and will be used in this handbook.

where:

Q = discharge, m3/sec (or cfs).
A = cross-sectional area of ditch, m2 (ft2).
R = hydraulic radius - area divided by the wetted perimeter, m (ft).
s = longitudinal slope, m/m (ft/ft).

n = Manning roughness coefficient, m1/6 or ft1/6 (same value for both metric and English units).

C = 1.0 when using metric units, 1.49 for English units.


Figure 5.1 A Farm Irrigation System (Ref. 41)

The Manning roughness coefficient, n, varies from 0.010 for smooth concrete to over 0.10 for channels with weeds and brush. Table 5.1 lists values of n that can be used for design of earthen and lined channels. The value for n should be chosen only after a careful study of the field situation.

The channel design problem is usually determining the width and depth required for a given flow with a measured slope in a given material or with a selected lining of a predetermined shape. In other situations, an estimate of the discharge is required while one knows the ditch size and slope, with an estimate of the roughness (Manning n from Table 5.1). Figure 5.2 gives a solution for the Manning Equation that can be used to make estimates of the ditch shape and flow. Two examples using Figure 5.2 follow.

Example 1: Earth canal in clay loam after weathering, clean; n = 0.022 (Table 5.1).

Assume:

Bottom width, B = 0.45 m (1.5 ft)


Longitudinal slope, s = 0.001


Side slope, z = 1.5 (1.5 horizontal to 1 vertical)


Discharge, Q = 0.10 m3/s (3.5 cfs)

Problem:

Determine the depth of flow.

Solution:

Solve for the Em in Figure 5.2.


Em = (Qn/1/2)/B8/3 = [(0.10)(0.022)/(0.032)]/(0.12) = 0.57


From Fig. 5.2, if z = 1.5 and Em = 0. 57, then D/B = 0.60 m


Because B = 0.45 m (1.5 ft), then D = 0.27 m (0.89 ft)

TABLE 5.1 Values of Manning Roughness Coefficient, n, for Earthen and Lined Channels (Ref. 41)


Roughness coefficient n

Type of Channel and Description

Minimum

Normal

Maximum

A. Excavated earthen channels


a. Straight and uniform



1. Clean, recently completed

0.016

0.018

0.020



2. Clean, after weathering

0.018

0.022

0.025



3. Gravel, uniform section, clean

0.022

0.025

0.030



4. With short grass, few weeds

0.022

0.027

0.033



5. With long grass and weeds

0.030

0.040

0.045


b. Winding and sluggish



1. No vegetation

0.023

0.025

0.030



2. Grass, some weeds

0.025

0.030

0.033



3. Dense weeds or aquatic plants in deep channels

0.030

0.035

0.040



4. Earth bottom and rubble sides

0.028

0.030

0.035



5. Stony bottom and weedy banks

0.025

0.035

0.040



6. Cobble bottom and clean sides

0.030

0.040

0.050


c. Channels not maintained, weeds and brush uncut



1. Dense weeds, high as flow depth

0.050

0.080

0.120



2. Clean bottom, brush on sides

0.040

0.050

0.080



3. As c.2., highest state of flow

0.045

0.070

0.110



4. Dense brush, high stage

0.080

0.100

0.140

B. Lined or built-up channels


a. Cement



1. Neat, smooth surface

0.010

0.011

0.013



2. Mortar

0.011

0.013

0.015


b. Concrete



1. Trowel finish

0.011

0.013

0.015



2. Float finish

0.013

0.015

0.016



3. Finished, with gravel on bottom

0.015

0.017

0.020



4. Unfinished

0.014

0.017

0.020


c. Brick



1. Glazed

0.011

0.013

0.015



2. In cement mortar

0.012

0.015

0.018


d. Masonry



1. Cemented rubble

0.017

0.025

0.030



2. Dry rubble

0.023

0.032

0.035


Figure 5.2 Manning Equation Solution for Determining Canal Design (Ref. 41)

Example 2: Brick with vertical wall, mortar trowel finished surface, n = 0.013 (Table 5.1).

Assume:

Bottom width, B = 0.45 m (1.5 ft)


Depth of section, 0.45 m (1.5 ft)


Freeboard, 0.15 m (0.5 ft)


Depth of flow, D = 0.30 m (1.0 ft)


Longitudinal slope, s = 0.001


Side slope, z = 0

Problem:

Determine the discharge.

Solution:

From Fig. 5.2 for D/B = 0.67 and z = 0, then Em = 0.28 m


Em = (Qn/s1/2)/B8/3 or m


Q = Ems1/2B8/3/n = (0.28)(0.032)(0.12)/0.013 m


Q = 0.083 m3/s (2.93 cfs)

Note that the amount of flow is inversely proportional to the roughness, n; i.e., an increase in roughness decreases the discharge in direct proportion, with shape, slope, and depth remaining the same. If the discharge remains constant and the roughness increases (such as from growing vegetation), then the depth of flow must increase.

So that water does not overflow the ditch, there should be a freeboard (distance from the maximum water surface to the top of the banks) of at least 15 cm (6 in) for small canals. The banks tend to lower with seasoning, aging of the canal, and use of the banks by traffic.

Earth Ditches

Unlined earth ditches are the most common means of conveying irrigation water to farm fields. Unlined ditches are preferred by many farmers because they can be built cheaply and easily and maintained with farm equipment. Also, unlined ditches provide flexibility - it is easy to change the layout, increase capacity, or even eliminate ditches after a rotation and rebuild them the next season. Unlined ditches have many disadvantages, however, that make them less desirable than lined ditches or underground pipe:

· They occupy more land than lined ditches.
· They usually lose more water due to seepage, leakage, and spillage.
· Rodents can cause leakage.
· If weed growth is a problem, frequent cleaning is needed.

· Earth ditches can erode and meander, creating problems in maintaining straight or proper alignment.

The slope for an earth ditch may be as low as 0.00018. (Egyptian irrigation canals generally have slopes ranging from 0.00018 to 0.00020.) Small slopes result in slow flow velocities, large cross sections, and possible sediment deposition on the bed.

It is customary to use a gradient of 0.001 in many areas. The slope of the ditch should be such that the bed does not erode and the water flows at a self-cleaning velocity, i.e., there is no deposition. A heavy clay soil will allow fairly high velocities without eroding (Table 5.2). At times it is necessary to insert drops into the ditch to reduce velocities and prevent scour and erosion. For soils normally encountered, the maximum velocities given in Table 5.2 should not be exceeded. For Example 1 above, the average velocity for an earth canal in clay loam is 0.43 m/s (1.4 ft/s). For Example 2, and lined ditch, the velocity is 0.61 m/s (2.0 ft/s). Both of these velocities are in the safe range. For unlined ditch side slopes, the lower value (steeper slopes) given in Table 5.2 should be used for cuts and the higher value (flatter slopes) for canals excavated in a fill section.

The approximate sizing of earth ditches with a side slope of 1.5:2 (Z = 1.5) is given in Figure 5.3 and can be used for preliminary design. With an estimate of slope, roughness factor, and desired discharge, several possible ditch sizes can be determined. Conversely, with a known ditch shape (bottom width), roughness, and discharge, the required depth and slope can be estimated. By using the Manning Equation, tables similar to that in Figure 5.3 can be developed for other ditch shapes, roughness, and slopes.

Ditch locations should be carefully planned to serve the irrigated area adequately. If adjacent fields are being levelled, any needed fill material for the ditch can be easily obtained. Earth ditches can be formed manually or with pulled ditchers. The animal-powered V-ditcher can be run in furrows opened by a moldboard-type plow. Two furrows are made adjacent to each other, with the furrow slice thrown in opposite directions. The V-ditcher then moves the soil to form a berm on each side. It is usually necessary to plow a second or third time to obtain more earth for the banks.

Table 5.2 Suggested Maximum Flow Velocities and Side Slopes for Lined and Unlined Channels

Type of Surface

Maximum Flow Velocities m/sec

Side Slopes ft/sec

Range (z)a

Unlined Ditches, Seasoned

Sand

0.3 - 0.7

1.0 - 2.3

3


Sandy loam

0.5 - 0.7

1.6 - 2.3

2 to 2.5


Clay loam

0.6 - 0.9

2.0 - 3.0

1.5 to 2b


Clays

0.9 - 1.5

3.0 - 5.0

1 to 2b


Gravel

0.9 - 1.5

3.0 - 5.0

1 to 1.5


Rock

1.2 - 1.8

4.0 - 6.0

0.25 to 1

Lined Ditches


Concrete



Cast-in-place

1.5 - 2.5c

5.0 - 8.2c

0.75 to 1.5



Precast

1.5 - 2.0

5.0 - 6.5

0 to 1.5d



Brick

1.2 - 1.8

4.0 - 6.0

0 to 1.5d


Asphalt



Concrete

1.2 - 1.8

4.0 - 6.0

1 to 1.5



Exposed membrane

0.9 - 1.5

3.0 - 5.0

1.5 to 2



Buried membranee

0.7 - 1.0

1.6 - 3.3

2


Plastic



Buried membranee

0.6 - 0.9

2.0 - 3.0

2.5

a z is the horizontal unit to one (1) vertical unit.
b Side slopes of 1:1 for small canals in clay and clay loam are common.
c Flows in this velocity range may be supercritical (see definitions) and difficult to control. They are not recommended except for special uses.
d Small precast and brick channels may have vertical walls (z = 0).
e Maximum flow velocities will depend on the cover over the membrane.

Many ditches erode and deteriorate. It is better to remove old ditches and form new ones. Figure 5.4 gives a procedure for doing so that will result in a new, more stable channel that will lose less water than the old one. The compaction and forming of the new channel can be done manually or with a machine. Soil that has a high percentage of silt and clay will form the best channel from all standpoints.

Flat bed before ditch is formed

Finished ditch section















s = 0.00005

s = 0.001

s = 0.002

s = 0.003















n = 0.03

n = 0.04

n = 0.03

n = 0.04

n = 0.03

n = 0.04

n = 0.03

n = 0.04

B

D

F

W

T

A

R

Q

Q

Q

Q

Q

Q

Q

Q

m

ft

m

ft

m

ft

m

ft

m

ft

m2

ft2

m

ft

m1/s

ft1/s

m1/s

ft1/s

m1/s

ft1/s

m1/s

ft1/s

m1/s

ft1/s

m1/s

ft1/s

m1/s

ft1/s

m1/s

ft1/s

0.15

0.5

0.30

1.0

0.15

0.5

0.30

1.00

2.6

1.5

0.19

2.00

0.15

0.49

0.04

1.4

0.03

1.0

0.05

1.9

0.04

1.5

0.01

2.7

0.06

2.1

0.10

1.4

0.07

2.5

0.30

1.0

0.30

1.0

0.15

0.5

0.46

1.50

3.0

10.0

0.23

2.50

0.16

0.54

0.05

1.8

0.04

1.4

0.07

2.6

0.06

2.0

0.10

3.7

0.08

2.1

0.13

4.5

0.10

3.4

0.46

1:5

0.30

1.0

0.15

0.5

0.61

2.00

3.5

11.5

0.21

3.00

0.11

0.59

0.07

2.3

0.05

1.7

0.09

3.3

0.07

2.5

0.11

4.7

0.10

3.5

0.16

5.7

0.12

4.3

0.61

2.0

0.30

1.0

0.15

0.5

0.76

2.50

4.0

13.0

0.33

3.50

0.19

0.62

0.08

2.8

0.06

2.1

0.11

4.0

0.01

3.0

0.16

5.6

0.12

4.2



0.15

5.2

0.10

1.0

0.17

1.2

0.21

0.7

0.31

1.25

3.4

11.3

0.31

3.16

0.19

0.61

0.0

2.7

0.06

2.0

0.11

1.9

0.01

2.9

0.16

5.5

0.12

4.1



0.14

5.0

0.46

1.5

0.37

1.2

0.21

0.7

0.46

1.50

1.7

12.1

0.17

3.96

0.21

0.61

0.10

3.4

0.07

2.5

0.14

4.8

0.10

3.6



0.14

5.1



0.18

6.2

0.61

2.0

0.17

1.2

0.21

0.7

0.61

2.00

4.2

13.8

0.42

4.56

0.22

0.72

0.12

4.1

0.08

1.1

0.16

5.7

0.12

4.3



0.17

6.1





0.46

1.5

0.41

1.13

0.24

0.8

0.51

1.75

4.1

11.6

0.43

4.65

0.23

0.74

0.12

4.2

0.09

3.2



0.13

4.5









0.61

2.0

0.41

1.33

0.24

0.1

0.61

2.00

4.5

14.6

0.49

5.31

0.24

0.71

0.14

4.9

0.11

3.1



0.15

5.3









0.61

2.0

0.46

1.5

0.10

1.0

0.46

1.50

4.7

15.5

0.59

6.31

0.26

0.86

0.18

6.4

0.14

4.1













0.91

3.0

0.46

1.5

0.30

1.0

0.61

2.00

5.3

17.5

0.73

7.11

0.29

0.94



0.18

6.3













A - cross sectional area

R - hydraulic radius

n - Manning's roughness coefficients


0.03 - soil with gravel


0.04 - soil with grass

!s - slope

Q - ditch flow capacity


Figure 5.3 Earth Irrigation Ditch Sizes for Different Slopes, Roughness, and Discharges (Ref. 41)

The importance of good construction for earth channels depends a great deal on expected ditch usage. Some ditches, such as those run on a contour for grain and rice, are used only one season and then filled in. Other ditches are relatively permanent and should be constructed with more effort and care. Ditches intended for furrow or border irrigation directly from the ditch need substantial banks, and the banks might be higher for using spires and siphon tubes than for open ditch bank cuts. In this case, the top of the banks should be a minimum of 25 cm (10 in.) above the surrounding field surface. Banks must be high enough to allow the water level to be increased by checks, if needed. If seepage is excessive, compaction of the banks or deposition of a clay blanket can be tried.

5.1.3 Control structures

Good control structures are required to reduce labor requirements and simplify irrigation by providing easy and positive control of the water. Structures are used to control the water as it is conveyed from the main canal or lateral headgate to its destination on the field. Structures may be required to control the channel or ditch itself. These water control structures ensure adequate water levels, dissipate energy, provide accurate distribution, and deliver water to the field without erosion.

Grade control structures (Figures 5.5, 5.6, and 5.7) are used to prevent erosive velocities where unlined canals are on steep slopes. The water is lowered over drops and carried down the slope in a series of "stair steps." Stair steps basically consist of either vertical or inclined drops and a stilling pool or other means of dissipating the energy. Stair steps may also be used in combination with check structures, which are used to control the water level in field ditches. Drop heights in conveyance channels should generally be limited to 1 m, with the recommended height at about 0.6 m.

Drop heights in field distribution ditches are more limited because of delivery capability requirements; the maximum height should be limited to 0.3 m, with the recommended height at about 15 to 20 cm. For small drop structures, prefabricated structures may be used. Erosion control is improved by the use of wide stilling basins with low end sills and/or gravel lined stilling basins. Where steep ditch slopes require the use of many closely spaced drop structures, a lined ditch or a buried pipeline may be an economical alternative. A pipe drop is a commonly used structure for grade control, particularly when a combined road crossing is needed.

Figure 5.4 Suggested Procedure for Mechanical Reconstruction of Earthen Channels with a Tractor-Drawn Scraper, Ditcher, and Compactor (Ref. 41)


1) Remove the Old Banks and Pile the Organic, Vegetation Filled Bank Soil away from the New Channel Site.


2) Build a Pad of Clean, Moist Soil on the New Channel Site and Compact the Pad in 10-15 cm (4-6 in.) Layers.


3) Pull the Ditch in Stages, Compacting the Bank Soil between each Excavation In 10-15 cm (4-6 in.) Layers.


4) Continue Enlarging the Channel and Compacting the Moist Soil Deposited on the Banks in Layers.


5) Trim and Shape the New Compacted Banks to the Design Crass Section


Figure 5. 5 Examples of Small Drop Structures (Ref. 41)


Figure 5.6 Sloping Rock Drop Structure (Ref. 41)

Note:

1. Gravel (if available) should be used to fill between Rock.
2. Rocks con be Grouted


Figure 5.7 Drop Structure with Gravel/Rock Stilling Basin (Ref. 41)

Check structures (Figures 5.8, 5.9, and 5.10) are used to maintain or increase the water level in an open channel above the normal flow depth. If provided with flashboards, drop structures can also serve as check structures. When a constant water level is desired upstream from the check structures, an Overflow-type check is generally used. The structure is usually fitted with grooves to accept flashboards or stop logs that permit water to flow over them while maintaining the water level upstream. Flow over such a type of check may be estimated by the general weir equation.

Discharge control structures (Figure 5.11), also referred to as outlets or turnouts, are used to control the release of water to laterals or ditches, or from a field irrigation ditch to borders, basins, or furrows. One of the most frequently used turnout is a concrete or metal pipe with a slide gate on the inlet or upstream end. For unlined ditches, the headwall and slide gate are usually vertical. Discharge through these pipes can be estimated by the orifice equation. Weir type openings are also used, and their capacity can be estimated with the general weir equation.

Siphon tubes are widely used for distributing water from field ditches into furrows, basins, or borders on the field. The flow through these siphon tubes depends on their length, diameter, number of bends, and material from which they are constructed.

Siphons eliminate the need for cutting the ditch bank, thus reducing labor and ditch maintenance. Spiles are sometimes used in place of siphon tubes; they consist of short pipes or other small conduits through the ditch bank.

Division boxes (Figure 5.12) are used to divide water into two or more ditches. Flow measurement of both streams assures the most equitable and accurate division, but flows can be considered to be proportional to the size of the divisions if approach and exit conditions are adequate. Some boxes are designed to give fixed proportional division while others have a moveable splitter to vary the proportions. Usually, more accurate proportions can be obtained by dividing the flow at a control section where super critical flow occurs, such as at a free overfall. Satisfactory division can also be made without the critical flow section if the approach channel is long and straight and the flow conditions downstream do not favor one channel or the other.

Sediment, trash, and weed seed can cause serious problems in irrigation systems by clogging sprinklers, pipelines, and siphon tubes. Trash racks, screens, settling or distilling boxes, and sediment traps of various design can be utilized to minimize the effects of these problems.

Figure 5.8 Wooden Ditch Checks with Different Openings (Ref. 41)


(a) Top-opening Gate with Removable Section Cover


(b) Center-opening Gate with Unit Slide Cover


(c) Bottom-opening Gate with Swinging Cover


Figure 5.9 Small Concrete Ditch Check (Ref. 41)


Figure 5.10 Wood, Single Well Check with Turnout (Ref. 41)


Figure 5.11 Turnout Structures


dimensions (cm)

Q

W

H

P

A

30-60 1/sec

30

40

30

80


Figure 5.12 Flow Dividers


dimension (m.)

FLOW

W

H

30- 60 1/sec

.5

.5

All water control structures must be periodically checked and maintained. Follow these recommendations:

· Remove weeds and trash that restrict the flow of water. Clean the canals and raise their banks when necessary.

· Don't put more water in the canal than it can carry.

· Fix seeps and leaks in the canal.

· Don't cut ditchbanks just anywhere where water is needed. Select a few places and use the appropriate outlet structure.

· Keep animals away from the canal and its banks. They destroy the banks and protective vegetation.

· Fix breaks in the structures promptly before they become serious.

<<TOC3>> 5.2 Pipeline hydraulics and design

The design of pipelines requires an understanding of hydraulics. Hydraulics describes water and its behavior in closed conduits (pipelines). The most basic concepts that irrigationists should understand and be able to work with include:

· the continuity equation,
· head, pressure, and energy,
· friction, and
· water hammer.

5.2.1 Continuity equation

The continuity equation is often written as:

where:

Q = flow rate.
V1 = velocity of the water at Point 1.
A1 = cross-sectional area of the flow at Point 1.
V2 = velocity of the water at Point 2.
A2 = cross-sectional area of the flow at Point 2.

The equation assumes that the water is incompressible and there are no fluid losses between points 1 and 2. For example, a pipeline that carries 2.5 liters/sec, or 2500 cm3/sec, will have a velocity of 178 cm/sec in a 50 mm diameter pipeline, and 112 cm/sec in a 63 mm diameter pipe.

Example: If we need to carry 2.5 liters/sec at a maximum velocity of 2.0 m/sec, what size pipe would be needed?

Step 1. Find the area of the pipe required.

Step 2. Find the inside diameter of the pipe required. (Remember the area of a pipe A = pr2, with p = 3.142 and D = 2r, where r is the radius and D is the diameter.)

Thus, the diameter,

D = 2r = 2
.79 = 1.58"

D = 2r = 2
2 = 4 cm

Step 3. Choose the closest pipe size available. (Remember, pipe is usually available in 1/2, 3/4, 1, 1 1/2, 2, 2 1/2, 3, 4, 5, 6, 8, 10, 12, and 15 inches.)

We would normally ask for a 1 1/2" pipe.

5.2.2 Pressure, head, and friction losses

(Adapted with appropriate modifications from Ref. 21)

Pressure Exerted by a Column of Water

A column of water exerts a force due to the weight of the water. The pressure, or force per unit area, is dependent on the height of the column of water. Therefore, head, or water pressure, is usually expressed in terms of the equivalent height of water needed to exert that pressure. The pressure under static conditions is not dependent on pipe diameter. (See Figure 5.13)


Figure 5.13 Example of Static Head (Ref. 21)

The pressure at the bottom of each column of water is the same. It is 10 meters of head, or 1.0 kg/cm2. The pressure midway in each column would be 5 meters of head, or 0.5 kg/cm.

Pressure in a Static System

In a system under static conditions, the pressure at any point is dependent on the difference in height between the point in question and the highest point in the system. If an opening is made in the pipe in any part of the system and a tube connected to it, the water level will rise until it is the same as the highest point. (See Figure 5.14)


Figure 5.14 Head on System with No Flow (Ref. 21)

The system in Figure 5.14 is static, and no flow occurs. The pressure or head at Points B, C, F, and H is the same; i.e., 10 meters. The pressure or head at Point E is 5 meters, or the difference in height between Points A and E. If the pipeline were opened and a tube connected to it at Point C or F, then the water would rise 10 meters and would be at the same level as Points A, D, and G.

Pressure in a Flowing System When water in the pipeline is flowing, the pressure is no longer dependent solely on the height difference with respect to the highest point. There is a loss of pressure or head due to friction between the water and the pipe. The pressure or head at any point is equal to the static head (relative height difference) minus the head loss due to friction and is known as the dynamic head level. Because of the head loss, the water will not rise to the same level as the highest point but only as high as the pressure or head at that point. Head loss occurs only when water is flowing. (See Figure 5.15)


Figure 5.15 Hydraulic Gradient Line With and Without Flow (Ref. 21)

Under flowing conditions, the pressure is no longer the same, and the pressure at Point C or C1 is not sufficient to raise the water level to Points D or F. The height difference between Points D and E, or Points F and G, is the head loss due to friction in the pipeline. If the flow were stopped, the water level would return to Points D and F.

5.2.3 Factors influencing head loss

The amount of head loss is influenced by the following factors:

a. The length of pipe.

The longer the pipeline, the greater the head loss. This loss is directly proportional to the length; i.e., the head loss for 200 meters of pipe would be twice that for 100 meters under the same conditions.

b. The diameter of the pipe.

The smaller the diameter of the pipeline, the greater the friction will be for the same flow of water. The differences are not proportional.

c. The velocity of water in the pipe.

The higher the flow rate of water in a given pipe, the greater the head loss due to friction. Friction increases as the square of the velocity.

d. The nine material.

The smoother the inner surface of the pipe, the lower the head loss. Thus, since PVC pipe is smoother than steel or cast iron, it has a lower head loss for identical conditions.

e. The number of fittings or bends in the pipeline.

A straight pipeline would have a lower head loss than one of the same length with fittings or bends.

5.2.4 Pine design

In designing a gravity flow pipeline, several factors are of primary importance. The design flow is calculated by the designer to fit the needs. A pipe size is then chosen that will result in adequate flow and pressure at the discharge after head losses are accounted for. The following data are required as a minimum for a design:

· flow rate required,
· pressure or head required at outlet,
· length of pipeline,
· elevation profile of the land where the pipeline will be laid from source to discharge,
· availability and cost of materials,

· description of soils and terrain on which pipe will be laid and storage and inlet facilities located.

5.2.5 The hydraulic gradient line (HGL)

The Hydraulic Gradient Line (HGL) is determined by subtracting the head loss in the pipeline from the static head. The difference between the ground profile and the HGL is the pressure in the pipeline while the water is flowing. If an opening were made in the pipeline and a tube connected to it, then the water would rise to the level of the HGL. The HGL should always lie above the profile. If it does not, the water may still flow but in sections where the profile lies above the gradient, a negative pressure ensues that can cause air or contaminants to enter the pipeline. Sections of the pipeline where negative pressures occur should be redesigned. Figures 5.16 and 5.17 illustrate redesign to eliminate negative pressure.


Figure 5.16 Hydraulic Gradient Line with Uniform Pipe Size (Ref. 21)

In Figure 5.16, if the pipeline followed ground profile A, the choice of pipe with the given HGL would be acceptable. If the pipeline followed ground profile B, negative pressure would exist in section C, so the pipeline should be redesigned.


Figure 5.17 Hydraulic Gradient Line with Change in Pipe Diameter (Ref. 21)

In Figure 5.17, the pipe diameters have been changed with larger pipe in the initial section (less head loss), thus changing the HGL. Use of a larger diameter pipe near the source ensures that the HGL lies entirely above the ground profile and is acceptable. Note that two pipe diameters are now used between the source and reservoir. For each diameter, the HGL has a different slope. The slope is directly dependent on the head loss, so a smaller diameter pipe has a steeper slope.

5.2.6 Pipeline design sample problems

Plotting the pipeline profile is a process of trial and error. The calculated values for head losses from different sizes of pipe are compared to the available head on the profile drawing. The smallest diameter pipe that results in acceptable flow and pressure is chosen for each continuous section of the pipeline.

Example 1: A spring with a flow of 0.5 L/s is 1,000 meters from the farmer's field, and the available head is 20 meters. It is planned to convey the entire flow to a small reservoir. What size pipe is recommended?

Add 10% additional friction losses to the friction losses estimated from using Tables such as 5.3 and 5.4 or the Hazen-Williams equation (see section 5.2.8, "Calculating Friction Losses”). Using Table 5.4 with an additional 10% friction loss and assuming the use of galvanized iron pipe, a flow of 0.5 L/s, and a length of 1,000 meters, a 1.5-inch pipe results in a head loss of 11 meters. For 1.25-inch pipe, the head loss is 26 meters. Thus, the required flow will not be obtained with a 1.25-inch pipe. A 1.5-inch pipe could be used; however, the most economical solution is a combination of two pipe sizes. A 1.5-inch GI pipeline of 500 meter length with a flow of 0.5 L/s has a head loss of 6 meters, and 500 meters of 1.25-inch GI pipe has a head loss of 13 meters. Thus, the total head loss for the 1,000 meter pipeline is 19 meters, which closely matches the available head. The pipeline profile and HGL are plotted in Figure 5.18.


Figure 5.18 Example: Sketch for Pipeline 1 (Ref. 21)

Table 5.3 Rigid PVC Frictional Bead Loss Factors (Ref. 21)

RIGID PVC FRICTIONAL HEADLOSS FACTORS

These are the approximate headless factors, in m/100 m (%), for new rigid PVC pipe. Flows are in liters/second.

FLOW

1/2"

3/4"

1

1 1/4"

1 1/2"

2"

2 1/2"

3"

4

0.1

4.2

1.0

0.25

0.08






0.15

8.8

2.2

0.53

0.17

0.07





0.2

15.0

3.7

0.9

0.28

0.12





0.25

22.0

5.5

1.35

0.44

0.18





0.3

31.0

7.8

1.9

0.6

0.25





0.35

41.0

10.0

2.45

0.8

0.34





0.4

53.0

13.0

3.1

1.0

0.43





0.45

66.0

16.3

4.0

1.25

0.54

0.13




0.5


19.0

4.8

1.5

0.65

0.16

-



0.55


23.5

5.6

1.8

0.78

0.19




0.6


27.5

6.6

2.1

0.9

0.22




0.65


32.0

7.8

2.4

1.04

0.25




0.7


36.0

8.7

2.7

1.19

0.28




0.75


41.0

9.9

3.1

1.32

0.33

0.1



0.8


45.0

11.0

3.5

1.5

0.37

0.12



0.85


52.0

12.5

4.0

1.7

0.41

0.14



0.9


57.0

14.0

4.5

1.9

0.45

0.15



0.95


63.0

15.0

4.9

2.1

0.5

0.17



1.0



16.5

5.4

2.25

0.55

0.18

0.08


1.05



18.0

5.8

2.5

0.6

0.20

0.09


1.1



19.5

6.3

2.7

0.67

0.22

0.1


1.15



21.5

6.9

2.95

0.71

0.24

0.11


1.2



23.0

7.3

3.2

0.78

0.26

0.12


1.3



26.5

8.6

3.75

0.9

0.29

0.13


1.4



30.0

10.0

4.25

1.0

0.34

0.15


1.5



35.0

11.2

4.9

1.15

0.39

0.17


1.6



39.0

12.5

5.5

1.3

0.43

0.19


1.7



44.0

14.2

6.05

1.45

0.49

0.21


1.8



49.0

15.9

6.9

1.6

0.54

0.24


1.9



55.0

17.4

7.5

1.8

0.6

0.26


2.0



60.0

19.0

8.0

2.0

0.66

0.28


2.2




22.5

9.7

2.35

0.79

0.34


2.4




26.8

11.5

2.75

0.9

0.4


2.6




31.0

13.3

3.2

1.05

0.45


2.8




35.1

15.2

3.7

1.2

0.52


3.0




40.0

17.0

4.2

1.36

0.6


3.2




45.0

19.3

4.7

1.52

0.68


3 4




50.0

21.9

5.25

1.6

0.75


3 6




56.0

24.0

5.8

1.9

0.84

0.2

3.8




62.0

26.0

6.3

2.1

0.9

0.22

4.0




69.0

29.0

7.0

2.3

1.0

0.24

4.5





36.0

8.8

2.8

1.2

0.3

5.0





44.0

10.5

3.5

1.5

0.37

5.5





62.0

12.5

4.2

1.75

0.44

6.0






14.7

4.9

2.1

0.52

6.5






17.0

5.6

2.4

0.6

7.0






19.5

6.5

2.8

0.7

Table 5.4 GI Frictional Head Loss Factors (Ref. 21)

GI FRICTIONAL HEADLOSS FACTORS

These are the approximate headless factors, in m/100 m (%), for new GI pipe. Flows are in liters/second.

FLOW

1/2"

3/4"

1

1 1/4"

1 1/2"

2"

2 1/2"

3"

4"

0.1

5.9

1.58

0.38

0.12






0.15

12.25

3.4

0.82

0.26






0.2

21.45

5.65

1.4

0.44

0.19





0.25

31.65

8.5

2.1

0.68

0.28





0.3

44.91

11.9

2.9

0.92

0.4





0.35

58.2

15.8

3.8

1.2

0.52





0.4

75.5

19.9

4.8

1.55

0.67





0.45

91.9

25.0

6.0

1.93

0.84





0.5


30.0

7.3

2.35

1.0

0.25




0.55


36.0

8.7

2.75

1.2

0.3




0.6


42.0

10.2

3.25

1.4

0.35




0.65


48.0

11.9

3.8

1.63

0.4




0.7


55.0

13.6

4.35

1.82

0.46




0.75


63.0

15.4

4.9

2.15

0.52

0.17



0.8



17.4

5.55

2.4

0.59

0.19



0.85



19.4

6.15

2.65

0.68

0.21



0.9



21.8

6.9

2.9

0.74

0.23



0.95



24.0

7.5

3.25

0.82

0.26



1.0



26.2

8.2

3.6

0.88

0.28

0.12


1.1



31.0

9.8

4.2

1.05

0.34

0.15


1.15



34.6

10.6

4.6

1.15

0.37

0.16


1.2



36.0

11.3

5.0

1.25

0.39

0.17


1.3



42.5

13.3

5.7

1.45

0.45

0.2


1.4



48.0

15.3

6.6

1.65

0.52

0.23


1.5



55.0

17.5

7.65

1.9

0.59

0.26


1.6



62.0

19.5

8.45

2.1

0.67

0.29


1.7



69.0

22.0

9.5

2.35

0.75

0.33


1.8




24.2

10.5

2.6

0.82

0.36


1.9




26.5

11.7

2.85

0.9

0.4


2.0




29.5

12.8

3.2

1.0

0.44


2.2




35.0

15.3

3.8

1.2

0.52


2.4




42.0

17.9

4.45

1.4

0.61


2.6




48.5

20.5

5.15

1.6

0.71

0.17

2.8




55.0

24.0

5.95

1.85

0.82

0.2

3.0




62.5

26.7

6.7

2.1

0.92

0.22

3.2





30.0

7.6

2.35

1.02

0.25

3.4





34.0

8.4

2.65

1.15

0.28

3.6





38.0

9.4

2.95

1.28

0.32

3.8





41.0

10.3

3.25

1.42

0.35

4.0





45.0

11.2

3.55

1.55

0.38

4.5





56.0

14.0

4.45

1.95

0.46

5.0






17.0

5.45

2.25

0.56

5.5






20.0

6.5

2.8

0.68

6.0






24.0

7.5

3.35

0.8

6.5






28.0

8.65

3.9

0.92

7.0






32.0

10.0

4.45

1.05

Example 2: A water source is 1,000 meters from Farm A, and it is 1,000 meters farther to Farm B. The available head between the source and Farm A is 20 meters, and between Farm A and Farm B it is also 20 meters. The design flows are 2.0 L/s from the source to Farm A, and 0.5 L/s from Farm A to Farm B. What are suitable pipe diameters?

A suitable selection of GI pipe would be 2.5-inch pipe for the first 1,000 meters and 1.25-inch pipe for the second 1,000 meters. The total head loss is then 37 meters, which closely matches the total available head of 40 meters. Note that the second 1,000 meters has a head loss of 26 meters and an available head of only 20 meters. This deficit is allowable because there is excess head available from the first 1,000 meters of the pipeline, and the HGL is always above the pipeline profile. See Figure 5.19.


Figure 5.19 Example: Sketch for Pipeline 2 (Ref. 21)

5.2.7 Pipes and pipeworking

Types of Pipe

The proper selection and use of pipe is a vital component of all gravity and pumped water systems. Therefore, it is important for all water technicians and engineers to be familiar with the characteristics of various types of pipe and learn the correct methods of working with pipes in the field. The three types of pipe that are widely distributed around the world and commonly used for piped water systems are discussed below.

Galvanized iron pipe (GI) is regular iron pipe that is coated with a thin layer of zinc. The zinc greatly increases the life of the pipe by protecting it from rust and corrosion. GI usually comes in 6-meter (21-foot) lengths, and is joined together by threaded connections.

Plastic polyethylene pipe (PE) is black, lightweight, flexible pipe that comes in large coils 30 meters or more in length. The pipe varies in density and is joined by inserted fittings with clamps or heat fusion.

Plastic polyvinyl chloride pipe (PVC) is a rigid pipe, usually white or gray in color. It comes in 3 or 6 meter lengths and is joined primarily by solvent cement but can also be threaded. The pipe varies in density and, when buried, is extremely resistant to corrosion.

Table 5.5 lists some of the characteristics of the three types of pipe.

5.2.8 Working with pipes

Galvanized Iron

Before the advent of plastic pipe, GI was the primary type of pipe used in water systems. Much of it is still in use today. GI has several advantages in a water system: it is very durable in the field, able to withstand high pressure heads, and resistant to water hammer. Leakage is also rare because the pipe is very hard to puncture, and the threaded joints tend to seal themselves over time. GI pipe may be laid above ground, under roads, or across streams, performing well under all these conditions. The threaded joints, however, can be broken much more easily than the solid pipe and, therefore, must always be well supported.

GI pipe also has a number of disadvantages: its weight makes it difficult to transport, threaded joints are difficult and time consuming to make, certain kinds of water can corrode and rust the pipe, and it is difficult to repair in the field or tap in new branch lines.

The tools necessary for working with GI pipe are costly. If such tools are properly maintained, however, they can last a lifetime. The basic tools for such work are: pipe vise or clamp, pipe threader, pipe reamer, cutting oil, pipe dope or Teflon tape, pipe cutter or hacksaw, steel file, wire brush, and large pipe wrenches (14", 18").

A variety of fittings are used to connect the pipe. Pipe threads are called "male" for outside threads, and "female" for inside threads. The pipes themselves usually come in 21-foot (6-meter) lengths and are factory threaded at both ends; usually one coupling is also provided. A variety of diameter sizes are available, from small (3/8", 1/2", and 1") to large (4" and 6"); these sizes always refer to the inside diameter of the pipe. The outside diameter would measure 1/4" larger because of the wall thickness. The typical procedure for cutting, threading, and joining GI pipe is as follows:

Table 5.5 Characteristics of Different Pipe Materials (Ref. 21)


GI

PE

PVC

Life Expectancy Resistance to Corrosion Underground

Very long life expectancy of 30 years or more. However, joints are subject to rust and may break if not properly supported.

Generally good life expectancy. However, has low stress resistance and poor rigidity.

Long life expectancy if properly laid and backfilled.

Resistance to Corrosion or Chemicals Inside Pipe

Will corrode in acid, alkaline, and hard water.

Very resistant. However, very soft or very hard water can corrode.

Very resistant. However, very soft or very hard water can

*Safe Working Pressures (PSI)

Adequate for all pressures found in small scale water systems.

Rating from 80-160 PSI.

Ratings from 80-600 PSI.

Resistance to Puncturing and Rodents

Very high resistance.

Low resistance.

Good resistance,

Effect of Sun and Weather

No effect; however, threaded ends may rust.

Weakens with exposure.

Weakens with exposure.

Ease of Joining, Laying, Bending

Difficult to join, lay and bend. Very heavy.

Easy to join and lay because of few joints and light weight. B ends readily, but will collapse on short bends.

Easy to join and lay. Rigid, but will bend on long radius. Can be bent by heating.

Cost

Very high cost, especially in larger diameters.

Low cost.

Moderate cost.

* Always check pressure ratings with local manufacturer.

1. Clamp the pipe securely in the vise, with 6"-8" protruding from the vise jaws.

2. If the pipe needs cutting, cut with a pipe cutter or hacksaw. Make the cut straight, and clean all burrs with a pipe reamer or steel file. It is very important to remove all burrs.

3. Carefully place the pipe threader on the end of the pipe. Make sure that you are using the correct size pipe die, and that you have the teeth facing the correct way. The large end of the tapered teeth should go on first. Start the guide onto the pipe by firmly holding the die with one hand and turning the ratchet handle clockwise several turns with the other hand. Check to make sure that the threader is on straight and the die teeth are cutting properly. Squirt cutting oil generously onto the end of the pipe as you continue to rotate the threader. Every couple of turns, back off the threader a quarter turn to clear off the burr, especially if the teeth bind. Keep going one full turn past the point where the pipe emerges from the die. Stop, reverse the ratchet, and back the threader off the pipe. The threads should be clean, sharp, and continuous, with no broken points or burrs. Clean the threads with a wire brush and cloth rag.

4. At this point, you should always test the thread size by putting on a fitting. If you plan to install a fitting, start by applying pipe dope compound to the male thread, not the fitting itself. This will allow the fitting to be easily installed and, more importantly, removed at a later date if necessary. Teflon tape can be used instead of pipe dope. The tape is wrapped tightly, 1 1/2 layers around the threads in a clockwise direction.

5. Start the fitting with your hand by turning clockwise. Make sure that it is on straight and not cross-threaded. It should turn two or three rotations with your hand before it becomes tight. Now tighten with a pipe wrench, pulling the handle towards the open jaws, not away from the jaws. This will make the teeth bite and hold. Keep turning until the fitting is tight. Usually, the fitting is tight when two or three threads are left exposed. Be careful not to apply too much force. The threads are tapered and too much pressure can split the fitting. If you are using wrenches to install the fitting, place the second wrench facing in the opposite direction on the pipe, close to the fitting. You may turn either the pipe or the fitting, whichever is easier.

Plastic Pine

Plastic pipe has become the preferred type of pipe for small water systems around the world. It has several advantages when used in a water system: it is lightweight and easily transported; simple to join, cut, and lay; low in cost relative to GI pipe; very resistant to corrosion inside the pipe; and its smooth inner walls reduce friction loss factors. It also has some disadvantages, however: it is more easily punctured, will withstand only moderate pressure heads, weakens when exposed to weather, and must often be laid underground in a particular manner in order to perform satisfactorily.

The two most common types of plastic pipe are Polyethylene (PE) and Polyvinyl chloride (PVC).

Polyethylene pipe

The tools necessary for working with PE pipe are few. The basic tools are: handsaw, file or rasp, pliers, and screwdriver. Fittings consist of adapters to inside and outside threads, couplings, elbows, and tees. They are made of either plastic or steel. The pipe comes in large coil rolls, and size is based on inside diameter. PE pipe also comes in various densities that correspond to the amount of pressure head that it will withstand. Its light weight and flexibility make it easy to work with and, because it comes in large rolls, very few joints are needed when laying the pipe. Of the three pipes discussed here, however, PE is the weakest. It has poor rigidity, is the most easily punctured, and handles the lowest pressure heads.

When joining PE, the pipe slips over serrated fittings and is clamped with stainless steel worm drive clamps or secured tightly with thin steel wire if clamps are not available. The typical procedure for cutting and joining PE pipe is as follows:

1. With a hand saw, make a straight cut on the pipe ends that are to be joined.
2. Remove all burrs with a file, rasp, or knife.
3. Slide the clamps onto the pipe ends.
4. Position the fitting on the pipes and join the ends together.

5. Tighten the clamps with a screwdriver, or securely wrap steel wire around the pipe and tighten with pliers.

Polyvinyl chloride (PVC)

PVC is the most versatile pipe used in small rural water systems. The tools necessary for working with PVC are: handsaw or hacksaw, file or rasp, clean, dry rags, and PVC solvent cement and applicator. Fittings consist of couplings, reducers, elbows, adaptors, tees, and caps. They are joined together by the use of solvent cement. The pipe comes in 3- or 6-meter lengths and is usually gray or white. It also comes in various densities that correspond to the amount of pressure head that it will withstand. It is lightweight, but its rigidity makes it quite strong. It is very resistant to corrosion and, when properly laid in a trench, will last indefinitely. Cutting, joining, and laying PVC pipe is a simple process. The typical procedure is as follows:

1. With a hand or hacksaw, make a straight cut on the pipe ends that are to be joined.

2. Remove all burrs inside and out with a file, rasp, or knife.

3. Clean all pipe surfaces that will be joined. The pipe and fitting must be clean and dry. You may rough up the pipe surface with a file or rasp for better contact.

4. If available, apply pipe cleaner to the pipe and fitting.

5. Apply a liberal coating (it should not be dripping off, however) of solvent cement to the pipe surface and fitting. Coat all the way around the pipe and fitting. Work quickly because some types of solvent cement set up very quickly. Also, do not expose any cement to direct sunlight, if possible.

6. Join the two surfaces together firmly, making sure that the pipe is pushed all the way into the fitting.

7. Gently set the joint down and do not disturb it until it has reached its initial set. (This set time will vary with different types of solvent cement; check the label.)

Trenching

Both PVC and PE should be buried underground to provide long and trouble free service. Therefore, it is necessary to dig a trench the entire length of the pipeline. Trenching is no easy job, even under the best of conditions; consequently, digging the trench is usually the most time consuming and labor intensive task in a water project. If trenching and pipe laying are done properly, the life span of the system will be greatly increased and maintenance problems greatly reduced.

The trench itself should be of uniform depth and gradation. The standard acceptable depth is one meter, but shallower depths are acceptable if only light traffic is expected and frost is not a problem. The trench should have no sharp corners nor run in a zigzag manner. The bottom should be relatively smooth and free of rocks or sharp objects that could damage the pipe.

Join and lay the pipe as described above. When backfilling, the pipe should first be completely covered with dirt alone (no rocks or sticks) up to 1/3 of the trench depth. This earth should be compacted to protect the pipe from surface pressures. The trench is then completely backfilled with the remaining soil. Rocks may be placed towards the top of the trench. Remember to compact the soil while backfilling - this will help stabilize the trench. Also, the top of the trench, when complete, should have a slight crown to allow water to run off the trench, rather than down it.

Once the pipe has been laid in the trench, the trench should be backfilled as soon as possible. Therefore, one should not lay more pipe in one day than can be backfilled in that same day. The pipe should be completely backfilled except for a 2 to 3-meter area at each joint. The joints should be only partially covered until the line has been tested for 24 hours with working pressure.

The course of the trench should follow, whenever possible, the route of the original survey. Some detours may be necessary to avoid such things as heavy erosion areas, extremely rocky areas, or steep gullies. If detours occur, however, care should be taken so that the route does not change the hydraulic gradient of the system.

At times, GI pipe may be needed to cross streams, roads, or other areas where trenching is impossible. These areas should be marked out when the original survey is conducted. In determining the rest of the route, the surveyor should select the easiest course for trenching.

Calculating Friction Losses

The friction loss, hf in a pipeline may be estimated using Tables such as 5.3 and 5.4. If such tables are not available, however, the losses may be estimated based on the flow rate, size of pipe, type of pipe, and the types of fittings and valves used in the pipeline. The friction losses for a pipeline without fittings may be estimated by the use of the Hazen-Williams equation as follows:

where:

Q

is the flow rate in the pipe (Q = L/sec or gpm).

d

is the inside diameter of the pipe (d = mm or in).

c

is the roughness coefficient and is dependent on the material of the pipe. Approximate values are as follows:

Material

c

PVC or polyethylene

150

new steel pipe

135

old steel pipe

100

Friction losses in pipe fittings and valves may be computed by determining the equivalent length of pipe that will create the same loss. To obtain the equivalent length, we can multiply the nominal diameter by a factor. Some common fittings and their factors (Ref. 27) are:

Fitting

Length Multiplier

Union

7

Elbow (90°, short radius)

33

Gate valve (fully open)

7

Free entrance

29

Tee (straight run)

27

Tee (90 degree side)

68

For example, a 5 cm close radius elbow has the same loss as an equivalent length of pipe of 165 cm, i.e. (533).

For long pipelines normally used in irrigation, an increase of 10% in the friction losses calculated without fittings will account for fitting losses.

5.2.9 Water hammer

Water hammer is the pressure pulsation that occurs in a pipe when a valve is suddenly opened or closed. The shock pressure, or water hammer, may be several times greater than the working pressure of the pipe. Water hammer is a frequent cause of failure in PVC lines when valves are closed suddenly. The effect is greater with longer pipelines, higher velocities, and rapid valve closing. The maximum pressure rise in PVC pipe that will result from rapid valve closure can be determined with the following equation:

metric: P = 0.053 with P(kg/cm2); V(m/sec); L(m); T(sec)

English: P = 0.070 with P(psi); V(ft/sec); L(ft); T(sec)

where:

P = pressure rise above the static pressure.
V = liquid velocity in the pipe.
L = length of pipe ahead of valve causing the hammer.
T = time required to close the valve.

Example: Given a pipeline with a length of 61 m (200 ft); water velocity at 1.07 m/sec (3.5 ft/sec); valve closure in 0.1 sec; static pressure of 1.4 kg/cm2 (20 psi); then P = 34.6 kg/cm2 or 490 psi from the equations presented. Next add 1.4 kg/cm2 (20 psi) to 34.6 kg/cm (490 psi) to get 36 kg/cm (510 psi) as the instantaneous pressure under the selected service conditions.

The water hammer analysis indicates how important it is to close valves slowly in long pipelines.

5.2.10 Air relief. Vacuum relief, and pressure relief

Air vents or air relief valves are necessary for allowing air to escape from pipelines as they fill with water, thus preventing air pockets, which restrict flow. Vacuum relief valves are necessary to prevent vacuum pressures (sub-atmospheric pressures) from collapsing pipelines (especially plastic lines) at times when pressures may fall below atmospheric.

Air and vacuum relief are often combined into one valve and are often called air/vacuum relief valves. Air and vacuum relief valves should be installed at the beginning and end of a pipeline, at high points or summits in a line where air could be entrapped, or periodically where air could otherwise be entrained by the water.

In low pressure lines, the air and vacuum relief can be accomplished simply with a stand pipe that is taller than the expected maximum operating pressure, expressed as the height of a column of water with a freeboard of 30 cm (1 ft). (See Figure 5.20 for dimensions.) For higher pressures, a commercial valve for air/vacuum relief is available.


Figure 5.20 Air Vent for Low Pressure Pipelines (Ref. 41)

The sudden escape of large amounts of air from a pipeline as it fills can cause high momentary pressure rises - as high as 4 times the operating pressure - when automatic air relief valves close suddenly. Thus, it is important that pipelines be filled slowly. The velocity at which an empty pipeline should be filled should not exceed 0.7 m/sec, or 2 ft/sec. The orifice of a pressure/vacuum relief valve should be at least 1/2 inch for pipelines of 5 inches or less, and 1 inch for pipelines of 6 to 10 inches.

Pressure relief valves prevent pressures that would be significantly above normal operating pressures from building up. This helps to protect against breakages in the pipeline from pressure build-up due to blockages.

Open vent stands can be used for pressure relief; however, pressure relief valves are often used for pressurized systems. They are often made for a specific pressure, which should be marked on the valve.

Some pressure relief valves are adjustable and can be set for different pressures. The valves should be capable of discharging the design flow rate without elevating pipeline pressures by more than 50% above the working pressure. One valve at the lowest point in a pipeline, or the point of maximum expected pressure, is usually sufficient to provide necessary protection.

Pressure relief valves do not provide sufficient protection against water hammer. The Soil Conservation Service suggests that pressure relief valves be no smaller than 1/4 inch nominal size for each diameter inch of the pipeline. Pressure relief valves should be set to open at pressures no greater than 5 psi above the design working pressure of the pipe.

5.2.11 Other pipeline structures and accessories

Pipelines require couplers to join pipeline sections together. Reducers are required when one size pipe is coupled to a pipe of a smaller diameter. Elbows are required for directional changes. Tees and crosses are required where the flow will be split. Globe, gate, and check valves are used to control the flow of water.

Valves should be capable of being closed slowly to prevent water hammer. Check valves are installed between the pump discharge and pipeline wherever necessary to prevent backflow. Foot valves are used with pumps to prevent the loss of prime in a pump or siphon. Hydrants are used to allow the withdrawal of water from a pipeline. Thrust blocks are used to prevent the pipe from coming apart at places where the flow velocity changes direction. Drain valves at low points in the system may be necessary to permit drainage. A valve at the very end of the system is often required to allow for flushing of sediment periodically from the lines.

5.2.12 Pipeline materials

Aluminum, rubber, plastic, or metal pipelines are often used in irrigation pipelines. The pressure rating of a pipeline is an important aspect of pipeline design. The Soil Conservation Service suggests that the working pressure should not exceed 72% of the rated pipeline pressure, and the design flow velocity should not exceed 1.5 m/sec (5 ft/sec).

The most common type of materials used in small Peace Corps projects are PVC and polyethylene plastic. Pipe made from these materials is sold based on the IPS (Iron Pipe Size) or PIP (Plastic Irrigation Pipe) size. IPS pipe sizes start at 1/2 inch in diameter and can be used with iron fittings without special adapters. For example, a 3/4 inch plastic pipe can use 3/4 inch metal fittings. PIP plastic pipe is sold in diameters of 4 inches and greater.

To connect PIP pipe to other PIP pipe requires PIP fittings, and to connect to IPS pipe requires special adaptors. It is important to remember in pipeline design that a nominal size of pipe will have different I.D.'s (inside diameters) and O.D.'s (outside diameters) depending on the system used. Thus, it is important that the system be specified.

<<TOC3>> 5.3 Land leveling

Land leveling for irrigation is a process by which the surface relief of a field is modified to a desired grade to provide a more suitable surface for efficiently applying irrigation water. Land leveling requires moving a lot of soil and for practical purposes is not done on a large scale without special machinery. Land leveling is probably the most intensive practice that is applied to agricultural land and is very costly. Considering the high investment required for this activity, only deep and fertile soils should be considered for leveling. The land should already be fairly level to reduce construction costs and earth movement, which greatly increase with slope. Land smoothing, a rough grading, is less intensive than land leveling and much less expensive. It may result in dramatic improvements in the ability of water to spread evenly over the surface.

The factors that influence the design of a land-leveling project are land slope, depth of soil, topography, crops to be grown, and method of irrigation. A major problem with land leveling is the removal of the topsoil and the influence of that removal on plant growth. Therefore it is important to determine the soil depth of a field so that after the leveling is done, an adequate depth of topsoil covers the entire field.

Because of the high cost and need for heavy machinery, farmers on a small scale may be more inclined to rough grade their fields. This is the practice of removing knolls, mounds, or ridges and filling pockets and depressions. This requires less soil movement and simpler, less costly tools and implements can be used. Figure 5.21 shows a wooden land-smoothing implement and a buck scraper. They can easily be constructed in the field and revisions in the design and materials can be made to adapt to local conditions. The implements can be pulled by a team of oxen across the field. The back plank on the buck scraper is hinged to the box so the operator can make cuts or fills by lowering or lifting the handle. The rectangular box is filled when making cuts and emptied when making fills. The addition of rocks or some other heavy object on the implements often improve their cutting and carrying ability.

Before leveling a field, several different locations in the field should be checked to assure sufficient soil depth and good uniformity. A topographical map will provide a basis for planning the irrigation system. Lastly, the land should be plowed to loosen the soil. This will make the smoothing process easier and more effective.


Figure 5.21 Land Smoother and Buckscraper

Using the buck scraper requires that several passes be made across the field. Elevation differences need to be checked periodically against a permanent benchmark. Remember that the grade must be taken into consideration when taking field elevation shots from a set point in the field. Rough grading is time consuming and may also require final touches done manually. This process can be accomplished over several years with the grade being refined with each planting season.

<<TOC3>> 5.4 Irrigation methods

There are a number of irrigation methods in use throughout the world. They may be broadly classified as:

1. Surface - water is spread over the land surface;

2. Sprinkle, Drip, or Low Volume Irrigation - water is applied as artificial rainfall;

3. Localized - water is applied as droplets or very small streams of water to specific points on the surface; and

4. Sub-irrigation - water is supplied to the root zone of the crop by maintaining a high water table.

The methods may be broken down further. For example, surface methods include graded or level border, basin, graded furrow, level furrow, contour furrow, contour ditch, wild flood, or corrugation methods.

Factors to consider when selecting and planning an irrigation system are:

· slope and topography of the field;

· crops to be grown - water requirements, tolerance to salt, moisture stress, wetness of surface, waterlogging, value of the crop, crop height, and cultivation required;

· field size and shape;

· soil texture, structure, depth, infiltration characteristics, water-holding capacity, erosivity, and variability within a field;

· soil and water salinity;

· availability and quantity of water, and availability of time;

· amount and intensity of rainfall;

· economics - initial costs, amortized costs, operating costs (fuel, labor, water maintenance), availability of capital, marketability of crop, and net profitability; and

· farmer, social, and institutional constraints.

5.4.1 Characteristics of irrigation systems

(Adapted from Ref. 57)

A. Surface Irrigation (Figure 5.22) - level basins, level borders, and level furrows

1. Water is ponded on an enclosed level field and allowed to infiltrate in basins, borders, or furrows.

2. Advantages

a. Management is very easy.
b. Adapts easily to flat topography.
c. Low cost required.
d. Can function with no outlet drainage facilities.
e. Allows easy leaching of salts.
f. Allows full utilization of rainwater.
g. High application efficiencies can be achieved.
h. Adapts well to moderate to low infiltration rates.
i. Works well with short-term water supplies.
j. Adapts well to small land holdings.

3. Disadvantages

a. Requires level land to achieve high efficiencies (maximum land elevation fluctuation shouldn't be greater than half the applied irrigation depth).

b. Soils with high infiltration rates require small field sizes, which interferes with mechanization.

c. It is difficult to remove excess water, particularly during times of excess rainfall.

d. Plants are partly covered with water sometimes offer extended periods (in low infiltration rate soils).

e. It is difficult to apply small irrigations.

f. Small basins require extensive delivery channels.

g. Small basins are not easily adaptable to tractor mechanization.

B. Surface Irrigation (Figures 5.22 and 5.23) -graded systems.

1. Water is put on the high end of a field and allowed to run slowly to the low end. Types of graded surface irrigation are:

a. furrows

- straight on medium slopes,
- contour on steep slopes;

b. borders;
c. corrugations; and
d. unguided (wild flooding).

Figure 5.22 Surface Irrigation Methods 1 (Ref. 10)


Level border (basin) irrigation


Graded border irrigation


Contour ditch irrigation


Contour levee Irrigation

Figure 5.23 Surface Irrigation Methods 2 (Ref. 10)



Level furrow irrigation


Graded furrow irrigation


Corrugation irrigation


Contour furrow irrigation

2. Advantages

a. Requires low capital and energy costs.
b. Allows irrigation on sloping land (as is found in many irrigated areas).
c. Allows irrigation of long fields with relatively small flows.
d. Is applicable to soils with moderate to fairly high intake rates.
e. Field drainage of excess rain is made possible.

3. Disadvantages

a. To get relatively high efficiencies, a high degree of management and water control is required.

b. To get relatively high efficiencies, the land must be uniformly graded and shaped.

c. With moderate to slow infiltration rates, long irrigation times are required. Irrigation time must be close to the required intake opportunity time.

d. Except for soils with high infiltration rates, a drainage outlet must be available from every field to dispose of tailwater and rainwater.

e. Is labor intensive.

C. Sprinkler Irrigation

1. Pressurized water flows through pipes to outlets that spray the water over the area to be irrigated.

2. Types of sprinkler systems:

a. permanent,
b. semi-permanent, and
c. movable.

3. Advantages

a. Can achieve high efficiencies.
b. Applicable to most terrains - land leveling not required.
c. Applicable to soils of all infiltration rates.
d. Can have low labor requirements.

4. Disadvantages

a. Requires high capital and energy costs.
b. Requires moderately high technology.

D. Localized Irrigation

1. Water is constantly applied at very low rates through small holes in plastic tubing or from emitters to points or small areas in the field. Only part of the field is wetted.

2. Advantages

Has many of the same advantages as sprinklers, plus very high water efficiencies can be achieved, and it can be successfully utilized with highly saline waters.

3. Disadvantages

a. Requires high capital costs.
b. Requires high technology level.
c. Salinity may cause problems.

<<TOC3>> 5.5 Surface irrigation systems

Surface irrigation is the predominant method of irrigation throughout the world and has been used for thousands of years to irrigate a wide range of crops. Through the years, great improvements have been made to the wild flooding practice. Today there are a wide range of surface irrigation practices that allow the farmer an opportunity to select the practice that best fits conditions in the field.

Since the distribution of water with a surface system is dependent on the natural flow of water over the area to be irrigated, the land slope becomes very important. Some types of systems are for level land while other types are for land with some slope.

5.5.1 Criteria for design and operation

1. Factors that are important in designing surface irrigation systems include:

a. flow rates available,
b. soil and topography,
c. length water must run on given plot of irrigated land,
d. depth of water application,
e. slope of the land surface in the direction of the flow,
f. uniformity of the land (relief),
g. erodibility of the soils to be irrigated,

h. form of distribution of water from the conveyance ditch to the cultivated land (e.g turnout, siphon)

i. area and geometry of the land, and
j. system management with regard to the design and operation.

2. Criteria for the efficient use of irrigation water are:

a. Storing the required amount of water in the root zone. This amount depends primarily on the water-holding capacity of the soil and the root depth of the grown crop.

b. Obtaining a relatively uniform application of water. This requires that the amount of time the water remains on different parts of the field does not vary appreciably. Generally, the variation in depth infiltrated should not vary by more than 25%.

c. Minimizing erosion. Although erosion cannot be totally eliminated, it should be minimized as much as possible. This requires good management and adjustments made in the field.

d. Minimizing runoff. One efficient method to reduce losses in furrows is to reduce the flow when it has arrived, or is about to arrive, at the end of the furrow. This reduction is not as important when the runoff water can be reused in another plot.

e. Minimizing overwatering and percolation losses, with the exception of the need to leach salts.

f. Minimizing the land surface occupied by ditches, paths, and other components of the irrigation system.

g. Adapting the system to the geometry and dimensions of the field. When the physical, social, and legal factors permit, plots can be combined to eliminate boundaries, which allows design and management of the entire system with greater efficiency.

h. Adapting the system to the soils, topography, crop, and other physical factors that influence and determine the best design.

5.5.2 Description of different surface irrigation methods

(See Figures 5.22 and 5.23)

Wild Flooding

Wild flooding is a system used primarily for low income crops on steep land where the uniformity of water distribution is not an important factor. Water is delivered at several points along a head ditch that runs along the upper edge of a sloping field. The water advances on to the field without any attempt to control or restrict flow. The distribution is generally not uniform, with too much water applied on some parts of the field and inadequate amounts on others.

The converging of water on irregular topography can create concentrations of flow that can initiate erosive forces, and this must be monitored and controlled. This type of irrigation is used in places where there is an abundance of water and water costs are low. It is most suitable for close-growing perennial forage grasses and low value permanent pasture crops that will protect the soil from erosion. The spacing between the supply ditches is usually between 15 to 45 m, depending on the land slope, water flow rate, texture and depth of soil, and the crop grown. The minimum amount of land preparation and low cost of the system installation are the main advantages.

Level Basin Irrigation

The basin method of irrigation involves dividing a field into small, level plots. It is, therefore, most suited to flat lands but can be used on sloping land, provided that the soil is deep enough to allow leveling without exposing the subsurface. Small ridges or dikes of earth 30 to 50 cm high are constructed around the area to form the basin. The basins are filled with the amount of water required to fill the.i.pond and infiltrate into the soil. Basins may be square, rectangular, or irregular in shape, and may vary in size.

The size of basins depends primarily on the flow rates available and the texture of the soil. The practice involves flooding the plot as rapidly as possible so that all parts are uniformly covered with water. Therefore, for a given flow rate, basins in sandy soils will be smaller than basins in heavy clay. Table 5.6 gives suitable areas for basins with relation to flow rates and types of soils.

TABLE 5.6 Suitable Areas for Basins (m2) (Ref. 47)

Flow


Soil Type


Liters/sec

Sand

Sandy Loam

Clay Loam

Clay

10

65

200

400

700

20

130

400

800

1,400

50

325

1,000

2,000

3,500

100

650

2,000

4,000

7,000

When land is sloping, basins should be constructed in steps or terraces following the contour of the slope. Normally, the width of the terraces will depend on the depth of topsoil. A minimum topsoil depth of 30 cm is recommended to ensure good plant growth. Where soils are shallow and slopes are steep, very little land levelling can be done, and terraces will be narrow.

Basins adapt well to pre-irrigation or water harvesting that diverts stream flow during rainfall to the plots to store moisture in the soil profile. In areas of intensive rainfall, it may be necessary to provide for some means of drainage to prevent overtopping of the levee.

This type of irrigation is used with crops that can withstand contact with water for long periods of time (such as rice and cotton), with close-growing crops with deep rooting (such as alfalfa), and with some types of orchards, where each tree may have its own basin.

Border Irrigation

The layout of graded borders is similar to that of basin, except that the irrigated surface has a slight slope to it. The method is designed so that a sheet of water advances down the border and covers all the plot uniformly. A field is divided by borders into a series of strips 3 to 30 m wide and generally from 60 to 800 m long. See Table 5.7 for suitable dimensions for border strips.

Cross slopes should be eliminated whenever possible. This allows the water to spread evenly over the entire surface.

The levees or ridges forming the borders to the strips should be 20 to 25 cm high. When irrigating, each strip is flooded at the upper end. The amount of flow must be sufficient to reach the end of the strip. When the sheet of water has advanced 80%-85% of the length, the supply flow can be reduced. Several field tests, with variations in the flow rate and time of irrigation, may be necessary to determine the optimum combination that assures a uniform distribution with minimum runoff losses. A tail ditch is generally needed to remove the excess water at the end of the border.

This type of irrigation is best suited for close growing crops, such as small grains, alfalfa, and grasses.

TABLE 5.7 Suitable Dimensions for Border Strips (Ref. 47)

Type

Soil Infiltration Rate
(mm/hr)

Slope
%

Width
m

Length
m

Flow
liters/sec

Sands

25 and over

0.2

15-30

60-90

220-450



0.4

10-12

60-90

100-120



0.8

5-10

75

30-70

Loams

7 to 25

0.2

15-30

250-300

70-140



0.4

10-12

90-180

40-50



0.8

5-10

90

12-25

Clays

2.5 to 7

0.2

15-30

350-800

45-90



0.4

10-12

180-300

30-40

5.5.3 Contour ditch

This method of irrigation utilizes a system of contour ditches that are laid out over the entire field, and which divide the land into several strips. The water from the head ditch is directed into one ditch at a time and fills the entire ditch.

The overflow spills over the ditch edge and flood- irrigates the strip of land below. There are no ridges or dikes to restrict the water flow. The water flows across the strip of land to the next contour ditch. Any excess water is collected in the next lower ditch to be used on the next strip. This process is repeated until the entire field is irrigated. The field ditches must be placed closely together to keep the water distribution even throughout the field. The spacing depends on the slope, intake rate of the soil, and the amount of water needed to irrigate each area.

The contour ditch method is often used for close-growing crops in pastures on sloping and rolling land that cannot be easily levelled for other methods of irrigation.

5.5.4 Contour levee

The contour levee type of surface irrigation is similar to the level border method, except it is adapted to sloping land. The strips have been graded until they are level, and instead of rectangular fields bordered by ridges, the fields are bounded on the contour by levees at the lower edge of the strip.

This method of irrigation is often used on fields where the slope is greater than 0.2% but less than 4%, and where land leveling would be impractical or too expensive. The distance between levees depends on the slope and crop to be irrigated, but typically the difference in elevation between levees should not exceed 10 cm; therefore, the distance between levees varies between 3 to 15 m.

When irrigating, the procedure is the same as for the level border system. Water is applied from a head ditch, and it spreads rapidly over the area where it infiltrates into the soil. Since contour levee systems consist of leveled areas on the side of a slope, there are more strips below the highest one. Therefore, water can be reused from the higher strips to the lower strips. The excess water from a strip is directed into the one immediately below. Good drainage must be provided at the lowest strip to remove all excess water. This type of irrigation is well suited to rice cultivation and can be used with crops that can tolerate being submerged in water for long periods of time, such as cotton and some forage grasses.

5.5.5 Furrow irrigation

With this type of irrigation, the water is applied to small channels, known as furrows, that are between the rows of plants. Furrow irrigation adapts better than any other method to crops that are grown in rows with more than a 30 cm spacing, such as vegetables, tomatoes, maize, and potatoes. In contrast to basin and border irrigations, furrow irrigation wets only part of the ground surface. This reduces evaporation losses, improves aeration of the root zone, and permits earlier cultivation after irrigation.

Furrows are usually V-shaped in cross section, 25-30 cm wide at the top, and 15-20 cm deep. Wider, U-shaped furrows with a greater wetted area are sometimes used on soils with slower water intake rates.

There are three different types of furrow irrigation:

· level furrow,
· graded furrow, and

· contour furrow. (The furrows are laid out across the slope on a uniform grade. This type of furrow irrigation can be used on hillside slopes of up to 25%.)

Table 5.8 suggests maximum furrow lengths for different soils, slopes, and water applications.

TABLE 5.8 Suggested Maximum Furrow Lengths (meters) (Ref. 47)

Furrow Slope %

Clays

Loams

Sands

0.05

400

270 - 400

90 - 190

0.10

450 - 500

340 - 470

120 - 220

0.20

510 - 620

370 - 530

190 - 300

0.30

570 - 800

400 - 600

220 - 400

0.50

540 - 750

370 - 530

190 - 300

1.00

450 - 600

300 - 470

150 - 250

1.50

400 - 500

280 - 400

120 - 220

2.00

320 - 400

250 - 340

90 - 190

Water is admitted to the head of each furrow, and the rate of flow is adjusted so that the furrow flows full without overtopping. As the water reaches the end of the furrow, the required amount of water has infiltrated into the soil to satisfy the irrigation requirements. The rate of flow into the furrow depends primarily on the intake rate of the soil and the length of the furrow. Table 5.9 gives infiltration rates for various soils textures and suitable furrow flow rates per 100 m length of furrow.

To determine the correct flow rate per furrow requires testing in the field. A simple advance and recession test can be done. To do this, the irrigationist marks off three points along the furrow - a point near the beginning, the midway point, and a meter from the end of the furrow. The water is directed into the furrow at the desired operating flow rate, and the times when the water passes the three markers are noted. At the end of the irrigation, the irrigationist, using the same points along the furrow, notes the time that it takes the water to infiltrate and regress from the end of the furrow to the beginning. With these two sets of data, the irrigationist plots the advance and recession curves for the flow rate in the furrow (x-y axis graph: x-axis is length of furrow and corresponding marks; y-axis is time) on the same graph paper. If the two curves are more or less parallel, this indicates that the flow and time for the length of furrow gave a good water distribution. If this is not the case, the flow rate and/or time of irrigation should be changed. This test should be done for each alteration until the desired results are achieved. This field test is easy to perform and should be done regularly to assure a good water distribution application.

Table 5.9 Soil Infiltration Rates and Suitable Furrow Inflows per 100 Meters of Furrow Length. (Furrow spacing 1 meter) (Ref. 47)

Soil

Infiltration Rate
mm/hour

Furrow Inflow
L/sec/100 m Length.

Clay

1-5

0.03-0.15

Clay loam

5-10

0.15-0.30

Silt loam

10-20

0.30-0.50

Sandy loam

20-30

0.50-0.80

Sand

30-100

0.80-2.70

5.5.6 Corrugation irrigation

This method of irrigation is very similar to graded furrows, except that the furrows are much smaller in size. The corrugations or rills are evenly spaced in the field. The spacing and size of the corrugations vary with different soil textures. The method is generally more applicable on fine-textured soils that intake water slowly. The more porous the soil, the closer spaced the corrugation should be because of the lateral movement of water. This method is also used on soils that seal over and crust when other types of flood irrigation are used.

Table 5.10 gives suggested maximum lengths and spacings for corrugations for different slopes, and for deep and shallow soils.

TABLE 5.10 Length and Spacing of Corrugations (Ref. 47)



CLAYS

LOAMS

SANDS


Slope

Length

Spacing

Length

Spacing

Length

Spacing


%

m

m

m

m

m

m

Deep

2

180

0.75

130

0.75

70

0.60

soils

4

120

0.65

90

0.75

45

0.55


6

90

0.55

75

0.65

40

0.50


8

85

0.55

60

0.55

30

0.45


10

75

0.50

50

0.50

--

--

Shallow

2

120

0.60

90

0.60

45

0.45

Soils

4

85

0.55

60

0.55

30

0.45


6

70

0.55

50

0.50

--

--


8

60

0.50

45

0.45

--

--


10

55

0.45

40

0.45

--

--

5.5.7 Operation and maintenance of farm surface irrigation systems

1. Clear the head ditch and conveyance canal of weeds and debris on a regular basis. Obstructions greatly influence the flow rate.

2. Check for washouts around the edges of all control structures along the conveyance system to the field.

3. Conduct micro-levelling in the border or basin: the need will become evident after the first couple of irrigations.

4. Regulate the flood gate or other means of turning in the water so that the water advances across the plot(s) as quickly as possible without creating erosion.

5. Make sure the operator knows how much water is being placed on the field.

6. Reform the ridges, levees, or furrows when necessary. Conduct land smoothing often.

7. Keep animals off of field.

8. Fill in or plug any rodent or ant holes.

9. If it is necessary to slow down velocity in canals, place heavy objects (e.g. rocks) in the canal to form a small check.

10. Pre-irrigate land before transplanting on to furrow beds.

11. Check field one to two days after irrigation with shovel or soil auger to assure uniform distribution of water.

5.6 Sprinkler irrigation systems

In the 1980s, the use of sprinkler irrigation in developing countries increased greatly as new technology and equipment were introduced. This large jump in technology (from not irrigating to sprinkler irrigation) has led to many problems for small-scale farmers in applying new technology correctly and efficiently. To address these problems, this section will expand upon the operation, evaluation, and maintenance factors involved in sprinkler irrigation practices and, to a much lesser extent, design procedures. As more farmers use sprinkler irrigation, understanding of system operation and methods for applying water more efficiently will become more important for the irrigationist.

Sprinkler irrigation is applicable in a wide range of circumstances. It can be used on steeper slopes than most surface systems. It is usually the most efficient irrigation method for shallow, permeable, and uneven ground. The greatest limitation of this practice with small-scale farmers is the large initial investment for the equipment. Other limitations include high energy costs for pumping stations (if required), the effects high winds have on water distribution patterns, and the need for water that is free of any debris (e.g. sand, leaves, sticks) that clog or damage the system.

5.6.1 Principal components

As shown by Figure 5.24 the main components of a sprinkler system are:

· pumping station (if required - in a gravity flow system, no pump is needed),
· main line pipe,
· lateral pipe,
· riser,
· sprinkler, and
· accessories.


Figure 5.24 Typical Sprinkler Irrigation System Components

- The pumping station is located at the water source, and the pump lifts the water and makes it available under pressure to the system. The pump is required to overcome elevational differences between the water source and the field, counteract frictional losses within the system, and provide adequate pressure at the nozzle for good water distribution. A gravity flow system uses the potential energy in an elevational drop to create pressure for its operation.

- The main line delivers water from the water source to the field. It may be either permanent or movable.

- The lateral pipe delivers the water from the main line to different sections of the field. The lateral line is usually movable.

- The riser delivers the water from the lateral line to the sprinkler. The length of the riser depends on the crop, although a minimum value of 30 cm is recommended to assure a good distribution pattern.

- The sprinkler is the unit that sprays the pressurized water through an orifice and rotates to distribute water on to the field.

- The accessories are parts of the system that generally connect all of the other units together to form a watertight system. These parts are very important to an efficient system and should be installed whenever possible. Examples of accessories are tees, unions, elbows, and reducers.

5.6.2 Pine specifications

1. Types of materials are:

· galvanized steel,
· aluminum,
· polyvinyl chloride (PVC),
· polyethylene, and
· soft plastic (e.g. garden hose).

2. Pressure limits are:

a. PVC Standard Dimension Ratio (SDR): pressure rating

- 51:80 psi
- 41:100 psi
- 32.5:125 psi
- 26:160 psi

(above ratings are for PVC tubing 1120, 1220 or 2120; other materials will have lower pressure ratings.)

b. polyethylene - 30 to 100 psi for SDR 41 to 17 depending on material.

5.6.3 Sprinkler heads and nozzles

Types of materials

· metal (generally brass)
· plastic
· combination metal and plastic

Types of Sprinkler

· microjets - a mist-type sprinkler used primarily in seedbeds and underneath foliage of certain crops;

· one-nozzle sprinkler -- components include hammer, range nozzle;
· two-nozzle sprinkler -- components include hammer, range nozzle, and spreader nozzle.

NOTE: One-nozzle sprinklers are notorious for poor water distribution. They create a doughnut of wetted soil because they don't have a spreader nozzle. Therefore, a special effort must be made in their installation so that they overlap properly to compensate for unwetted areas.

General Sprinkler Specifications

Type

Class

Pressure Range

Capacity

Wetted Diameter

Microjet

Low pressure

5-15 psi

5-30 gpm

2-6 m

One-nozzle

Medium pressure

15-45 psi

1-3 gpm

8-12 m

Two-nozzle

High pressure

45-95 psi

3-8 gpm

12-60 m

Nozzle Sizes

Nozzle size, pressure, and type affect application rates, size of wetted diameter, and water droplet size. Higher pressures and smaller nozzle size result in smaller water droplet sizes. Certain nozzle sizes are difficult to find or simply not available in developing countries Frequently, sprinklers are sold with the nozzle already in place. In this case, the irrigation system capacity must be designed around a specific sprinkler. See Figures 5.25 and 5.26. The manufacturer will give specifications for the sprinkler for different applied pressures.

Figure 5.25 Example 1 of Sprinkler Characteristics

20JH/20EJH
1/2 In (15 mm) Full Circle Impact Sprinklers

· Full circle brass impact sprinklers
· Standard trajectory angle for maximum distance of throw
· Small nozzle sizes; low flow rates
· Low water application rate; ideal for heavy soils and slopes
· "E" model has non clog vane in nozzle for greater distance of throw
· Durable brass construction; excellent for many types of field applications
· Superior "H" bearing for longer life
· New die cast spoon drive for greater durability on portable pipe systems


20 JH

Application:

· Undertree
· Overtree
· Overvine
· Field Crops
· Row Crops
· Permanent Systems
· Solid Set Systems
· Portable Systems
· Vegetables
· Nurseries

Common Spacings Range:

2035 ft to 4050 ft

6
9 m to 1215 m

Models:

Sprinklers

20JH

20EJH

Nozzles

SB

RFN-1

Specifications:

Bearing Size/Type -1/2 in (15 mm) male NPT
Body trajectory - 23°

Materials:

Body: Cast Bronze
Arm: Cast Bronze
Bearing Sleeve: Brass
Bearing Nipple: Brass
All Springs: Stainless Steel
All Washers: Chemically resistant

20JH/ST. BORE NOZZLES

Stream Height 6 ft*

U.S. UNITS


Nozzle 7/64" **

Nozzle 1/8"

Nozzle 9/64"

Nozzle 5/32"

PSI @ Nozzle

Dia. GPM

Dia. GPM

Dia. GPM

Dia. GPM

35

76

2.05

77

2.68

79

3.39

79

4.19

40

77

2.19

78

2.86

79

3.62

79

4.41

45

78

2.32

79

3.03

80

3.82

80

4.73

50

78

2.45

79

3.20

81

4.05

81

5.00

55

78

2.57

80

3.35

81

4.24

81

5.23

60

79

2o8

80

3.50

82

4.43

82

5.47

Stream Height-1,8m*

METRIC UNITS

Bars

Nozzle 2,78 mm ** 7/64"

Nozzle 3,57 mm 9/64"

Nozzle 3,57 mm 9/64"

Nozzle 3,97 mm 5/32"


Rad. M

Flow M3/h

Flow L/s

Rad. M

Flow M3/h

Flow L/s

Rad. M

Flow M3/h

Flow L/s

Rad. M

Flow M3/h

Flow L/s

2.5

11,6

0.47

0.13

11.8

0.62

0.17

12.0

0.78

0.22

12.0

0.97

0.27

3.0

11.8

0.52

0.14

12.0

0.68

0.19

12.2

0.86

0.24

12.4

1.06

0.29

3.5

11.9

0.56

0.16

12.1

0.73

0,20

12.3

0.93

0.26

12.3

1.14

0.32

4.0

12.0

0.60

0.17

12.2

0.78

0.22

12.4

0.99

0.27

12.5

1.22

0.34

20EJH/ST. BORE NOZZLES

Stream Height 7 ft*

U.S. UNITS


Nozzle 7/64" **

Nozzle 1/8"

Nozzle 9/64"

Nozzle 5/32"

PSI @ Nozzle

Dia. GPM

Dia. GPM

Dia. GPM

Dia. GPM

35

77

2.05

79

2.68

82

3.39

84

4.19

40

78

2.19

80

2.86

83

3.62

85

4.41

45

79

2.32

81

3.03

84

3.82

85

4.73

50

80

2.45

82

3.20

85

4.05

86

5.00

55

81

2.57

82

3.35

86

4.24

87

5.23

60

81

2.68

82

3.50

87

4.43

88

5.47

Stream Height-2,1m*

METRIC UNITS

Bars

Nozzle 2,78 mm ** 7/64"

Nozzle 3,57 mm 9/64"

Nozzle 3,57 mm 9/64"

Nozzle 3,97 mm 5/32"


Rad. M

Flow M3/h

Flow L/s

Rad. M

Flow M3/h

Flow L/s

Rad. M

Flow M3/h

Flow L/s

Rad. M

Flow M3/h

Flow L/s

2.5

11.8

0.47

0.13

12.1

0.62

0.17

12.5

0.62

0.17

12.8

0.97

0.27

3.0

12.0

0.52

0.14

12.3

0.68

0.19

12.8

0.86

0.24

13.0

1.06

0.29

3.5

12.2

0.56

0.16

12.5

0.73

0,20

13.0

0.93

0.26

13.1

1.14

0.32

4.0

12.4

0.60

0.17

12.5

0.78

0.22

13.2

0.99

0.27

13.3

1.22

0.34

1 bar = aprox. 100 kPa.

GENERAL NOTE: Performance data are obtained under ideal test conditions and may be adversely affected by mind, hydraulic conditions, and other factors

SHADED AREAS: Nozzle/pressure combinations in shaded area of chart result in marginal water distribution.

* Shown for standard nozzle at mid-range operating pressure
** Standard nozzle

NOTE A: Distance of throw data are based on a 30 in (76 cm) riser height.

Figure 5.26 Examples 2 of Sprinkler Characteristics

30H/30WH/30WSH/30EH/30EWH
3/4 in (20 mm) Full Circle Impact Sprinklers

· Full circle brass impact sprinklers
· Standard trajectory angle for maximum distance of throw
· Durable brass construction; excellent for many types of field applications

· "H" models have rear spreader nozzle; "WH" models have plugged spreader outlet; "WSH" model has single nozzle body

· "E" models have non-clog vane in nozzle "for greater distance of throw


30H

Applications:

· Overtree
· Field Crops
· Row Crops
· Vegetables
· Nurseries
· Permanent Systems
· Solid Set Systems
· Portable Systems
· Wheel Lines

Common Spacings Range:

3050 ft to 6060 ft

12
15 m to 1818 m

Models:

Sprinklers

30H

30WSH

30EWH


30WH

30EH


Drive nozzles

SB

LPN-3

RFN-3

Spreader nozzles

slotted (7° or 20°)

Specifications:

Bearing Size/Type - 3/4 in (20 mm) male NPT
Body trajectory - 27°

Materials:

Body: Cast Bronze
Arm: Cast Bronze
Bearing Sleeve: Brass
Bearing Nipple: Brass
All Springs: Stainless Steel
All Washers: Chemically resistant


30WSH

30H / ST. BORE DRIVE NOZZLE
3/32 in (2,38 mm) 7° SPREADER

Stream Height 9 ft*

U.S. UNITS


Nozzle 9/64" 3/32" - 7°

Nozzle 5/32" 3/32" -7°

Nozzle 11/64" 3/32" - 7°

Nozzle 3/16" 3/32" - 7°

PSI @ Nozzle

Dia. GPM

Dia. GPM

Dia. GPM

Dia. GPM

25

80

4.2

82

4.8

83

5.6

85

6.4

30

81

4.6

85

5.3

88

6.1

91

7.0

35

82

4.9

87

5.7

90

6.6

94

7.5

40

83

5.2

88

6.1

92

7.0

96

8.1

45

84

5.6

89

6.5

93

7.4

98

8.5

50

85

5.9

90

6.8

95

7.9

100

9.0

55

86

6.1

91

7.1

96

8.2

101

9.4

60

87

6.4

92

7.4

97

8.6

102

9.9

65

88

6.7

93

7.8

98

8.9

103

10.2

70

89

6.9

94

8.1

99

9.3

104

10.7

75

90

7.2

95

8.3

100

9.6

105

11.0

80

91

7.5

96

8.7

101

10.0

106

11.5

Stream Height-2,7 m*

METRIC UNITS

Bars

Nozzle 3,57 2,38 mm 9/64" 3/32" - 7°

Nozzle 3,97 2,38 mm 5/32" 3/32" - 7°

Nozzle 4,37 2,38 mm 11/64" 3/32" - 7°

Nozzle 3,97 2,38 mm 3/16" 3/16" - 7°


Rad. M

Flow M3/h

Flow L/s

Rad. M

Flow M3/h

Flow L/s

Rad. M

Flow M3/h

Flow L/s

Rad. M

Flow M3/h

Flow L/s

2.5

11.8

0.47

0.13

12.1

0.62

0.17

12.5

0.62

0.17

12.8

0.97

0.27

3.0

12.0

0.52

0.14

12.3

0.68

0.19

12.8

0.86

0.24

13.0

1.06

0.29

3.5

12.2

0.56

0.16

12.5

0.73

0,20

13.0

0.93

0.26

13.1

1.14

0.32

4.0

12.4

0.60

0.17

12.5

0.78

0.22

13.2

0.99

0.27

13.3

1.22

0.34

30H / ST. BORE DRIVE NOZZLE
118 in (3.18 mm) 20. SPREADER

Stream Height 9 ft*

U.S. UNITS


Nozzle 9/64" 1/8" - 7°

Nozzle 5/32" 1/8" -7°

Nozzle 11/64" 1/8" - 7°

Nozzle 3/16" 1/8" - 7°

PSI @ Nozzle

Dia. GPM

Dia. GPM

Dia. GPM

Dia. GPM

25

80

5.2

82

5.8

83

6.6

85

7.4

30

81

5.6

85

6.4

88

7.2

91

8.1

35

82

6.1

87

6.9

90

7.7

94

8.7

40

83

6.5

88

7.3

92

8.3

96

9.3

45

84

6.9

89

7.9

93

8.8

98

9.9

50

85

7.3

90

8.2

95

9.3

100

10.4

55

86

7.6

91

8.6

96

9.7

101

10.9

60

87

7.9

92

9.0

97

10.1

102

11.4

65

88

8.2

93

9.4

98

10.5

103

11.8

70

89

8.5

94

9.7

99

10.9

104

12.3

75

90

8.8

95

10.0

100

11.3

105

12.7

80

91

9.1

96

10.3

101

11.6

106

13.1

Stream Height-2,7 m*

METRIC UNITS

Bars

Nozzle 3,57 2,38 mm 9/64" 3/32" - 7°

Nozzle 3,97 2,38 mm 5/32" 3/32" - 7°

Nozzle 4,37 2,38 mm 11/64" 3/32" - 7°

Nozzle 3,97 2,38 mm 3/16" 3/16" - 7°


Rad. M

Flow M3/h

Flow L/s

Rad. M

Flow M3/h

Flow L/s

Rad. M

Flow M3/h

Flow L/s

Rad. M

Flow M3/h

Flow L/s

2.0

12.3

1.27

0.35

12.8

1.42

0.40

13.1

1.61

0.45

13.5

1.81

0.50

2.5

12.5

1.41

0.39

12.9

1.60

0.44

13.7

1.79

0.50

14.3

2.02

0.56

3.0

12.8

1.54

0.43

13.0

1.75

0.49

14.1

1.96

0.54

14.9

2.21

0.61

3.5

13.0

1.66

0.46

13.3

1.89

0.53

14.5

2.11

0.59

15.3

2.38

0.66

4.0

13.2

1.77

0.49

13.5

2.02

0.56

14.8

2.26

0.63

15.6

2.54

0.71

4.5

13.4

1.87

0.52

13.9

2.13

0.59

15.0

2.39

0.66

15.8

2.69

0.75

5.0

13.6

1.97

0.55

14.2

2.24

0.62

15.2

2.52

0.70

15.9

2.83

0.79

5.5

13.9

2.06

0.57

14.6

2.33

0.65

15.3

2.65

0.74

16.0

2.97

0.83

Angle of Nozzle Flow

Generally, water flows out of a sprinkler at different angles relative to the horizontal plane. This angle depends on the particular sprinkler, and the range of angles are 0° to 23°. The lower angles will result in a smaller wetted diameter, the water droplets will be larger, and the impact force with which the droplets strike the foliage or the ground surface is greater. Generally, the higher angle sprinklers are used in vegetable or cash crop irrigation because the droplet impact is less damaging to the leaf or fruit. Lower angle sprinklers are used under trees in orchards because their almost flat trajectory avoid wetting the foliage and gives better water distribution in windy areas.

5.6.4 Sprinkler system design

Preliminary Study

1. Topographical study.
2. Soil

- texture,
- water-holding capacity,
- intake rate, and
- density/compaction or filth.

3. Crop

- density of planting,
- depth of rooting, and
- height of growth.

4. Climatic conditions

- precipitation,
- temperature, and
- consumptive use/hydraulic balance.

5. Available materials and equipment.
6. Quality of water

- salts and
- contaminants

Design Parameters

1. Maximum daily requirement for peak crop water use.

2. The system application efficiency (65%-85%).

3. Peak use/efficiency = the depth of water that the design of the system must achieve.

4. Frequency of irrigation. This is determined by the amount of available water in the root zone. When water in the root zone is depleted by 50% by plant consumption, it needs to be replenished by irrigation. The amount of water applied should bring the soil at rooting depth to field capacity. Applying more water than is needed is not recommended because the excess water passes through the root zone and leaches plant nutrients downward out of reach of the plant. (See Table 5.11)

5. Time required to irrigate. The time required to irrigate is directly related to the rate of water infiltration into the soil. (See Table 5.12) To design an efficient application time, the rate of application should be as close as possible to the soil infiltration rate without exceeding it (this would cause run-off and possible erosion). The required irrigation time is then minimized. The amount of time to irrigate varies with plant water needs during different stages of growth.

TABLE 5.11 Available Water-Holding Capacities for Various Soil Texture a (Ref. 10)

Texture

Available Water-Holding Capacities cm of Water/Meter of Soil

Coarse sand and gravel

2 to 6

Sands

4 to 9

Loamy sands

6 to 12

Sandy loams

11 to 15

Fine sandy loams

14 to 18

Loams and silt loams

17 to 23

Clay loams and silty clay loams

14 to 21

Silty clay and clays

13 to 18

TABLE 5.12 Infiltration Rates and Associated Soil Textures (Ref. 10)

Final Infiltration Rate

Cm per Hour

Probable Soil Texture

High

3.0 to 8.0

Sandy loam, sandy clay loam

Medium High

1.5 to 3.0

Loam, silt loam

Medium Low

0.5 to 1.5

Clay loam, clay, silty clay loam

Low

0.2 to 0.5

Clay, adobe clay

Area and Geometric Shape of Field

Farmers will often want to irrigate much larger areas than are economically, socially, or technically feasible. The factors that determine the area that can be irrigated by a small-scale system are the application rate, how many hours it takes to restore the field to field capacity during peak consumptive use, and how many times the farmer is willing and able to rotate the system to different plots for the determined irrigation interval during peak water use of the plant. The majority of sprinkler systems adapt well to fields that are either square or rectangular in shape because the lateral lines rotate well within these shapes and give a uniform water distribution to the entire field.

5.6.5 Lateral design

It is recommended that the total pressure variance in the lateral should not exceed 20% of the operating system pressure. This standard maintains a uniform water application efficiency along the entire lateral. The total allowable pressure variance must also take into account elevational differences along the lateral sprinkler settings. For this reason, the lateral should be placed at the contour or on a slightly decreasing slope to compensate for the friction loss (when possible). Such placement will maintain acceptable uniform sprinkler applications. If there is too much variance in the design of a lateral, the diameter of the pipe will have to be increased to lower frictional loss.

The Hazen-Williams equation is used to compute frictional losses in pipes. The formula is:

where

hf = frictional loss per 100 m of pipe (m per 100 m).
c = coefficient of friction based on the pipe material (see Table 5.13).
Q = the flow of water in the line (L/sec).
d = the pipe diameter.

If d is in inches then KF = 1.76 105.

If d is in mm then KF = 1.19
1012.

TABLE 5.13 C Values for Given Pipe Materials

15-year-old steel pipe

C = 100

Aluminum pipe with couplers

C = 120

Polyethylene

C = 140

PVC (polyvinyl chloride)

C = 150

To calculate the frictional loss in the lateral:

1. determine the length of pipe in the lateral and the number of outlets for the lateral;

2. calculate the frictional loss using the Hazen-Williams equation as if the lateral carried the full flow through its length; and

3. multiply the calculated frictional loss by the factor (F), which is a function of the number of outlets (Table 5.14).

TABLE 5.14 Friction Factor (F) for Multiple Outlets (Approximate)

No. of Outlets

1

2

3

4

5

6

8

10

F1

1.0

.63

.53

.48

.45

.43

.41

.40

F2

1.0

.51

.43

.41

.39

.38

.37

.36

NOTE: Use F1 when the first sprinkler on the lateral is one full sprinkler spacing from the beginning of the lateral. Use F2 when the first sprinkler on the lateral is 1/2 of one normal sprinkler spacing from the beginning.

The slide rule calculator for frictional losses also calculates the losses in lateral pipes and can be used without the calculations performed above. If the slide rule calculators are available, they are quick and easy to use.

Sprinklers should generally be spaced closer than 65% of the wetted diameter to achieve adequate uniformity. Lateral spacings, or the spacings between successive positions of the sprinkler, should take into account wind conditions. The following are recommended spacings:

Wind Conditions

Lateral Spacing as a % of Wetted Diameter

No wind

65%

8 Km/hr or 5 mi/hr

60%

8-16 Km/hr or 5-10 mi/hr

50%

Above 16 Km/hr or 10 mi/hr

30%

Sprinkler spacings along the lateral should generally be less than 50% of the wetted diameter for adequate coverage.

5.6.6 Sprinkler system installation

At the Water Source

A check dam or diversion should be constructed in the stream to pond water where the pump suction line or gravity flow inlet will be placed. The ponded water should be non-turbulent so that the majority of air from the turbulent flow in the stream will be released. Also, sediment and debris should either float to the top or sink in the ponded area so that the water will be clean of debris. It is recommended that the suction line be suspended by tying it to an airtight container (e.g. motor oil container) and weighing the container to the bottom with a rock or heavy object. Suspending the suction line assures that it is not taking water off the bottom, where sediment and debris settle. In the case of the gravity flow inlet, it is recommended the end of the pipe be submerged at least 50 cm deep in the pond to create a head of water. This head will then actually push water into the tube. The inlet should be placed at least 20 cm off the bottom so that debris does not enter. A metal or plastic screen can be tied over the entrance so that no large particles that would clog the sprinkler nozzles enter.

Main line Placement

The main line should be placed on a continually ascending path from the pumping station to the field (a descending path for gravity flow systems), with as many depressions and rises eliminated as possible. The pipe can be tied to stakes or gauged wire to cross difficult areas. The main line should also be placed on a shortest distance path from the water source to the field to reduce the length of the main line and frictional losses.

If the main line will be stationary, the possibility of burying it should be discussed with the farmer. Burying helps protect the main line from sunlight, vehicle or animal traction damage, or sabotage. The pipe should be buried at least 50 cm deep. It is important to use all needed accessories, in particular unions, if burying the main line, so that there are no leaks in the line. Check for leaks before burying the line.

Installation of Gate Valve

A gate valve should be installed at the end of the main line if economically feasible. This valve is used to shut off the water flow if there is any break in the lateral line so that the operator does not have to go to the water source to shut off the pump or stop flow into the inlet pipe. It should be noted that closing the valve should be done slowly so that water hammer does not occur and break unions in the main line. The gate valve is also the easiest place to make a reduction in pipe diameter size from main line to lateral line.

Installation of Lateral Line

To ensure adequate uniformity, the lateral line should be placed on the contour so that water pressure does not vary by more than 20%. Lateral line placement greatly affects the water distribution of the system. If possible, lateral lines should be installed perpendicular to the expected wind direction.

Placement of Risers

The sprinklers should be adjusted so that they are perpendicular to the ground surface to assure correct water distribution. Taller risers may need to be tied to well-anchored vertical poles to keep sprinklers perpendicular and reduce vibration.

Accessories

All required accessories should be installed with the system if they can be found and purchased. Although a very minimum part of the system, accessories are imperative to high operational efficiency. They should be securely installed in the system with pressure clamps or by some other mechanical means. Inner tube or strips of plastic bags can be used to wrap nipples that are inserted into the pipe for a snug fit.

Water Distribution Test

Once all parts of the sprinkler system are properly installed, a water distribution check should be conducted to assure that the designed sprinkler spacings give a uniform distribution. Perform this test during a calm time of the day or observe the wind direction to better interpret data. Stake out a rectangular grid about the same size as your riser lateral spacings. Set cans in a square grid 3 to 4 meters apart; containers should be weighed down or anchored so that they do not tip over. Turn on the irrigation system until the cans have an average of 1.5 cm or 0.5 inch of water in them. Afterward, with a ruler or graduated cylinder (a syringe works well also), measure the depth of water (or volume) in each container and record the data. Arrange the data from smallest to largest values. Determine the average depth of water in the lowest 25% of the cans, and the average in all the cans and determine the distribution uniformity.

The Distribution Uniformity (DU) is the ratio (expressed as a percent) between the depth of water applied by sprinkler irrigation in the low quarter (lowest 25%) to the mean depth applied.

Example: A farmer has sprinklers set up on a 40' 40' (12 m 12 m) grid. He or she sets up 16 cans in a grid at 10 feet (3 m) apart. The values in the cans are as follows:

Can:

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

Depth:

20

16

17

22

25

22

18

26

28

23

21

23

22

16

15

20

Ordered depth:

15

16

16

17

18

20

20

21

22

22

22

23

23

25

26

28

The average water measured in the 4 cans (1/4 of all the cans) with the lowest water application was 16 mm. The average water applied in all the cans is 20.9 mm. The DU is then:

This uniformity is very acceptable in most cases.

In sprinkler irrigation, the infiltrated depths may be represented by applied depths. Acceptable DU values depend on the value of the irrigation water, crop, energy costs, and labor. Generally, DU values below 67% are unacceptable; however, with low value water and crops, a 50% DU may be acceptable. With high value, moisture sensitive crops and high water costs, DU values of 80% or higher may be considered acceptable.

5.6.7 System operation and maintenance

1. Sprinklers must always have nozzles. Often farmers will remove nozzles because they can see more water exiting sprinklers without nozzles. Removing nozzles greatly affects the water distribution and water droplet size, and frequently the sprinkler will not even rotate without the nozzle, since the flapper arm is not hitting pressurized water.

2. Move the irrigation system as a unit to irrigate plots. Attempt to irrigate fields in geometric patterns of rectangles or squares. Squares and rectangles will give a more uniform water distribution over the entire field, and crops will also grow and mature uniformly.

3. Remove the suction line or inlet pipe from the stream if there are chances of rain. The stream flow will often rise considerably with rainfall and will wash out any equipment on or near the banks.

4. Operate the system when there are no high winds in order to reduce droplet drift and pattern distortion.

5. Operate the system before fumigating so the pesticide is not washed off.

6. Operate the system after fertilizing to move the fertilizer nutrients into the root zone.

7. In areas where plant fungus is a problem, it is preferable to irrigate in the morning hours so that the plant leaves can thoroughly dry during the day hours.

8. Maintain the sprinklers so that they rotate properly. Water is the lubricating medium in the majority of sprinklers, so do not apply oil based lubricants. Frequently, the rotation problem arises from malfunction of the bearing washer, which has to be cleaned of sand or grit regularly.

9. The screen tied over the entrance of the inlet pipe in gravity systems should be checked and cleaned frequently. Large particles can be sucked into the screen. Particles plug sprinkler nozzles and create more frictional losses in the system.

<<TOC3>> 5.7 Localized irrigation systems

Localized irrigation is a method of applying water that results in wetting only a small area of the soil surface and sometimes only part of the root zone. Water is applied near the base of the plant so that the application is concentrated in the root zone. Water is generally applied at a low flow rate, in small amounts, and frequently. The application devices may be small tubes, orifices, nozzles, or perforated pipes. The water may either be applied above or below the soil surface. The main components of a localized irrigation system are the water supply (including flow and pressure regulators), the filtration system, main lines, sub-main lines, laterals, and distributors. Figure 5.27 shows some basic components of a localized irrigation system.

The primary advantages of localized irrigation systems are the high efficiency rates that can be achieved, sometimes as high as 90%. High efficiency may result in very significant water savings. Often a localized irrigation system will allow a farmer to irrigate twice the area possible with surface irrigation. Precise control of water and nutrient application often results in much higher yields and quality. Control of weeds and pests may be better as the entire soil surface is not wetted nor is the foliage. A localized irrigation system may allow the use of more saline water, and can be used effectively with low infiltration soils that cannot be sprinkler irrigated. Some disadvantages are the higher initial costs of the systems, salinity buildups, more limited root development, and higher technology requirements. Later savings may be offset by higher maintenance costs. There are low cost methods, however, for irrigating garden sized plots with localized irrigation.

Drip or trickle irrigation is a localized irrigation method that applies water in very low flow rates. Pressures required in drip irrigation are typically between.7 and 1.4 kg/cm2 (10 and 20 psi). Drip irrigation is suitable for most soil types and most types of topography. It is very well suited to situations of limited water supplies or high water costs. Its high cost ($2,000-$5,000/hectare) can be justified only in orchard crops or other high-value crops.

There are two basic methods of drip or trickle application line source (used mainly for row crops) and point source (used primarily for tree crops). The line source method consists of one- or two-chamber polyethylene-type plastic tubing, 4-15 mm thick, and with small holes (usually laser cut) every 20 to 60 cm. The single outlet type uses emitters, usually a button-type plastic device with a barbed fitting that attaches to a polyethylene lateral hose having a diameter of 1/2" to 1".


Figure 5.27 Basic Components of a Localized Irrigation System

Potential clogging or mineral deposition are significant disadvantages because of the very small water passages and the slow water velocities. Filtration and clean water is, therefore, a very important consideration. Periodic maintenance to prevent clogging is required. Thus, more operator knowledge is required than for most other types of irrigation.

A typical soil-wetting pattern under a single emitter is shown in Figure 5.28. Inadequate wetting of the root zone can seriously limit crop growth of crops such as avocados or bananas, which have a shallow, expansive root system. For this reason, a minimum of 33% wetting of the total surface area is required. Line source systems will usually wet about 70% of the surface area.

5.7.1 Characteristics

Field slope can significantly affect the uniformity of performance due to the fairly low operating pressures. Some design characteristics for drip or trickle systems are listed below.

1. Emitters should be spaced so that at least 33% of the total field area is wetted.

2. Commonly used emitter flow rates are 4 to 8 L/hr (1 to 2 gal/hr).

3. Depending on the tree crop layout, use one or two lateral lines per tree row.

4. For the line source method, one line per row of crop is usually sufficient. Wider, raised beds of strawberries or carrots may require two lines.

5. Try to run line source or lateral line for emitters on contour or slightly downhill slopes in order to maintain a high degree of uniformity.

6. Manifolds should be strategically located to reduce lateral lengths; therefore, smaller line sizes can be used, resulting in lower costs and better long-term operation. Typically, on flat ground the lateral lines would be split in half by placing the manifold in the center of the field.

7. Allow for shrinkage and expansion of PE lines. The smaller diameter lines used for laterals will absorb the sun's heat, causing considerable expansion and contraction. Therefore, leave some slack (at least 3%) in the laterals as they are laid out; otherwise, the emitters will not stay where they are placed. Such displacement can lead to quite a disappointment.


Figure 5.28 Typical Pattern of soil Wetting Under Trickle

8. Provide for flushing: make allowance at the ends of the laterals or line sources to facilitate occasional flushing. Flushing is imperative immediately after installation. Flushing can be done easily by folding or doubling the end of the line 45" back on to itself, and then either slipping a piece of PVC pipe (approximately 4-5") over the folded end or tying the folded end with wire or string. When the system needs flushing, simply pull off the PVC pipe or untie the wire/string and run water through the open line until all debris is flushed out of the system. This should be done on a regular basis.

9. Operating pressure differentials between the maximum and minimum pressure emitters should not exceed 20%-25%, if at all possible.

Low cost localized irrigation methods that incorporate some of the advantages and avoid some of the clogging problems of drip irrigation have been used in developing countries for small garden plots. These systems may operate with pressures as low as 0.1 to 0.2 meters. One such low flow rate system uses a small 1/2" to 3/4" diameter hose with 2 to 4 mm holes drilled in the wall next to the location of the trees to be irrigated. Small basins constructed around the tree are filled every few days with this system. The larger orifice sizes do not plug easily and can easily be cleaned. However, a sedimentation basin and a screen at the inlet are required to keep out bigger particles that could plug the orifices if surface supplies are used.

5.7.2 Operation and maintenance

1. Maintain clean water through periodic checking and cleaning of the filters.

2. Flush after installation and every few months, or more often if source water is fairly dirty.

3. Check pressures periodically to ensure that they are close to the design pressure.

4. If algae growth within the lines is a problem (typical with stream or pond water), chlorine should be used during flushing.

5. Irrigation times should be long enough to refill the root zone and achieve at least 33% wetting of the entire area, unless the orchard is young.

6. Walk the lateral lines weekly or biweekly to ensure good emitter operation and that none have plugged up or popped out.


<<TOC2>> Chapter 6 - Farm water management

References

Primary:

(11), (42), (44)


Other:

(12), (22), (34), (43), (51)

6.1 Farm water management

6.1.1 General concepts

On-farm irrigation water management involves the manipulation of such factors as the timing and amounts of irrigation applied to the crop, the flow rates to be used, and the methods of controlling the water. These and other parameters can be modified to achieve desired crop production goals within the restrictions imposed by soils, crop, climate, water supply, and economics, as well as social and other constraints.

The benefits of good on-farm water management are many. A good on-farm water management program generally helps to maximize or optimize yields and crop quality. It can help reduce water and energy consumption, thus making these supplies available for irrigating more land, as well as decreasing the cost for a given irrigation system. It can reduce the loss of fertilizer which is leached out with excess water application, which in turn decreases the amount of fertilizer necessary to achieve desired yield goals. A good management program ensures that root zone salinity is controlled at desired levels and that waterlogging of soils and excess deep percolation losses are either diminished or eliminated. It can help to eliminate problems such as erosion and control crop diseases related to excessive or deficient water application. Water control can reduce machine and labor time. A good farm water management program takes the guess work out of irrigation so that the farmers can consistently make the best use of their water resources.

In setting up an effective irrigation water management program, the engineer, technician, or farmer must first be intimately familiar with the irrigation system. He or she must be aware of the different possible modifications in design and management and must know how changing one or more design or management factors will affect the crop and soil system and other operational characteristics. For example, it may be possible to change the control structures in order to ensure a more flexible and responsive system. It may also be possible to change the rotation and supply of water so that the availability is more in tune with crop requirements.

On-farm design and management factors that should be considered for modification are: irrigation method, irrigation system geometry (width, length, depth, spacing), slope and topography, crop, tillage practices, flow rates, and irrigation timing and duration. Changes in the tools used for irrigation, such as the use of siphons, can ensure the ability to apply uniform, non-erosive irrigations. In sprinkler and trickle irrigation, it may be possible to change pressures, pipe sizes, or other system components.

In all cases, a good maintenance and monitoring program is essential to good management. Before undertaking any modifications or setting up a management program, certain data must be obtained and analyzed. These include crop data such as varieties, length of growing seasons, crop water requirements, and acceptable soil moisture levels at different stages of growth. For a given crop and area, the manager must know when the crop will reach certain stages of growth, when the critical periods for moisture stress are, how stress or excessively high moisture levels affect the crop in terms of yield and quality, and how moisture levels and irrigation frequency affect the resistance to disease or the propagation of diseases. Also important is how the roots develop through the season and what the moisture extraction pattern is. In the case of poor drainage, saline soil, or saline water, it is necessary to know how yield and quality will be affected by different management practices. For example, maintaining a high moisture level may dilute salts enough so that they will not affect yield significantly. Leaching to control salt levels in the soil profile can minimize the effect on the crop.

Soil data that must be known include the soil(s) water-holding capacity, depth(s) of the different soil layers, and the infiltration characteristics of the soil. Knowledge of soil texture, structure, and organic matter content will also help to determine whether the moisture-holding capacities or the intake rate can be improved. Knowledge of soil salinity and how to control the salt levels is essential. Drainage conditions such as depth to water table and capillary rise of water to the root system should be known. Topography (slope and relief) and soil erosivity must also be known if changes in the irrigation system, land leveling, or even changes in the flow rates in furrows, borders, or basins are contemplated.

Irrigation system data, such as conveyance and application efficiencies and uniformity of water application and penetration in the present system, are necessary, as well as the effects of wind, evaporation, and infiltration rate on these. The geometry of the irrigation system, control structures, system flexibility, energy requirements, water availability and cost, the cost of system design (or redesign) and construction, and the return on investment are all essential data. The quality of the irrigation water may be important in determining proper system capacity. If the system is also going to be used for crop cooling or heating, these requirements must be accounted for.

Climatic data on the time and space distribution of precipitation, solar radiation, temperature, and wind are necessary in determining crop irrigation requirements, and even in determining what crops are suitable to a given set of conditions. Hydrologic data, including stream flows, reservoir capacities, and water supply data, are necessary for determining the amount of irrigable area, possible crops and planting dates, and how the water supply will be distributed to that irrigated area.

Of course, we must not forget the very important data on the farmer. We must know their preferences and schedules, educational and economic level, and progressiveness, as well as the restrictions imposed by the community, religion, irrigation district, or any of the institutions that must be dealt with.

Those in charge of developing the water management program should be critically aware of the consequences that changing the management in any given area may have on the other areas. They should consider how the management in one area will impact on other water users downstream or in low-lying areas. For example, the more efficient use of water in a project upstream may mean that more land can be irrigated in that area. Users downstream, however, may need to count on the return flows from upstream users to continue irrigating.

The implementation of a successful on-farm water management program is a very definite and deliberate process consisting of certain steps. Typically, the person(s) setting up such a program will need to:

1. Conduct an evaluation of the farming system, including soils, crops, and irrigation methods. This helps identify major problems associated with the present management system and the constraints imposed by soil, crop, irrigation system, water supply, and social considerations. The evaluation also helps estimate what the benefits of implementing the required changes will be.

2. Determine the design, redesign, and management alternatives for the system and select the best alternative(s).

3. Ensure that the system is modified according to the selected design and/or redesign.

4. Set up a detailed management program for the system; it should include irrigation schedules, maintenance, and other management parameters.

5. Train the technicians and farmers so that they are capable of implementing the program that was set up.

6. Monitor the system in order to make necessary changes through the season(s) and change the management program to reflect changing conditions. The system monitoring phase is an essential part of good water management. Initially, after the program is implemented, monitoring is conducted both by technical personnel and by the farmer. After a good working program is in place, however, the farmer does the day-to-day monitoring of his or her own irrigation system, with perhaps occasional technical assistance.

A good water management program requires that the technical personnel who design and monitor the program, and the farmer who ultimately implements the program, be well trained. Those technical personnel who conduct the training, as well as those who set up the water management programs, should have extensive field experience in working with people and irrigation at the farm level, or they should be given the opportunity to acquire this experience before they are allowed to conduct training or develop the programs. These personnel should be selected for their knowledge of irrigation, as well as their ability to understand the farmer and his or her system and develop rapport with farmers.

The training itself should consist of some classroom instruction, with extensive hands-on field exercises. The types of subjects to be covered would include:

· Irrigation control structures and tools (selection, construction, and maintenance).

· Basic irrigation concepts: crop water requirements, water-holding capacities, infiltration, uniformities, efficiencies, runoff, and erosion.

· Irrigation methods: advantages, disadvantages, selection, construction, and maintenance of irrigation systems.

· Design and redesign of existing irrigation systems for better management.

· Management of new or existing systems: how to determine correct flow rates and timing and amount of each irrigation; evaluate uniformities and efficiencies in an irrigation system; monitor the soil profile and the crop to ensure good irrigations; minimize runoff; detect simple visual indicators of poor or good irrigations; evaluate the impact of changing one or more management parameters on the soil (erosion, infiltration characteristics, and water-holding capacities), the crop, or the overall distribution of water in the field (uniformities, efficiencies, and amount of deep percolation and/or runoff).

The training of both the technical personnel and farmer can cover essentially the same subjects, but the farmer training will be on a more basic level. The training should, however, give farmers enough fundamentals so that he or she can monitor the system well and make changes in timing, duration, flow rates, and other operational characteristics as needed. The farmer should also have the ability and motivation to perform the necessary maintenance.

Extensive hands-on field training is a must. Classroom exercises, slide shows, videotapes, training bulletins, and classroom instructors are important, but they are no substitute for hands-on training.

This chapter summarizes technical information and procedures for developing farm water management programs. It avoids theoretical developments and presents primarily methods that are compatible with data and equipment available to Volunteers working in small farm situations in developing countries. It includes a section of a training manual, "Water Management on Small Farms," which has been used successfully in a number of countries for training small-scale farmers and the technicians who assist them.

<<TOC3>> 6.2 Farm irrigation scheduling

The questions of when to irrigate and how much water to apply cannot be answered simply in a textbook approach with a series of equations. To provide guidelines for answering these questions, however, we do resort to equations whose results must be tempered with experience and knowledge of the field situation. To avoid oversimplification, we begin with a discussion of the factors that farmers should consider when they determine the irrigation schedule.

6.2.1 Factors affecting irrigation scheduling

Outside the growing season, a farmer's need to irrigate will depend on several factors. The farmer must consider irrigation requirements for tillage and leaching of salts or to ensure adequate moisture for planting. He or she may consider filling the soil profile to avoid having to irrigate early in the growing season or to ensure an adequate soil water reservoir in times of drought or insufficient water availability during the growing season. A farmer may also irrigate to germinate weeds that can then be removed through cultivation before planting.

After planting and during germination, irrigation requirements are a function of the soil moisture around the seed. Shallow or variable planting depths may require several light irrigations to obtain a good stand while deeper planted crops may require one irrigation or possibly no irrigations, if good moisture is available at planting. Other factors, such as the need to push salts below the seed and seedling, may influence the number of irrigations and amounts of irrigation water to be applied. Crusting of the soil surface may inhibit plant emergence, thus one or two irrigations specifically for the purpose of softening the soil to allow uniform seedling emergence may be required.

After emergence and during the vegetative development of the crop, the irrigation schedule is a function of the rate of crop water use (evapotranspiration), the soil water reservoir available to the roots, the specific crop, and the flexibility of the system. As the crop grows and the rooting system develops, the soil water reservoir accessible to the plant increases. As the plants grow, however, they require higher moisture levels in the root zone. Thus, during vegetative development, the schedule should take into account the change in the soil water reservoir, water use, and crop sensitivity. In addition, factors such as the unevenness of the soil surface, surface methods of irrigation, intake characteristics, and water control may limit the maximum or minimum efficient depth of application. Thus, a farmer may increase or decrease irrigation intervals and application depths to avoid deep percolation or runoff with less concern for the level of soil moisture at which yield decreases begin.

Irrigation intervals may be increased to avoid leaching of nutrients at this stage. Soil salinity considerations may require greater depths of application for leaching, or closer intervals to dilute the salts in the root zone. Other factors such as the need to germinate weeds, prevent wind erosion, and modify soil temperatures may be important.

During mid-season, the soil water reservoir is fully or almost fully developed. Most crops are especially sensitive to moisture stresses at this time, and their potential to use water is greatest. The irrigation schedule during this part of the season can often be a constant if weather conditions are constant. It could, however, vary with changes in crop water use or the amount of effective precipitation. The same factors (previously mentioned) that influence the maximum or minimum application depths should be considered. Nutrient leaching and soil salinity may be considerations. Disease or pest problems that result from frequent irrigations or wet conditions are often reasons for lengthening irrigation intervals. Crop cooling and frost protection may also be factors influencing a schedule, especially in sprinkler irrigation. In irrigation systems that are used for application of fertilizers, pesticides, or other chemicals, the timing of these may be the overriding consideration for certain irrigations. The fruit formation stage of many crops occurs sometime during mid-season. This is usually the most sensitive period of the whole crop season; thus, proper irrigation scheduling during mid-season is a key to good production.

Late in the growing season, irrigation schedules are influenced by the same factors as during vegetative and mid-season periods. During this time, however, crop yields are not as easily affected as during mid-season. The soil water reservoir in the root zone is developed to a maximum, water use is decreasing, and the crop is less sensitive to high soil moisture depletions. As a result, irrigations can typically be less frequent and of greater depths. The quality of the crop can often be greatly influenced by the irrigation schedule during this time. The protein content in grains can be increased, potato storage ability is improved, and the sugar content of cane is increased by withholding water as these crops mature. The color of some crops can be altered. Harvest moisture is a definite consideration, since the last irrigation usually must be scheduled with enough time to allow the soil and crop to dry for harvesting. For root and tops, however, sufficient soil moisture to allow harvesting must be assured. Post-harvest tillage moisture requirements may also be considered at this time.

For any irrigation, the availability of water and the flexibility of the delivery and farm system are often the most important factors in scheduling. The importance of a farmer's personal preferences, religious and social obligations, and other cultural aspects should also be taken into account.

6.2.2 The practice of irrigation scheduling

In theory, farmers should be able to apply irrigation water when they consider their soils to have reached an acceptable soil water depletion, or Management Allowed Deficit (MAD). At that time, the farmer should be able to apply an amount of water that will either fully or partially refill the soil profile. The farmer may opt to only partially fill the soil profile if the water supply is limited, if he or she wants to avoid deep percolation and leaching, or if he or she wants to leave room for precipitation storage.

MAD may be equal to the depletion at which crop yields begin to decrease, or it may be greater or lesser than this depth. It is also common to allow a lower MAD on trickle irrigation systems as labor and flexibility do not pose a problem, or to insure adequate down time during system malfunctions. Sprinkler systems may require a smaller MAD and application amounts to avoid runoff problems or to allow for application of chemicals. Irrigating with a lesser MAD may also be practiced if the water is delivered frequently, but in small amounts. On the other hand, a farmer may select a MAD that corresponds to significant crop stress. The farmer may do this to increase his or her irrigation efficiencies by building up larger deficits before irrigating, or by spreading the water over a greater area, thus maximizing yield over the farm.

6.2.3 Techniques for preparing irrigation schedules

Determinations on the timing and amounts of water to apply are made by the farmer in a variety of ways. Farmers may visually inspect the crop for signs of stress and then apply an amount of water consistent with their experience, availability of the supply, or by other means. The farmer may inspect the soil in the root zone by feel and by visual methods to determine timing and/or amounts. Another farmer may use moisture sensing or stress sensing devices such as tensiometers, neutron probes, or infrared sensors. Still others may rely on soil water budget models that use soil, crop, climate, and irrigation system factors. Many will use combinations of the different methods, and many will use no method but irrigate when water is available.

A good scheduling technique should be able to predict with some anticipation the timing and amount of irrigation required so that the farmer can plan water deliveries. In the following sections, two common and useful scheduling techniques are presented.

6.2.4 Useful relationships in irrigation scheduling

Knowledge of the soil, crop, climate, and system parameters can be integrated into useful relationships for irrigation scheduling. Irrigation intervals, net application and gross application depths, volume of water required for irrigation, and irrigation duration versus flow rate are some of the essential results that can be derived by using the following basic relationships:

1. Net Irrigation Requirement

Irn = ET - Pe - Gw - Wb,

where:

Irn

is the net irrigation requirement for the time period under consideration for a given crop.

ET

is evapotranspiration for the crop being considered. For non-stressed conditions, ET = ETc.

Pe

is the effective precipitation, or that portion of precipitation that provides water for the ET requirements of the crop or for tillage and other beneficial uses related to crop growth.

Gw

is the ground water contribution to the crop requirements. Gw is generally negligible if water tables are not within a meter of the surface.

Wb

is that portion of stored soil water at the beginning of the period that can be used to satisfy ET requirements of the crop. With shallow soils and shallow rooted crops this is usually negligible.

2. Management Allowed Depletion

MAD = p' AW D

where:

p'

is the allowable depletion of the soil water that will not result in unacceptable yield reduction, expressed as a fraction of the moisture that a soil can hold between field capacity (fc) and percent wilting point (pwp). For example, a value commonly used is 0.5, although for a farm with moisture-sensitive crops and high water availability, lower values usually around.35 would be appropriate. For stress-tolerant crops and limited water availability, higher values (0.6 to 0.7) may be better.

AW

is the amount (depth) of water that a soil can hold between field capacity (fc) and percent wilting point (pwp) per unit depth of soil.

D

is the root zone depth, which varies with crop, stage of development and soils.

Example: A sandy loam soil holds 140 mm/m between fc and pwp. The root depth is 30 cm, and the depletion not resulting in yield decrease is 35%. What is the allowed depletion if moisture stress is to be avoided?

Solution: MAD = 0.35 140 mm/m 0.3 m = 14.7 mm

3. Typical Irrigation Interval Required

Assume the soil is filled at least to field capacity at each irrigation.

I = MAD/(ETc - Pe - Gw) + Ts

where:

I

is the allowable interval between irrigations if the crop is irrigated when the management allowable depletion has been reached.

ETc

is the daily crop water use, assuming that the crop is kept at near optimum moisture conditions.

Pe

is the mean daily effective rainfall.

Gw

is the mean daily contribution of the ground water or saturated zone below the root system to crop requirements. Neglect this component unless water tables are within one meter of the surface.

Ts

is time (days) for soil to drain from saturation to field capacity after an irrigation if more water than that required to fill the soil to field capacity is applied. (Use 1 day for sandy soils, 2 days for sandy loams, and 3 days for loams, clay loams, and other heavy soils.)

Example: The daily crop water use of the lettuce crop in the previous examples is approximately 5 mm/day. What is a desired frequency, assuming that the soil will be filled above capacity at each irrigation to allow for a longer interval between irrigations? Assume no rainfall or ground water contribution.

Solution:

Comment: Coarse-textured soils can retain saturation moisture in the root zone approximately 1 day after irrigation, medium-textured soils 2 days, and fine-textured soils 3 to 4 days. Thus, the 4-day interval should be conservative.

4. Net Application Depth Desired (dn)

For a known irrigation interval and ET, assume the profile was filled to field capacity at the previous irrigation and will be filled again with the present irrigation.

dn = I(ET - Pe - Gw)

where:

I

is irrigation interval.

ET

is average daily evapotranspiration over the period.

Pe

is the average daily effective rainfall contribution.

Gw

is the average daily ground water contribution.

Comment: With

I = 4 days of previous example, then:

dn = 4 days
5 mm/day = 20 mm

In the case of limited water availability when the deficit will not be totally replenished, dn will be diminished by the amount of deficit allowed after irrigation.

5. Gross Application Depth

dg = dn/Ea

where:

dg

is the gross depth to be applied at each irrigation in order to apply net dn.

Ea

is expected application efficiency (fraction).

Example: We want to a apply net depth of 20 mm. The application efficiency with furrow irrigation is 40%, and with sprinkler irrigation is 65%. How much water do we need with furrow irrigation? With sprinklers?

Solution (sprinkler irrigation):

dg = 20/0.65 = 30.8 mm

Solution (surface irrigation):

dg = 20/0.40 = 50 mm

6. Continuity Equation

Va = Qt = dg A

where:

Va

is the gross volume applied.

Q

is the discharge or flow rate.

t

is the irrigation duration.

A

is the area over which water is applied.

Dg

is the gross applied depth.

Example: With a 28 liter per second (28 liters/sec) flow rate, how long will it take to irrigate a 1/2 hectare surface irrigated plot from the previous example?

Solution:

28 liters/sec = 101 m3/hr
dg = 20/0.4 = 50 mm = 0.050 m

Va = 0.050 m
5,000 m2 = 250 m3
t = Va/Q = 250 m3/101 m3/hr = 2.5 hours

6.2.5 The soil water budget approach

To schedule individual irrigations by the soil water budget approach, estimates of the net irrigation requirements during an interval are estimated with Equation 1 or similar equations. The depletions are then compared with MAD (Equation 2). Irrigation is effected when Irn is approximately equal to MAD. Equation 3 is used to estimate the desired irrigation interval. Equation 4 may be used for estimating net application depths when the irrigation interval is known. Equation 5 is used in establishing gross irrigation. Equation 6 is used to determine required flow rates or irrigation duration once the gross depth and area to be irrigated are known.

The depletion at the start of the season must be estimated or measured. Usually, excess rainfall or irrigation is assumed to be lost to deep percolation. Saturation moisture, however, is usable by plants (1 to 4 days after irrigation). Especially in heavier soils, saturation moisture should be considered by the scheduler. If water tables are deeper than 40 cm below the root zone, then ground water contributions may be neglected for scheduling purposes.

The ETc estimates are usually provided by private or public agencies or can be estimated from knowledge of weather and crop conditions. The budgets (balance) can be updated by hand, by computer, or hand programmable calculators.

Typical schedules based on normal climatic conditions, crop development, soils, and other considerations may be developed as an aid to irrigators.

Efficiencies may vary through the irrigation season. Predictions are affected by the accuracy of inputs. Because of this, the predictions of water balance techniques should be field verified periodically.

6.2.6 The feel and appearance method

The Feel and Appearance Method correlates the "when" and "how much" of irrigation to the feel and appearance of the soil (Table 6.1). This method is rapid and simple and, when used by experienced personnel, can be quite accurate. It permits a large number of samples to be analyzed quickly in the field. It requires only the use of a soil probe or shovel to obtain the samples. Proper location for sampling and correct depth of sampling are important in obtaining a true status of moisture in a field. Some considerations are:

1. Where are the critical soils, i.e., which soils have the greatest and the least moisture retentivity, which have the fastest and slowest infiltration rates? Keep in mind the proportion of the field that has these critical soils in order to determine whether or not it would be economical or practical to base the irrigation schedule on conditions at those locations.

2. Where is the application (or penetration) of water the least? For example, with sprinkler irrigation, this could be where the pressures in the system are the least. In border strip irrigation, it could be where the contact time of the water on the soil is the least (the head or the tail of the strip). In furrows, it is usually at the end of the furrow if the water has not ponded there.

3. In which part of the field is irrigation initiated, and where is it terminated? These two points generally show the greatest differences in moisture content if the soils are uniform in other aspects.

4. Where are plant population and development representative? Generally, where the plants have greater foliar area and where the foliar development is more active, is where the plants utilize more water. With other conditions being uniform, this will be where the soil will become the driest until full plant coverage is reached.

5. Where are different crops, or varieties of the same crop, planted? Even within different varieties of the same crop, there can be great differences of water use.

6. In which part of the field is access convenient for the farmer or a technician to take readings?

7. What is the root depth of the crop and what is the critical depth for moisture monitoring? For example, at effective cover, small grains have rooting depths of a meter or more if roots do not encounter obstacles. If the soil is uniform and deep, it is important to periodically verify the moisture status in the entire profile. However, if the good soil is very superficial (say 30 cm) and below is only coarse gravel, it is important to monitor in the top 30 cm. Critical depth may be quite different from total rooting depth. For example:

Potatoes can have roots to one meter. During tuber enlargement, however, it is very important to monitor the top 45 cm, especially the first 30 cm. Therefore, the soil moisture monitoring is conducted to 45 cm of depth. The root depth changes from the time of planting, so the important zone for monitoring also varies from planting time until about the effective cover stage.

8. Where is the greatest root concentration? In potatoes, for example, root development initiates at the seed tuber and the greatest water use begins there. Therefore, it is important to place the sensors near the planted tuber in the initial growth stage.

6.2.7 Summary of scheduling techniques

Many types of moisture sensors and scheduling techniques have been developed. Because of the limited technical background of many farmers, however, the acceptability of a scheduling technique by farmers may hinge on the simplicity of the method. An acceptable scheduling technique for them may consist simply of field probing and use of the feel method for determining when to irrigate and whether the irrigations are adequate. In spite of its simplicity, monitoring of root zone moisture by this technique may result in dramatic improvements in efficiencies and crop yields.

Table 6.1 Guide for Judging How Much Moisture is Available for Crops (Ref. 51)


Feel or appearance of soil and moister deficiency

Available soil moisture

Loamy Sand

Sandy Loam

Loam and Silt Loam

Clay Loam or Silty

remaining




Clay Loam


Course Texture

Moderately Course Texture

Medium Texture

Fine and Very Fine





Texture

0 to 25 percent

Dry, loose, single grained, flows through fingers

Dry, loose, flows through fingers

Powdery dry, sometimes slightly crusted but easily broken down into powdery condition.

Hard baked, cracked, sometimes has loose curmbs on surface.

25 to 50 percent

Appears to be dry, will not from a ball with pressure. 1*

Appears to be dry, will not form a ball. 1*

Somewhat crumbly but holds together from pressure.

Somewhat pliable, will ball under pressure. 1*

50 to 75 percent

Appears to be dry, will not form a ball with pressure.

Tends to ball under pressure but seldon holds together.

Forms a ball somewhat plastic, will sometimes slick slights with pressure.

Forms a ball, ribbons out between thumb and forefinger.

75 percent to field capacity (100 percent).

Tends to stick together slightly, sometimes forms a very weak ball under pressure.

Forms weak ball, breaks easily, will not slick.

Forms a ball, is very pliable, slicks readily, is relatively high in clay.

Easily ribbons out between fingers, has slick feeling.

At field capacity (100 percent).

Upon squeezing, no free water appears on soil but wet outline of ball is left on hand.

Upon squeezing, no free water appears on soil but wet outline of ball is left on hand.

Upon squeezing, no free water appears on soil but wet outline of ball is left on hand.

Upon squeezing, no free water appears on soil but wet outline of ball is left on hand.

1* Ball is formed by squeezing a handful of soil very firmly.

6.2.8 A comparison of scheduling criteria for surface, sprinkler, and drip irrigation

Surface Irrigation Scheduling

The inability of many surface irrigation systems to apply small amounts of water efficiently, and the inflexibility sometimes associated with them, often requires a different scheduling criteria than is associated with sprinkler or drip irrigation.

A farmer typically irrigates to fill the soil profile on the higher parts of his or her field or at the tail end of his or her runs. Unless a surface irrigation system is well designed, levelled and managed, it is difficult for farmers to irrigate with less than 8 or 10 cm of water application. Net irrigation requirements (deficits) of 2 to 4 cm at the time of irrigation can result in extremely wasteful and inefficient irrigations. Thus, even though yields may decrease due to water stress, a much higher MAD (perhaps 4 or 5 cm) might be most economical. With greater MAD, irrigation efficiency is often increased, leaching of fertilizers decreases, and total irrigated area often increases. Thus, yield per unit of water may increase significantly. For sensitive crops or light soils, or heavy soils with infiltration problems, the economically allowed MAD may be only 2, 3, or 4 cm. In this case, it may be necessary to consider well levelled basins or graded furrows that can be operated to apply 3 to 6 cm of water uniformly and efficiently.

Once MAD and irrigation interval are determined for a surface system, the task is to determine how to apply the desired net depth of application. Intake rates, surface roughness, geometry of furrows, and required net application depths may change from one irrigation to the next and from season to season. Thus, advising the irrigator on the flow rates and durations of irrigation requires a very good understanding of the hydraulics of the system and its variability, or monitoring and experience by the farmer. In the absence of technical help, the farmer may experiment with various flow rates and durations that are possible and convenient. Probing the soil one, two, or three days after irrigation, he or she can determine whether irrigation depth was adequate in the critical parts of the field (tail, heavier or lighter soils, high spots, etc.). The farmer can then make adjustments in future irrigations.

Sprinkler Irrigation Scheduling

Well-designed sprinkler systems can be managed to apply water with a minimum of runoff and deep percolation. Proper management can result in application efficiencies of 70% to 90%. Infiltration rates, soil surface storage, labor availability, automation, and various factors other than rooting depth, water holding capacities, and crop type are important in determining MAD.

With hand move systems (and others requiring significant labor), it is often economically desirable to make the MAD as high as possible and develop 8, 12 or 24-hour sets to minimize labor costs and maximize convenience.

Drip Irrigation Scheduling

Net irrigation requirements may be less with drip irrigation than with surface or sprinkler irrigation, primarily because the area wetted is reduced and evaporation is decreased. This reduction in water requirement usually occurs early in the growth stage of plants when the soil surface is bare. Once the ground is 50% to 75% shaded, the difference in water requirements is minimal. As most ET estimating methods assume a significant evaporation component early in crop growth, estimates of ET may need to be reduced in relation to the soil surface area wetted by the drip system. On the other hand, frequent irrigations associated with drip irrigation may increase ET of the wetted area; thus ET may not necessarily be much less than on surface or sprinkler irrigation. Observation of the wetted area, soil moisture levels, and plant response will provide the experience necessary for making such adjustments.

Crop water requirements are often converted from units of depth per unit time, to units of water volume per unit land area per unit time, e.g., liters/ha/day. Application rates are often expressed as units of water volume per plant per unit time.

In drip irrigation systems, the wetted volume of the root zone is usually only a portion of that wetted by other methods. The system is usually designed for continuous, or nearly continuous, operation during peak use periods. Cost factors do not allow for significant oversizing of the system. In addition, flexibility due to control and automation permit the crops to be maintained at high moisture levels conducive to maximum yields while maintaining high irrigation efficiencies. Thus, the concept of drip irrigation implies a high frequency (short irrigation intervals) and small amounts of application (small MAD) at each irrigation as compared with conventional methods.

Because of the limited wetted volume, the concept of MAD applies to that portion of the root zone that is wetted by the drip system. The scheduling process is simple. MAD is usually selected to be less than or equal to RAW (see Chapter 2.4) in the wetted volume. The wetted volume is replenished daily, or at the preselected MAD. A slight excess (10% to 20%) above that required to eliminate the deficit is applied to account for non-uniformities to application.

For a well-managed system, the efficiencies are a function of emitter flow variations caused by hydraulic design and manufacturing variations, as well as whether or not the entire system will be replenished to capacity or some deficit will be allowed in areas of least water application. Efficiencies of 85% to 90% can usually be used in well-designed and managed systems for scheduling without noticeable yield effects.

6.2.9 Rice irrigation scheduling

The water use (Et) by a non-stressed rice crop can vary between 450 and 700 mm per day, depending on climate and variety. Kc for the initial 2 months varies from 1.1 to 1.15, depending on the wetness of the soil surface. Mid-season Kc varies from 1.1 to 1.3 and, for the final month, it varies from 0.95 to 1.05.

Saturation or near saturation conditions must be maintained during most of the growing season. After transplanting, saturation is maintained by a film of water (about 10 cm) over the surface. Flowering and head formation are most sensitive to a lack of water. Some scheduling approaches for rice follow.

In continual saturation scheduling, water is maintained at approximately 10 cm depth for about one week after transplanting. Through tillering, a maximum depth of about 3 cm is maintained. From 30 days before head formation and flowering to the start of maturity, soil is covered with water often to a depth of 8 or 10 cm. A continual flow of water is frequently maintained. The fields should be drained completely 30 to 45 days before harvest to ensure that they will be dry enough for harvest. This method generally results in maximum potential yield.

Intermittent irrigation is another scheduling approach used on rice when water is more scarce. Irrigations are applied periodically to maintain the crops at near-saturation level when possible. Yield may be reduced significantly with this method if moisture cannot be maintained at saturation from heading to flowering.

In the controlled water savings method of irrigation, the field is maintained as close to saturation as possible, except that the field is flooded at transplanting and then for about 30 days (from heading through flowering). Maximum yields can be achieved with this method.

Rice is an aquatic plant and thrives on saturation conditions. Total submersion of the rice plant for extended periods of time, however, decreases yields. Thus, it is important that both the supply and the drainage be closely controlled. As rice is usually irrigated in basins, the basin bottoms should be well levelled to prevent excessive submergence in places and inadequate wetting in others.

<<TOC4>> 6.2.10 Scheduling and management strategies for limited water supplies

Intentional under-irrigation is the practice of deliberately applying less than the soil moisture deficit (SMD) over all or part of the field. This may be done to conserve water during peak use periods through less percolation and runoff losses. Application efficiencies are generally higher. It is sometimes done to allow for precipitation storage in the root zone. This practice does not necessarily reduce yield, but makes more effective use of water. It also makes good use of the total moisture stored in the root zone.

Stress irrigation or deficit irrigation is the practice of extending the interval between irrigations to the point that crop water use is limited below potential or peak use. Thus, MAD is selected at values that will limit crop production. This practice may reduce system capacity requirements and obtain maximum yields per unit of water or unit of capital cost. With a limited but flexible water supply, it may be possible to stress a crop at periods other than critical periods and still obtain near-peak yields. This practice may result in better root development and better ability to use deep moisture storage. Where a significant drying of the soil in the lower root zone develops, the nutrients may not be available for plant growth; thus, the whole root zone should be wetted periodically. In addition, a limited supply will cause less stress if applied in enough depth that soil surface evaporation is limited and the lower root zone reservoir is filled or almost filled. A smaller concentration of roots in the deeper root zone will result in slower moisture extraction and less severe stress than application of small amounts that are quickly withdrawn from the upper root zone by the plant.

During periods of short water supply, a choice needs to be made between conducting deficit irrigation (in which the crops will be stressed) or reducing the area to be irrigated. The choice may be made at the project level if the administration is capable of enforcing the decision or at the farm level if the farmer can foresee the extent of the shortage. In some areas, water supply forecasts published by different agencies may help in making the decision.

If there is significant water storage in project or farm reservoirs, the time of water delivery can be controlled. Each crop has critical periods during which water stress has maximum effect on the reduction of crop yields. This period is usually during the flowering, fruit setting, or yield formation stages. Thus, short water supplies might be best distributed by holding back on the normal irrigation supplies except during these critical periods.

When water is scarce, the various irrigation efficiencies should be evaluated. Much water is frequently lost in the conveyance system. Observational evaluations of losses at turnouts and other distribution points on the canal system, and of vegetation in and along the canal that may be using water and increasing seepage losses, should be conducted. Seepage losses in sandy soils or permeable stretches of canal can be evaluated to determine what parts of the canal have greater losses and what the magnitude of these losses are. The value of water lost, as compared with production losses, may provide the economic basis of whether or not to line parts of the canal or pursue other measures to reduce the losses.

Increasing irrigation efficiencies through evaluation and improvement may lessen the impact of shortages. Land leveling, reuse of runoff water, use of better water control devices, and use of management techniques are important. Continual evaluation and monitoring of the irrigation system to determine frequency and amounts of irrigation, along with necessary operational changes, are of great importance.

Techniques such as runoff interception by ridges on the contour or contour seepage furrows permit more water to be absorbed by the soil. Mulching to reduce evaporation from the soil can significantly improve moisture availability for plant use. Plant residues and even thin gravel layers have been used. Proper tillage of the soil for moisture conservation leaves the soil loose so that the upper soil layer becomes a mulch that impedes capillary movement. Filling the soil during periods of high water availability may provide necessary moisture for crops during shortages, especially on deep soils with good water-holding capacities.

If the irrigation supply depends on stream flows that are partially sustained by ground water contributions, then water infiltration needs to be promoted during years of excess. The same holds true when ground water provides a major supply of irrigation water through pumping. In both cases, the water tables can be raised; therefore, the water stored for future use can be enhanced by water spreading on idle lands, ponding in gravel beds, areas of permeable materials, ditches, and drains, and recharging wells directly.

Excess water availability for irrigation tends to result in over-irrigation by farmers and can be as detrimental as a water shortage. Proper timing and amounts of irrigation and drainage of excess rain and irrigation water is necessary for maintaining high yields. Thus, it is of primary importance that farmers recognize that good management is necessary even in times of excess.

Some of the key elements in setting up acceptable irrigation scheduling programs are as follows:

1. The schedules must be compatible with the farmer's other schedules (e.g. cultivation, herbigation).

2. The scheduling techniques must be simple and reliable and must not make a great demand on the farmer's time.

3. The scheduling program must be economically attractive.

4. To be most successful at the farm level, water supplies must be flexible in terms of timing and quantity.

6.2.11 Delivery system schedules

In irrigation projects where several users are served from the same water supply, it is often economically unfeasible to supply each irrigator according to optimal timing and amount. In such cases, the delivery system may be scheduled to meet the needs (timing and amount) of the major crops of the area.

The distribution of water within the delivery system is accomplished in different ways. The main methods are:

On-Demand: The farmer takes water at any time up to the capacity of his or her outlet or at a predetermined discharge.

Semi-Demand: The farmer generally requests water within 2 to 7 days before delivery.

Canal Rotation and Free Demand: The secondary canals receive water by turns, and when the canal is receiving water the farmers can take how much they need when they need it.

Rotational System: The canals receive water by turns and the farmers under each canal also receive water by turns when it is available in the canal.

Continuous Flow: The farmer receives a continuous supply of water sufficient to cover evapotranspiration needs of his or her crop.

Demand systems generally require higher level technology, very good communication, large design capacities, high initial costs, and a high degree of flexible operational control.

Rotation supply schedules are the most common methods for distributing water in irrigation projects. There are several ways in which a rotational supply may be scheduled:

1. Fixed interval-fixed depth (amount). The farmer receives water at a fixed interval (e.g. every 7, 10, 14, 21 days) with the same amount (usually same discharge and duration) at each irrigation of the season. The amount may be proportional to land holdings or cropped area.

2. Fixed interval-variable depth (amount). The farmer is given less water per turn early and late in the season when crop water use is less.

3. Variable interval-fixed depth (amount). The same amount of water is given at each irrigation, but the interval is varied (shorter during high water use periods and longer during low water use periods).

4. Variable frequency-variable depth (amount). Both irrigation intervals and amounts are varied as the root zone water reservoir expands and the water use of the crop varies through the season.

Method 1 is the simplest to manage and administer but is the least in tune with crop requirements and has the highest potential for inefficiency. Method 4 is most in tune with crop requirements and would theoretically result in higher efficiencies. However, standardization of crops, varieties, plantings, dates, are seldom possible. A more in depth discussion of each rotation method follows.

Fixed depth-fixed interval scheduling typically results in inefficient irrigation during initial stages when roots are shallow and water use is low and late growth stages when water use is low. This sheduling may result in reduced yields on shallower soils due to leaching and moisture stress, or waterlogging problems on heavier, deeper soils.

Fixed interval-variable depth schedules typically have problems because present design and construction of irrigation systems do not permit application of small amounts. For example, unless a field is well designed and well levelled, farmers may need to apply 8 cm or more per irrigation to cover the high and dry areas. Cropping patterns and sowing dates would need to be standardized for each project area; systems would need to be redesigned and reconstructed; land levelling would be essential; and farmer training would necessary if the farmer is to be able to apply varying doses uniformly. Unless the farmer is able to adjust flow rate and duration on border and furrow systems, then uniform depths with various doses are impossible. Uniform application of different doses is much more easily accomplished with level basins by simply varying irrigation duration.

A variable interval-fixed dentin schedule based on applying water each time a given net deficit developed would result in stress early in the season when applications may need to be light and frequent to assure germination and initial development when roots are shallow. As most surface irrigation systems are efficient when designed and constructed for a given net depth of application, this could be a very efficiently operated system. If crops, varieties, and planting dates were standardized, this system could be made very useful as long as some flexibility were allowed during germination and initial development, i.e. the first two or three irrigations. A change in irrigation interval will require good communication between project managers and farmers as irrigators will not receive water on the same day of the week each time. If systems are modified to allow smaller but uniform application depths, the fixed depth-variable interval schedule could be adapted to soils with significant variability without creating problems, i.e. a net depth of depletion allowed at each irrigation (MAD) could be selected that would not stress crops or leach nutrients on shallow soils or waterlog heavier soils.

Implementation of a variable interval-variable dentin schedule usually has several major constraints. A flexible water supply would be needed, such as that available to well irrigators. Water would need to be available at all times in the canal to satisfy a mix of crops, varieties, planting dates, and soils. Farm irrigation systems are not designed and constructed to allow variable application depths. Farmers would need to account for changes in the water needs of crops and other aspects of soil, plant, water, and irrigation system interrelationships to apply the variable doses efficiently.

This schedule requires great accuracy in management and more skilled labor. Communication between farmers and administrators would have to be excellent. Farmers are accustomed to receiving water on the same day each week. Additionally, since some farmers are unable to read, they may have problems measuring depths.

Although difficult, it may be possible to:

· standardize crops (i.e. block farming) and varieties;
· standardize planting dates within 10-14 days;

· redesign farm systems, including leveling and use of level basins, sprinklers, alternate furrow irrigations, contour furrows, or other means of allowing variable doses;
· train farmers in adjustment of flow rates and duration, depth measurement in basins, and other aspects of management;

· standardize irrigation blocks in areas of similar soils; and
· improve communication between project and farm levels.

It may be possible to adjust doses and/or intervals once or twice during the season as climatic, crop, and other conditions change. It may also be possible to shorten intervals or increase amounts during critical growth periods with a minimum of administrative difficulty.

6.2.12 Project scheduling a summary

Some attempts at improved project irrigation scheduling have succeeded and others have failed to produce the desired results. A careful evaluation of ET requirements does not ensure that water will be made available at the right time, in the right place, and in the right amount. Poor land preparation and uneven surface topography often make satisfactory uniformity of surface irrigation impossible. Inflexibility and lack of capacity often form a major constraint. Training of all persons involved in water management from project level to farm level is often nonexistent. Failure to monitor the conditions adequately and adjust the schedule accordingly can lead to either excessive or deficit irrigation.

The flexibility of irrigation systems can often be improved through better communications, addition of regulating reservoirs, and modified canal structures, among other means. Uniform project- wide scheduling can be altered to increase flexibility by considering the differences in soils, crops, and flexibilities of the delivery system in various areas. The most flexible schedule possible should be used in each canal command area. For example, canals closer to the water supply source may be provided with greater flexibility than canals farther away. Rigid schedules may be considered for the peak season, with more flexibility being allowed during off-peak periods.

Generally, increased flexibility at the project level increases the cost of the system but allows the farmer to maximize his or her yields. Rigid (rotational) systems minimize capital costs and can be scheduled in various ways to increase efficiencies and improve the productivity of water. As water is often in limited supply, it is often necessary to consider scheduling strategies for maximizing yield per unit of water or for objectives other than the farmer's own objectives.

<<TOC3>> 6.3 Evaluation of existing irrigation systems

6.3.1 Strategies for farm management

Good farm irrigation management assures:

· correct frequency of irrigations,
· correct application depth,
· uniform irrigation,
· minimum runoff,
· minimum deep percolation except for that required for salt management,
· minimum erosion, and
· optimal return on irrigation investment.

Proper design, construction, and management are essential for achieving high efficiencies. A key to good management is the periodic evaluation of the irrigation system. System evaluation provides information on the actual operation and management of existing systems and potential improvements. Improvements can result in water, labor, soil, and fertility conservation, decreased drainage and salinity problems, and increased yields or profitability.

Detailed evaluation criteria for evaluating and managing most farm irrigation systems is presented in Reference 34. It is often impossible due to economic and other constraints, however, to evaluate systems in great detail. Thus, simple evaluation criteria that can help determine the magnitude of on-farm management problems and possible solutions are presented here.

Properly conducted evaluations provide answers to the following questions:

1. Is the system properly designed? In other words, are lengths, widths, flow rates, pressures, slopes, and other design factors within acceptable ranges to permit good irrigation when the system is properly managed?

2. Are the irrigations conducted at the right time and in the right amount? If not, what is the proper timing and amounts of typical irrigation applications?

3. Are the irrigations acceptably uniform? If not, how do we improve them?

4. Are runoff losses acceptable? If not, how do we lower these losses?

5. Is soil erosion a problem? If so, then how can the problem be eliminated?

In recommending modifications for a given crop, climate, soil, and irrigation system through the irrigation season, the following information is required:

· Planting dates, crop development data, and climatic data;

· Typical irrigation frequencies and amounts of water applied or flow rates and irrigation durations;

· Water availability through the season from rainfall and irrigation;

· Irrigation water quality and soil salinity;

· Soil textures, water-holding capacities, and intake rates;

· Climatic, crop, soil, and ground water data that will permit development of typical good management models;

· A physical system inventory including irrigation structures and tools and their condition;

· Maintenance programs, available labor, and capital;

· Social and economic constraints to system modification.

For furrow, border strip, or basin irrigation the types of design or management modifications that may be considered for improved irrigation include:

1. Flow rate adjustment: to increase or decrease advance rates, minimize runoff after the advance phase, or minimize erosion.

2. Frequency of irrigation: to achieve better yields or quality or permit higher efficiencies due to adjustment in MAD.

3. Duration of irrigation and depth of irrigation: to ensure adequate infiltration depths but minimize runoff and leaching losses.

4. Dimensions of space, length, width and geometry: to match soil types, flow rates, and topography.

5. Land leveling and smoothing.

6 Orientation or slope of furrows and borders (e.g., contour furrows).

7. Tillage and cultural practices: to change soil characteristics such as infiltration, soil surface storage, and erosion.

8. Return flow or tail water reuse systems: to eliminate runoff losses.

9. Storage and regulating reservoirs: to provide a more flexible and stable water supply and sometimes permit adjustments in flow rates.

10. Maintenance program: frequency and types of maintenance should be clearly defined.

11. Crop selection and planting dates: to be compatible with water availability over time and to take into account the rooting and moisture extraction patterns of crops. For example, deeper rooted crops develop a much greater moisture reservoir than shallow rooted crops and can be irrigated more efficiently on coarse soils. Deeper rooted crops are also less affected by micro uniformity problems as they have a more extensive rooting system.

Sprinkler irrigation systems may require the same type of adjustments as surface systems. However, modifications of the following should also be considered:

1. Operating pressure: to assure adequate drop breakup, wind fighting ability, uniformity, and actual rotation of the sprinkler heads throughout the system.

2. Riser height: to assure clearance of stream above crop and adequate coverage.

3. Sprinkler and lateral spacings: to provide uniformity.

4. Lateral orientation: to minimize terrain effects on uniformity.

5. Pipe sizes: to assure sufficient capacity and economy of operation.

6. Trash elimination: to prevent sprinkler plugging.

7. Alternate set sequencing: to improve uniformity and water penetration.

8. Operation during low evaporative conditions, i.e., nighttime or non-windy conditions.

Trickle irrigation may require the same types of modifications as sprinklers. In addition, the density of emitters and wetted volume should be considered to ensure adequate capacity for down time and adequate nutrient storage. The selection of the proper type of emitters, pressure regulators, and filtering and chemical purification systems and the maintenance of these is essential to adequate, trouble free operation.

6.3.2 Rapid on-site evaluations

A very basic evaluation to determine the adequacy of a specific irrigation requires only that simple observations and measurements be made before, during, and after irrigation. A soil probe or auger for sampling, along with experience in judging soil texture and moisture, are basic requirements. The following simple steps can be followed:

1. Observe the uniformity of crop growth, soil surface relief, and soil texture variation. Obviously stressed (stunted, wilted, yellow) crops can indicate location and extent of over or under watering, while observations of relief, texture variation, and patterns of non-uniformity can help determine the cause.

2. Determine the SMD by probing the soil in different parts of the field. Comparison of SMD with the MAD for that crop, soil, climate, and irrigation system indicates whether the irrigation interval is approximately correct or not.

In trickle irrigation the wetted volume is taken as the point of reference. Thus SMD and MAD are both determined for this volume.

The percent of wetted area, P is defined as: P = wetted area at the surface (30 cm below the surface) divided by the total cropped area.

The area wet by each emission point is small at the surface. The volume of the root zone under this point is the wetted volume.

Usually, 1/3 to 1/2 of the area is wetted. In areas with substantial rainfall the most economical P might be only 20 percent while in arid areas 50 percent may be more appropriate so that a greater volume of soil moisture and nutrient reservoir are brought into action.

3. Establish any magnitude of uniformity problems. Observations of the advance and recession rates in surface irrigation during irrigation can indicate the magnitude of uniformity problems if soil variations are not extreme. The time between when water first reaches a point in the field until the water recedes from that point is the intake opportunity time (To). Noting To at the beginning, middle and end of the field provides a uniformity indicator. If the times that water remains over different parts of the field are within 25 to 30 percent of each other on coarse soils and 40 percent on fine soils, the uniformities are usually acceptable.

Some simple empirical criteria useful in rapid evaluation of uniformities in specific system follow:

1. In basin irrigation systems where the surface is not continuously flooded the total volume of water should be introduced into the basin in 0.2 to 0.4 times the time that is required for the required amount of water to infiltrate. If erosion would occur with high flow rates, the number of inlets may need to be increased or the size of the basin and flow rate reduced.

2. With graded furrow systems, the water should advance the length of the furrow in 1/3 (heavy soils) to 1/4 (coarse soils) the time of the total irrigation duration.

3. With border systems, the water should advance to 2/3 to 4/5 of the border length before being turned off. Recession time should be about equal to advance time.

4. In sprinkler irrigation, 20 percent differentials in operating pressure at extreme points in the system will provide adequate discharge uniformity through the system. Spacing of sprinklers closer than the wetted radius and lateral spacings up to 50 percent greater than the sprinkler radius of throw provide adequate uniformities. With winds in excess of 5 mph, the lateral should be placed more closely. For better uniformities the laterals are usually placed vertical to the direction of prevailing winds.

5. The emission uniformity (KU) in trickle irrigation is a measure of the distribution uniformity. It is defined as the discharge per plant in the low quarter (of emitters) divided by the average rate of discharge of the emitters. It is affected by the manufacturing and pressure variations as well as condition of the emitters.

Emission uniformity values greater than 90 are considered excellent, 80 to 90 good, 70 to 80 fair, and less than 70 poor. Generally about 10 percent more water than the SMD or evapotranspiration is applied to the least watered areas. The potential efficiency of the low quarter can thus be estimated as PELQ = 0.9 KU.

After irrigation, simple probing of the soil in different parts of the field (and laterally into the bed with furrows) will provide a measure of uniformity.

6. Determine whether optimum and actual schedules coincide. Use the equation presented in the scheduling section (6.2) to determine desired irrigation intervals, depth of water application, etc. Then, compare what the optimal would be with what the farmer is actually doing.

7. Conduct additional observations on erosion, unevenness of spread of the water over the surface, fluctuations in the water supply, lateral penetration of water into the furrow bed during an irrigation event. Check pressure to ensure that sprinkler heads are operating within recommended ranges. Check for puddling of water, which would indicate incompatibility of the sprinkler and soil system.

8. Evaluate the adequacy of irrigation. The extent to which a soil profile is filled can be assessed 1, 2, or 3 days after an irrigation on coarse, medium, or fine-textured soils, respectively, by probing the soil to determine if any dry layers remain.

In evaluating irrigation systems and management alternatives at the farm level, it is imperative that the constraints beyond the farmer's control be carefully weighed. The irrigation district delivery schedules, and availability of power, machinery, and irrigation equipment may make many alternatives infeasible.

6.3.3 Evaluation of multiple farm irrigation systems

Rapid observational evaluations by experienced engineers and technicians along with a quick analysis of available or easily obtainable data can permit the development of useful management programs for a group of farms within an irrigation project. Such a procedure could encompass the following phases.

1. Compile information on

· soils, crops, and climate;
· seasonal, monthly, and weekly water delivery schedules;

· inventories of the typical irrigated area: number, size, condition, and location of canals, drains, reservoirs, and farm structures;

· administrative control of water;
· typical farmer management practices -- irrigation, cultural, and fertility;
· availability of power, machinery, and irrigation equipment;
· farmer progressiveness and sociological aspects; and

· typical yields of irrigated vs. non-irrigated land and well-irrigated vs. poorly irrigated lands.

2. Conduct observational evaluations of typical areas:

· maintenance and condition of canals and structures;
· uniformity of crop growth in irrigated fields -signs of excess or deficit watering; and
· indicators of high water tables, salinity problems, and erosion.

3. Determine water requirements and availability through the season.

4. Determine typical optimal irrigation schedules and compare with existing irrigation schedules.

5. Compare system designs and management with typical well-designed systems.

6. Determine possible changes within the constraints of the system to improve potential efficiencies. Improvements may involve changes in irrigation district management or changes in irrigation method, delivery and storage systems, adjustments in crops and irrigated acreages, or improved scheduling in the delivery system. Changes in fertility and other aspects of farm management may bring about on-farm improvements. Some changes will necessitate farmer training programs.

7. Select the better alternatives based on short and long-term economic, environmental, and social benefits.


<<TOC3>> 6.4 Training small-scale farmers in irrigation management

The following section is taken from the Water Management Synthesis Publication, "Water Management on Small Farms: A Training Manual for Farmers in Hill Areas" (Ref. 42). The manual resulted from work with small-scale farmers in the Peruvian highlands. Much of the agriculture in Peru takes place on steep slopes, shallow soils, and small farm plots. Extension personnel found the bulletin to be useful in training literate and semi-literate farmers and technicians.

The irrigation workshops conducted in Peru consisted of:

1. An audiovisual presentation (slides) that interpreted pertinent parts of the bulletin.

2. Extensive, practical hands-on training in which the farmers learned to use the material presented.

3. Field days during which farmers could observe the techniques taught to them "in action" on demonstration sites.

4. An evaluation of the farmers' new skills with such tools as simple levels and siphons and in evaluating and improving their management practices.

5. The bulletin was given to the farmers for reference.

This material should serve as a source from which basic concepts can be taken and applied to specific situations such as those encountered in developing countries.

An instructor guide (Ref. 42) is a companion to this manual.

THE IMPORTANCE OF IRRIGATION WATER AND ITS MANAGEMENT IN CROP PRODUCTION AND SOIL CONSERVATION


Bad irrigate


Good irrigate

The Role of Water in Plant Development


Growing plants need water, sun, air and nutrients to form roots, stems, leaves, fruits and seeds.

- Water is a large part of the plant's structure. It carries food through the plant and it cools the crop during hot weather.


Roots can't get enough air if there is an excess of water.

- A lack of water makes plants unable to draw needed nutrients (food) from the soil.


Too much or too little water results in lower

For good production, crops need the right amount of water throughout their development.


Enough water


Needed water


Irrigation can insure a good yield even during drought if we have provided the crop with its other needs.


In many parts of the world irrigation allows the harvest of two or more crops each year.

Sufficient water throughout the growing season helps insure healthy crops, thus enabling the plants to resist insects and. disease better.


Good example


Bad example

Irrigation Can Have Harmful Effects


1. More frequent irrigations than necessary result in a root zone which stays too wet for too long. Insufficient aeration and root rot may result.


2. Excessively heavy water applications may wash fertilizers and other nutrients away from the root zone.

3. Some farmers waste water when they irrigate, which means that other farmers may not have enough water for their fields.


Good example


Bad example


4 Careless maintenance of canals can result in weed, silt, or erosion problems, and not having water where it is needed.


Erosion is the loss of soil through the action of water and wind.


5 Uncontrolled irrigation water can be a primary cause of erosion.


On steep slopes there is greater danger of erosion than on flat land.

BASIC IRRIGATION CONCEPTS


1. Water holding capacity


2. Water entry Into soil


3. Deep percolation


4. Uniformity of water penetration


5. Runoff


6. Erosion

Concept 1.

WATER HOLDING CAPACITY

The soil in which the crop's roots grow is the reservoir from which plants can take their water. The amount of water which a soil can store within reach of the plant depends on the texture and structure of the soil and the depth to which the roots grow.

Organic matter within the soil helps increase the soil's capacity to store water. The best plants and plant residue for improving a soil are green legumes such as alfalfa, clover, peas and beans.

WATER HOLDING CAPACITY


Sources of Organic Matter


Soils low in organic matter


Soils high in

The Effect of Soil Texture and Structure on Water Holding Capacity

SOIL TEXTURE

Sandy or Coarse-Textured Soils

· contain much sand
· are rough when rubbed between the fingers
· are easy to plow and till
· do not form clods when dry

· generally these soils have low water holding capacity

Medium-Textured Soils

· contain coarse, medium and fine (clay) particles in almost even amounts

· form clods when dry which are easily broken with a pick, shovel, disc or other tillage equipment

· generally these soils have good water holding capacities.

Clayey or Fine-Textured Soils

· contain many fine particles
· are hard to plow if they are not wet
· form cracks on soil surface when the soil dries
· form very hard clods when dry

· these soils have high water holding capacity

SOIL STRUCTURE

Structure is the way soil particles are held together. Soils with good structure have different sized clumps of soil particles held together by decomposed organic matter. Well-structured soils can hold more water and air for plants to use.


COARSE


MEDIUM


FINE


SOIL STRUCTURE

ROOT DEPTH


Root depth increases with plant development until the plant is full grown unless the plant roots encounter obstacles. Thus, the moisture reservoir available to the plants expands as the root depth increases.

The area where roots are present is called the root zone.


Plants use more water from the upper part of the root zone because that is where they have more roots.


The maximum depth of roots depends on the crop and on the depth of good soil.... Layers of rock, hard soil, gravel, salts or other (...)

Where roots do not encounter obstacles in soils, they will reach to at least the following depths when they are fully developed.


Example 1


Example 2

Concept 2.

RATE OF WATER ENTRY INTO SOIL
(infiltration or intake rate)

How quickly water can penetrate into a soil varies with the kind of soil, the amount of moisture already present in the soil and the condition of the soil surface.

Water penetrates faster

· through sandy soil than clay soil
· through dry soil than wet soil
· through well-structured soil than compacted soil
· early in the irrigation season than later.


RATE OF WATER ENTRY INTO SOIL

Concept 3.

DEEP PERCOLATION

The penetration of water below the root zone is called deep percolation. Sometimes it is necessary to apply excess water so accumulated salts can be washed from the root zone.

This excess water may be costly, and it carries nutrients away. Thus, it should be limited to the amount necessary to eliminate harmful salts from the root zone.


DEEP PERCOLATION

Soils with flat, impermeable layers or high water tables will have their drainage problems compounded by deep percolation in the following ways.

· soils can become waterlogged
· a rising water table can bring more harmful salts into the root zone


Undesired


Desired


When the water table rises to the point where crop production declines, the resulting drainage problem will have to be eliminated by constructing a costly drainage system.


The drainage system may consist of open drains or buried drains. The design and construction of these generally require technical assistance.

Concept 4.

UNIFORMITY OF WATER PENETRATION

In a uniform irrigation, water penetrates to the same depth or about the same depth over the entire field.

Uniform penetration of water results in a uniform crop.


Good Irrigation

Non-uniform penetration of water creates uneven growth and yield.


Bad Irrigation

Causes of non-uniform penetration:

· variations of texture within the field
· uneven soil surface
· improper water management


Causes of non-uniform penetration

When the terrain is very uneven

· low spots receive too much water
· high spots remain dry
· water flowing through low areas can quickly erode the soils


water flowing through low areas can quickly erode the soils


terrain is very uneven


Planing or leveling the field can eliminate problems of uneven water distribution due to uneven terrain, but technical assistance should be used when precision land leveling is required.

Concept 5.

RUNOFF

Runoff is water that does not penetrate the soil and flows out of the irrigated field, or water which runs off of the high spots in a field and accumulates in the low areas.

On steep slopes runoff can erode fields and drains.

If irrigation water is scarce, runoff may be an unnecessary loss of precious water.

There are ways, discussed later, to reduce or eliminate these losses.


RUNOFF

Concept 6.

EROSION

Erosion due to irrigation occurs when water moves so fast over the soil surface that it begins to move soil particles. Slope, crop type, and water control together determine the amount of soil loss.

Slope is the steepness, grade, or inclination of a field.

Slope is a factor which determines what irrigation system to use and what methods or structures of water control are necessary for avoiding erosion.

Nearly flat terrain has very little slope.


EROSION

SLOPE: The slope of a field is measured in percent.

As slope increases, the danger of erosion increases.


3%: Terrain rises or falls 3 meters in 100 meters.


10%: Terrain rises or falls 10 meters in 100 meters.


20%: Terrain rises or falls 20 meters in 100 meters.


30%: Terrain rises or falls 30 meters in 100 meters.

Canals, Drains and Other Farm Installations

1. Canals and drains transport water.


Unlined


Rock masonry


Concrete

2. Flow division structures proportion the required water to the irrigated areas.


Example 1


Example 2

3. Check dams raise the water in the canal for distribution to other canals and to the field.


Wood, rock masonry, concrete


Canvas or plastic


Other canal

4. Headgates control the flow of water from the canals.


Example 1


Example 2


Example 3

Conservation and Maintenance of Canals, Drains, and Other Installation

Canals can be eroded very easily, especially on steep slopes. The following methods can help prevent canal erosion.

1. Construct your canals with very little slope - usually with a slope of one-half percent or less (50 cm/100 m or less).

2. You may consider using the following:

· rock masonry or concrete lining
· grasses of a suitable kind to hold the soil in place.

· on steeper slopes drop structures to bring the water down in steps. Be sure to protect the downstream side of the structure.

Do not build drop structures that allow water to fall more than 30 cm without technical help.


Rock masonry


Grass


Wood


Rock or concrete

All water control structures must be periodically checked and maintained. Follow these recommendations:


1. Don't let weeds and trash restrict the flow of water. Clean the canals and raise their banks when necessary.


2. Don't put more water in the canal than it can carry.


3. Don't let water seep through the canal banks. This usually occurs over fractured rock, sandy soil, or where animals have dug holes in the banks. Line the canals with concrete or clay in places where they would lose much water through seepage.


4. Don't cut ditchbanks just anyplace when water is needed. Select a few places and use the appropriate outlet structure.


5. Don't pasture animals in the canal or on its banks. They destroy the banks and protective vegetation.


6. Don't let breaks in the structures go unrepaired. Fix them before they become serious.

IRRIGATION TOOLS


SHOVEL


SIPHON


LEVEL


HOE


LONG-HANDLED PICK


PROBE


SOIL SAMPLER


CANVAS OR PLASTlC CHECKDAMS


RAKE

Because the construction and use of siphons, soil probes and soil samplers are not known to many farmers, these simple and effective irrigating tools are explained here.

SIPHONS


Siphons are curved tubes used to take water from a canal into a field.


WITH SIPHONS

· it is easy to control the amount of water going into furrows, borders and basins.

· water can be distributed fast, evenly, and without damaging the canal banks.

To use a siphon, the water must be higher in the canal than at the outlet of the siphon.

Siphon sizes for different uses:

1.9 cm - very short furrows
2.5 cm - longer furrows
3.8 cm - long furrows or very sandy soils
5 cm - very large, long furrows or small borders and basins
7.5 to 10 cm - small borders and basins
12.5 to 15 cm - large borders or basins


Use a siphon

How Are Siphons Used?


1. Completely submerge the siphon in the water, with the curve lower than the ends so that the air escapes...


2. Place your hand over the outlet side of the siphon.


3. Take the outlet side of the siphon out of the water, keeping it covered. Leaving the other end in the water, place the outlet end of the siphon tube in the bottom of the furrow or border.


4. Take hands off the siphon, and water will flow by itself

There are easier ways of priming siphons but they require experience


To get water to flow in 7.5 to 15 cm diameter siphon (3, 4 and 5 inches), use a piece of rubber inner tube fastened over the outlet of the siphon. When the tube is full (primed), it can be twisted with one hand so that the tube stays full while the siphon outlet is pulled out of the water.

You Can Make Your Own Siphon


1. Buy as much plastic conduit of the correct size as you need at a hardware store. Conduit for electrical installations works very well. Cut the conduit to the proper lengths. 1.2 meters is usually adequate for 1 to 1 ½ inch siphons; 1.5 meters for 2-inch siphons.


2. Observe the form of the farm ditch banks.


3. Make a mold with nails on a board, according to your observations.


4. Fill a tube with sand and compact it.


5. Heat parts of the tube over a fire or in a very hot water (at least 92°C) or in hot sand.


6. Press the heated tube against the nails on the mold to give it shape. Continue the procedure until the tube has the desired form. Then remove the sand; the siphon is ready for use.

Siphons larger than 5 cm (two inches) are harder to make. They are usually made of metal and can be bought in irrigation equipment stores.

SOIL SAMPLER AND PROBE

The tools described on this page are very useful for sampling a soil to determine its moisture condition.

An auger-type soil sampler can be made easily from two metal rods welded into a 'T' and a 1 ¼ or 1 ½ inch drill bit as indicated in the drawing.

Weld the drill bit to the 'T' handle.

The sampler can then be introduced into the soil to obtain a soil sample from the depth desired.


An auger-type soil sampler can be made easily from two metal rods


An auger-type soil sampler

To determine how deep the water penetrates during an irrigation, construct a simple tool:

Weld a smooth round bulb to a 'T' handle.


The depth at which the round-tipped probe becomes significantly harder to push into the soil is the depth that the water penetrated during irrigation. The probe may not work on soils which are rocky or which have hardpans. Use a shovel or auger-type soil sampler instead.

FARM WATER MANAGEMENT


What is a good irrigation?


What are crop water needs?


How do I know when to irrigate?


How do I keep my irrigations uniform?


How can I keep my runoff losses down?


Figure

WHAT IS A GOOD IRRIGATION?

A good irrigation is when:


1. water is applied when the crop needs it...not when the plants look like this...


2. the right amount of water is applied to the root zone.


3. the penetration is uniform or almost uniform.


4. Irrigation does not result in excessive runoff and wasted water.


5. The water does not erode the soils.

To irrigate well, the farmer must know:


1 The water needs of the crops. & 2 The approximate time that should elapse between irrigations; how to tell with certainty if it is time to irrigate crops.


3 When to cut off the water.


4. If the irrigations are uniform. If not, how to improve them.


5. How to keep runoff losses low.


6. How to identify erosion problems and eliminate them.


WHAT ARE CROP WATER NEEDS?

- Water requirements vary as the crop develops, but in general:

MOST TOLERANT to low moisture

Maintain high moisture levels from flowering through grain formation on cereal crops so that yield is not affected.

Most crops have periods when a lack of water results in great reductions in yield. Make adequate moisture available especially during these stages!!


Stage 1


Stage 2


Stage 3


Stage 4

HOW DO I KNOW WHEN TO IRRIGATE?

The frequency of irrigation depends on the amount of water used by the crop and the amount of water which can be removed from the soil by the crop without stress to the plant.

Consider the same crop and the same soil depth.


Sandy soils hold less water, so irrigations must be more frequent than on fine soils - but less water should be applied at each irrigation.

Consider the same soil and different crops:

Crops with shallow roots have less water within their reach, so the irrigations should be more frequent and lighter (less depth per irrigation) than deeper rooted crops


On sunny, hot days, crops use more water than on cool, cloudy days and irrigations must be more frequent.


Crops with shallow roots

Apply only light irrigations during germination; just enough to wet the seed bed. Heavy irrigations wash the nutrients down from the upper part of the soil where they are needed for initial development.

Fill the soil profile with moisture early in the season so that you will have a reservoir for your plants to draw water from when the supply becomes deficient.,

Use the following procedure to determine if it is time to irrigate.

1. Take some soil from the root zone area.

· if the crop has 12 inches of roots, check at least to six inches, and sometimes to 12.
· if the crop has 24 inches of roots, check to at least 12 inches, and sometimes to 24, etc.


2. Squeeze the soil in your hand.

3. Open your hand and observe the soil. Irrigate most crops when the soil gets to the following state:

Coarse or sandy soil:

The soil falls apart (crumbles) when the hand is opened. (A)

Medium textured soil:

The soil retains the form of the compressed hand when opened, but crumbles when pressed by the finger. (B)

Fine textured soils:

The soil retains its form when the hand is opened. The ball will not crumble when pressed by the finger. However a thin cylinder cannot be formed with the soil. (C)


Crops which are not tolerant to stress should be Irrigated BEFORE they get to the conditions indicated above.

You can determine if the irrigation has removed the moisture deficit from the root zone


1. Dig down to the root depth:

1 day after irrigating coarse-textured soils
2 days after irrigating medium-textured soils
3 days after irrigating fine-textured soils


2. Check the moisture depth.

A change in moisture will usually be obvious if the water has not penetrated enough

WHEN CAN THE WATER BE CUT OFF?


In furrows:

Cut the water off when the round-tipped probe (see tool section) penetrates easily ½ to ¾ of the root depth along the whole run.

Water will penetrate the soil for a significant time after an irrigation. This is why it does not need to penetrate to root depth during the irrigation.


In borders or basins:

The round-tipped probe should penetrate from ½ to ¾ of the root depth, after the water disappears from the surface.

Adjust the irrigation duration, stream size, and how far it must travel along the surface before being cut off.

In border irrigation, the stream usually proceeds 2/3 to ¾ of the border length before being cut off.

HOW DO I MAKE SURE THAT MY IRRIGATIONS ARE UNIFORM?

First look for any of the following indicators of non-uniformity.


1. Variations in plant height through the field.


2. Plants wilting in some parts of the field and not in others.


3. Large differences in texture and depth of soil along the field


4. Large variations in the time that water remains on different parts of the field. (For example, if the water advances slowly over the field and disappears at almost the same time over the whole field, the water may have penetrated more at the head than at the tail end of the field.)


5. Fields with uneven surfaces...water ponds in low spots and high spots remain dry.


6. Sprinklers spaced so far apart that the water from one does not overlap with that of another, or does so very little.

There are usually solutions to the problem of non-uniformity.


1. An uneven soil surface can be leveled or planed


2. Soils of different textures or structures and depths may be irrigated separately.


3. If the difference in time that water stands on the surface at the head and tail end of the system is great, we may be able to:

a. cut down the length or width of furrows, borders, or basins
b. increase the size of stream at the inlet of the furrow, border, or basin.
c. increase the irrigation duration.


4. If the water did not advance evenly a all the furrows, your irrigations tools. Use siphons and adjust them so that the necessary amount goes into each furrow.


5. If some (...) are more compact then other because of tillage practices change so that each furrow gets the same amount of tillage traffic.


6. When water in a border tends to flow along one side, there is too much cross slope. Change the direction of the borders to that of the primary slope, or level or plane each border to eliminate cross slope.


7 Decrease sprinkler spacing and/or increase pressure.

To determine definitely if the irrigation was uniform or not:


1. Introduce the long, round-tipped probe into the soil along the irrigation run after an irrigation and note at what depth it meets significant change in resistance. This is the depth the water penetrated to. Check the depth of penetration in different parts of the field and compare.


2. On rocky soils this probe will not work. Excavate with a shovel, soil auger or other instrument to observe the depth of penetration.

HOW CAN I KEEP MY RUNOFF LOSSES DOWN?


1. Decrease the size of the stream into the furrows, borders, and basins.


2. Decrease the duration of each irrigation. Give lighter, more frequent irrigations. & 3. Reduce the inflow into the furrows when the water reaches the end.


4. Gather the runoff water in reservoirs and pump it back to the head or take it to other fields.


5. With borders, build small dikes at the end (outlet) to hold some of the water back.


6. Increase sprinkler pressure to break up drops so the water penetrates more easily.

HOW CAN I STOP EROSION?

First of all observe:


1. Does water enter the field, furrow, border or basin clear and come out looking muddy?


2 Do the shape of the furrows change? Are there galleys in the fields, furrows, borders or basins?


3. Is soil being deposited at the end of the furrow, border, basin, drain or outlet of the field?


ALL OF THESE ARE SIGNS OF EROSION.

To control erosion, consider one or more of the following:


1. Reduce stream size into the furrow border or basin You may need to cut run lengths when you cut stream size to maintain uniform irrigation.


2. Make wider borders or furrows so that the stream will not be as concentrated


3. Plant crops which will hold the soil in place--permanent pastures etc.


4. Change furrow slopes.


5 Construct terraces.


6. Level or plane the soils.


Especially on steep slopes, always guard against the dangers of erosion.

1 Plant crops which will fix the soil in place-permanent pastures, shrubs, trees, etc.

2. Don't pasture the hillsides constantly, as this destroys the vegetation that protects your sod. Rotate your livestock from one pasture to another.

3. If gullies start to form, eliminate pasturing totally. Plant protective shrubs and trees. You may have to place erosion barriers in the gullies.

4. If you must plant the hillsides, leave strips of permanent grasses and vegetation between cultivated plots.

5. Don't plant to the edge of steep banks. Leave strips of permanent vegetation.

6. Make sure excess water from irrigation and rain is taken away to adequately protected canals and drains.

7. Do not destroy existing protective vegetation.

Take care of your soils and insure food for your children!

We repeat these recommendations for insuring good irrigation management


Irrigate frequently enough so that you do not hurt your crop.


Make sure your irrigations are uniform.


Add enough water to your soil moisture reservoir, but don't over-irrigate.


Don't erode your soils.

To obtain good irrigations you can control many factors:


FREQUENCY & DURATION


TOOLS AND STRUCTURES


STREAM SIZE


SLOPE


RELIEF-SOIL SURFACE UNEVENESS


CROP


IRRIGATION SYSTEM

But remember, when you change one aspect of the operation to eliminate a problem, you may cause other problems. So be careful and make sure everything contributes to "good" irrigation.


<<TOC2>> Chapter 7 - Waterlogging and salinity

References

Primary:

(1), (53)

7.1 Basic concepts in waterlogging and salinity

Excess water in the plant root zone restricts the aeration required for optimum plant growth. It also may affect the availability of several nutrients by changing the environment around the roots.

Excess salts in the root zone inhibit water uptake by plants, affect nutrient uptake, and may result in toxicities due to individual salts in the soil solution. Excess exchangeable sodium in the soil may destroy the soil structure to a point where water penetration and aeration of the roots become impossible. Sodium is also toxic to many plants.

Waterlogging and salinity in the soil profile are most often the result of high water tables resulting from inadequate drainage or poor quality irrigation water. Adequate surface drainage allows excess irrigation and rainwater to be evacuated before excess soil saturation occurs or before the water is added to the water table. Adequate subsurface drainage insures that water tables are maintained at a sufficient depth below the soil surface to prevent waterlogging and salt accumulation in the root zone. Salinization of the soil profile is prevented because upward capillary movement of water and salts from the water table does not reach the root zone. Adequate subsurface drainage also allows salts to be removed from the soil profile through the application of excess irrigation water (leaching).

To understand how we may prevent, eliminate, or otherwise deal with a waterlogging or salinity problem, we must first understand how crops and soils respond to excess water and salts.

7.1.1 Waterlogging and high ground water tables

The growth of most crops is affected when ground water is shallow enough to maintain the soil profile in the root zone wetter than field capacity. This excess water and the resulting continuously wet root zone can lead to some serious and fatal diseases of the root and stem. Working the soil when overly wet can destroy soil structure and thus restrict root growth and drainage further. The chemistry and microbiology of waterlogged soils is changed due to the absence of oxygen. These changes can affect the availability of many nutrients. For example, nitrogen can undergo denitrification more readily and be lost to the atmosphere as a gas. The anaerobic (reducing) environment results in changes to metals and other cations that can result in deficiencies or toxicities. For example, sulfide, and ferrous and manganese ions will accumulate in waterlogged soils.

Crops vary in their tolerances to waterlogging and high water tables. Some crops, such as rice, are adapted to these conditions and can thrive. Table 7.1 presents the different tolerances of some crops.

TABLE 7.1 Tolerance Levels of Crops to High Ground water Tables and Waterlogging (Ref. 12)

GROUND WATER AT 50 CM

WATERLOGGING

HIGH TOLERANCE

sugarcane, potatoes, broad beans

rice, willow, plum, strawberries, some grasses

MEDIUM TOLERANCE

sugarbeet, wheat, oats, barley, peas, cotton

citrus, bananas, apple, pears, blackberries, onion

SENSITIVE

maize, tobacco,

peaches, cherries, olives, peas, beans, date palm

The capillary fringe is a saturated zone that extends some distance above the water table. Water moves into this zone by capillary movement. The roots on many crops do not generally penetrate closer than 30 cm above the water table. The capillary fringe is thinner in sandy soils than in loam or clay soils. Thus the following depths to ground water are suggested as a minimum for most crops:

Sandy Soils - - - - - - - - - -

Rooting Depth + 20 cm

Clay Soils - - - - - - - - - -

Rooting Depth + 40 cm

Loam Soils - - - - - - - - - -

Rooting Depth + 80 cm

7.1.2 Soil and water salinity

Crop yields decrease linearly with increasing salt levels above a given threshold level. This threshold level will vary according to the tolerance of the crop. Yield decreases in the presence of toxic salts such as boron are mainly due to the difficulties the crop has in taking up water when concentrations of salt in the soil solution are high. Often crops have a droughty or dry appearance in high salt soils.

Table 7.2 presents the tolerance of different crops to soil and water salinity levels and the effect that increasing salinity levels has on yield. In this table, the ECe (Electrical Conductivity of the Saturated Paste Extract) is a measure of soil salinity, and ECw (Electrical Conductivity of the Irrigation Water) a measure of water salinity. The Max ECe is the highest ECe that the plant can tolerate. "Yield Potential" is the percent of an optimum yield that can be attained under given growing conditions. Table 7.2 is used as follows: A farmer can produce 50 kg per hectare of corn on good soil. The farmer has a field with an ECe of 3.8, which gives him or her many problems. Using the table, an estimate can be made of an expected yield of roughly 37 kg per hectare (i.e. a 75% Yield Potential) for this field.

TABLE 7.2 Crop Salt Tolerance Levels for Different Crops as Influenced by Irrigation Water or Soil Salinity (Ref. 12)

FIELD

YIELD POTENTIAL

CROPS

100%

90%

75%

50%

0%


ECe

ECw

ECe

ECw

ECe

ECw

ECe

ECw

ECe

ECw

Barley

8.0

5.3

10

6.7

13

8.7

18

12

28

19

Cotton

7.7

5.1

9.6

6.4

13

8.4

17

12

27

18

Sugarbeet

7.0

4.7

8.7

5.8

11

7.5

15

10

24

16

Sorghum

6.8

4.5

7.4

5.0

8.4

5.6

9.9

6.7

13

8.7

Wheat

6.0

4.0

7.4

4.9

9.5

6.3

13

8.7

20

13

Wheat, Durum

5.7

3.8

7.6

5.0

10

6.9

15

10

24

16

Soybean

5.0

3.3

5.5

3.7

6.3

4.2

7.5

5.0

10

6.7

Cowpea

4.9

3.3

5.7

3.8

7.0

4.7

9.1

6.0

13

8.8

Peanut

3.2

2.1

3.5

2.4

4.1

2.7

4.9

3.3

6.6

4.4

Paddy Rice

3.0

2.0

3.8

2.6

5.1

3.4

7.2

4.8

11

7.6

Sugarcane

1.7

1.1

3.4

2.3

5.9

4.0

10

6.8

19

12

Corn (Maize)

1.7

1.1

3.4

2.3

5.9

4.0

10

6.8

19

12

Flax

1.7

1.1

3.4

2.3

5.9

4.0

10

6.8

19

12

Broadbean

1.5

1.1

2.6

1.8

4.2

2.0

6.8

4.5

12

8.0

Bean

1.0

0.7

1.5

1.0

2.3

1.5

3.6

2.4

6.3

4.2

VEGETABLE CROPS

Zucchini

4.7

3.1

5.8

3.8

7.4

4.9

10

6.7

15

10

Beet, Red

4.0

2.7

5.1

3.4

6.8

4.5

9.6

6.4

15

10

Squash

3.2

2.1

3.8

2.6

4.8

3.2

6.3

4.2

9.4

6.3

Broccoli

2.8

1.9

3.9

2.6

5.5

3.7

8.2

5.5

14

9.1

Tomato

2.5

1.7

3.5

2.3

5.0

3.4

7.6

5.0

13

8.4

Cucumber

2.5

1.7

3.3

2.2

4.4

2.9

6.3

4.2

10

6.8

Spinach

2.0

1.3

3.3

2.2

5.3

3.5

8.6

5.7

15

10

Celery

1.8

1.2

3.4

2.3

5.8

3.9

9.9

6.6

18

12

Cabbage

1.8

1.2

2.8

1.9

4.4

2.9

7.0

4.6

12

8.1

Potato

1.7

1.1

2.5

1.7

3.8

2.5

5.9

3.9

10

6.7

Sweet Potato

1.5

1.0

2.4

1.6

3.8

2.5

6.0

4.0

11

7.1

Pepper

1.5

1.0

2.2

1.5

3.3

2.2

5.1

3.4

8.6

5.8

Lettuce

1.3

0.9

2.1

1.4

3.2

2.1

5.1

3.4

9.0

6.0

Radish

1.2

0.8

2.0

1.3

3.1

2.1

5.0

3.4

8.9

5.9

Onion

1.2

0.8

1.8

1.2

2.8

1.8

4.3

2.9

7.4

5.0

Carrot

1.0

0.7

1.7

1.1

2.8

1.9

4.6

3.0

8.1

5.4

Turnip

0.9

0.6

2.0

1.3

3.7

2.5

6.5

4.3

12

8.0

FORAGE CROPS

Ryegrass, per.

5.6

3.7

6.9

4.6

8.9

5.9

12

8.1

19

13

Vetch, Common

3.0

2.0

3.9

2.6

5.3

3.5

7.6

5.0

12

8.1

Sudan Grass

2.8

1.9

5.1

3.4

8.6

5.7

14

9.6

26

17

Forage Cowpea

2.5

1.7

3.4

2.3

4.8

3.2

7.1

4.8

12

7.8

Alfalfa

2.0

1.3

3.4

2.2

5.4

3.6

8.8

5.9

16

10

Clover, Berseem

1.5

1.0

3.2

2.2

5.9

3.9

10

6.8

19

13

Other Clover

1.5

1.0

2.3

1.6

3.6

2.4

5.7

3.8

9.8

6.6

Date Palm

4.0

2.7

6.8

4.5

11

7.3

18

12

32

21

Grapefruit

1.8

1.2

2.4

1.6

3.4

2.2

4.9

3.3

8.0

5.4

Orange

1.7

1.1

2.3

1.6

3.3

2.2

4.8

3.2

8.0

5.3

Peach

1.7

1.1

2.2

1.5

2.9

1.9

4.1

2.7

6.5

4.3

Apricot

1.6

1.1

2.0

1.3

2.6

1.8

3.7

2.5

5.8

3.8

Grape

1.5

1.0

2.5

1.7

4.1

2.7

6.7

4.5

12

7.9

Almond

1.5

1.0

2.0

1.4

2.8

1.9

4.1

2.8

6.8

4.5

Plum, Prune

1.5

1.0

2.1

1.4

2.9

1.9

4.3

2.9

7.1

4.7

Blackberry

1.5

1.0

2.0

1.3

2.6

1.8

3.8

2.5

6.0

4.0

Strawberry

1.0

0.7

1.3

0.9

1.8

1.2

2.5

1.7

4.0

2.7

Table 7.2 includes general information about relative tolerances to salt, but varietal differences are also very important. Much effort has been put into developing salt tolerant varieties of many crops because of the worldwide salinity problem. In some cases, minor problems can be alleviated by selecting the correct variety.

Electrical Conductivity (EC) is the reciprocal of resistance (1/ohms) and is measured in mmhos/cm or in dS/m (dS/m = mmhos/cm). EC is measured with a salinity or conductivity meter, which is a standard piece of equipment in all soil labs and can often be purchased at a reasonable price for field use. ECw (salinity of water) is measured by simply inserting the conductivity meter in the irrigation water and adjusting for temperature. Measuring EC (soil salinity) is a little more complicated, requiring a saturated paste of the soil from which the water is then extracted and the salts measured.

Exchangeable sodium in the soil becomes a problem when the predominant salts in irrigation water or in the soil solution are sodium salts. Soil constituents that determine soil structure, such as clays and organic matter (soil colloids), have negative charges (exchange sites) on their outer surface that loosely attach to positive ions and molecules (cations), such as calcium (Ca++), ammonium (NH4+), and sodium (Na+) (see Figure 7.1). These cations can readily be replaced by other cations (they are exchangeable). If there is excessive sodium in the soil solution, it will occupy most of the exchange sites. Sodium is a small cation, so when present in large quantities on the exchange sites, it destroys the separation between soil particles. Then the clay or organic matter collapses on itself, leaving no air spaces or pores (deflocculation) (See Figure 7.1). In some cases, the structureless organic matter is dispersed and can be lost in the drainage water, hence the old-fashioned term for these soils is Black alkali soils.


Figure 7.1 "Normal" and "Sodium" Affected Clay Particles

Sodium is measured as the Exchangeable Sodium Percent (ESP) or as the Sodium Absorption Ratio (SAR). The ESP is simply the percent of all the exchange sites in the soil occupied by sodium. The SAR is more complicated and is merely an index of the extent of the problem.

Very high sodium levels not only affect soil structure but are toxic to many crops.

7.1.3 Classification of salt affected soils

Saline Soils

These soils contain sufficient amounts of soluble salts to interfere with germination, growth, and yield of most crop plants. They do not contain enough exchangeable sodium to alter soil characteristics. Technically, a saline soil is defined as a soil with an ECe greater than or equal to 4 mmhos/cm and an Exchangeable Sodium Percent (ESP) less than 15. The soil pH is usually less than 8.5. These soils may have a white crust or white salt crystal accumulation on the surface (salt blooms) so they are sometimes called "white alkali soils." Excess soluble salts can be removed by leaching if drainage permits, as will be discussed.

Saline-Sodic Soils

These soils contain soluble salts and exchangeable sodium in sufficient quantities to interfere with the growth of most crops. Technically, a saline-sodic soil is defined as a soil having an ESP greater than 15 and an ECe greater than or equal to 4 mmhos/cm. The soil colloids (charged particles) are collapsed (deflocculated), and drainage and aeration are very poor, pH is usually in the range of 8-10.

Sodic Soils

These soils contain sufficient exchangeable sodium to interfere with the growth of most crops but do not contain appreciable quantities of soluble salts. Technically, they are soils with an ESP greater than 15 and an ECe of less than 4 mmhos/cm. Drainage and aeration are very poor because soil colloids are very dispersed. The pH is generally above 8.5. These soils are sometimes called ''black alkali soils." High pH values generally can be used as a indicator of possible sodium problems, but this is not always true.

7.1.4 Evaluating waterlogging and salinity problems

The evaluation of the extent of waterlogging and salinity problems can usually be conducted through simple observation, communication, and possibly some soil analysis. The following steps can be followed:

1) Interview local agronomists, agricultural technicians, and agribusiness personnel. Ask them questions about water table depths, and salinity problems. If such problems exist, how are local farmers taking care of them?

2) Conduct a field reconnaissance to find out if the problem exists in your area. Wells, gravel pits, and deep channels that show the depth to ground water should be observed. If there are few of these, then install pits or auger small observation wells into the soil to depths of 30 to 80 cm below the expected rooting depths (30 cm for sandy soils, 80 cm for loams and fine-textured soils). If soil horizons are reached that are grey, wet, and possibly contain black or red mottles, you have hit "gleyed" or waterlogged horizons. You can assume at this point that soils are poorly drained at this level.

As part of the reconnaissance, observe fields for signs of excess water or salinity, such as:

a) White crusts on the soil surface. There may be a problem even when these are not present.

b) Plants that are stunted, appear droughty, or irregular even though the soil is fairly moist. In cases of high salinity, the leaves may be curled up and yellow. The margins of the leaves may burn, a reddish color is often seen and in some cases the plant may actually die during or shortly after germination and emergence.

c) Use of drainage water, tailwater, or water that has been used extensively for washing, irrigation, or industrial purposes before reaching the field. This problem may exist when the farmer is a tail end user on a major irrigation system. The water can accumulate salts.

d) Soils with poor structure, which appear sticky and plastic when wet and which do not grow a crop. Hard, structureless soil pans can develop at different depths in sodic soils.

e) Standing water or wet spots in parts of the field where crops grow poorly. Standing water in spots after a prolonged drying period are also useful indicators.

f) Soil that is dry and smooth or has slicked over areas without vegetation, sometimes with a thin peeled up skin, can indicate infiltration and sodic soil problems.

g) Absence of field drains for removing excess water.

h) Condition of field drains: Are surface drains full of vegetation or otherwise plugged? Are surface and subsurface drains operating properly?

i) If the opportunity presents itself, take soil samples and have them analyzed if you suspect a salinity problem or look at past samples if any are available.


<<TOC3>> 7.2 Control of waterlogging and salinity problems

7.2.1 Surface and subsurface drains

The first requisite in the prevention or elimination of waterlogging and salinity problems is an adequate drainage system. Very often the natural drainage in an area, along with good water management, is sufficient to eliminate excess water and preclude the need for expensive subsurface drainage systems. Almost every farmer who applies water by surface irrigation, however, or who deals with significant rainfall, should have adequate surface drainage facilities to remove excess water. This will allow the farmer to avoid waterlogging and possible salinity problems at the tail end of borders, furrows, or basins after irrigation or intense rainstorms. Drainage facilities will also allow the prevention of erosion associated with natural movement of excess water over the soil surface.

Surface drains are open channels that collect water as it runs off or into irrigated fields. These drains convey water to a stream or channel where it can be carried safely. The design procedures for these drains are the same as for any open channel (see Chapter 5). The main requirement is that they are able to convey the maximum expected flow rate without erosion. At the tail end of irrigated fields, these drains are often broad and shallow to allow farm machinery to operate efficiently.

Subsurface drainage may be accomplished either through the construction of open trenches or through buried clay or concrete tiles or perforated pipe. Subsurface drainage systems can be classified as Natural, Herringbone, Gridiron or Interceptor (Cutoff) types.

Natural systems are used in fields where there are small and isolated wet areas. The buried drain lines follow natural draws or depressions.

Herringbone systems are useful in situations where the land slopes toward a draw on either side. The main line follows the draw, and the laterals empty into this from both sides.

Gridiron systems are similar to the Herringbone except that they enter the main drain from only one side.

Interceptor drains are installed across a slope to intercept the passage from higher ground. These drains can prevent the waterlogging of soils below irrigation ditches, springs, or at the foot of a hill. They can be useful in collecting water for recycling into the irrigation system.

The design, drain size, spacing, and depth of these drainage systems are a function of the water table depth desired, the soil permeability (hydraulic conductivity), amount of water to be drained, and economics of construction. Generally, the deeper the drains are installed, the wider the spacing between drains can be. In humid regions, drain spacings of 10 to 50 meters (30 to 150 feet) are common. The closer spacing is used in heavier soils with higher value crops and greater rainfall. In more arid irrigated areas, spacings of 50 to 200 meters (150 to 600 feet) are common.

Tile drain is common in 10, 13, and 15 cm (4, 5, and 6 inch) sizes but can be obtained in greater sizes as can corrugated drainage pipe. Minimum grades are sometimes based on a minimum velocity of 0.45 m/s (1.5 feet per second) at full flow. Surface inlets, outlets and cleanouts, envelope filters, and other structures must be properly designed if the drain system is to operate correctly.

The design of subsurface drains is generally more complex than for surface drains and requires significant knowledge of ground water hydrology. Thus the reader should seek the assistance of a drainage engineer before undertaking the design of expensive subsurface drains. The one possible exception is the Interceptor drain, which can be installed as an open channel below the level of an irrigation canal to provide drainage to land that would otherwise be waterlogged by the canal.

7.2.2 Reclamation of salt affected soils

The chemical and physical analysis of soils provides a basis for the diagnosis, treatment, and management of salt affected soils. After diagnosing the problem but before actual reclamation, two steps must be observed.

1. Establish adequate drainage in the area. The water table should be lowered if it is high, and water should be at least 3 to 4 meters below the surface.

2. The land should be level or contour farmed so that the surface of the soil will be covered uniformly by water.

Saline Soil

If the soil is only saline, it can be reclaimed simply by leaching the excess salts below the root zone. The quantity of water required depends on the texture of the soils, the concentration of salts in the soil and in the leaching water (the higher, the more water needed), and the amount of salts to be leached. On the average, 0.5 to 1.25 meters of water are required.

Saline Sodic Soil and Sodic soil

If leaching is conducted on a saline-sodic soil, the soil will become sodic and could present more problems than it would have originally. Saline-sodic soils require the leaching process to be accompanied by the application of amendments. The amendments that are used are the same ones that would be utilized on a sodic soil. Sodic soils are generally very poor in infiltration, so amendments are slow to enter soil. For this reason, both compacted saline-sodic soils and sodic soils should undergo deep cultivation such as deep ripping to break up hardpans that prevent infiltration.

7.2.3 Correcting sodium problems with amendments

The presence of lime (free calcium carbonate) in soil allows for the widest choice of amendments. To test for this, a spoonful or clod of soil is treated with a few drops of sulfuric acid or hydrochloric acid. If bubbling or fizzing occurs where the acid drops fall, then lime is present. The greater the fizzing, the more lime is present. If the soil contains lime, any of the amendments listed in Table 7.3 can be used. If no lime is present, then only amendments containing soluble calcium are recommended.

TABLE 7.3 Commonly Used Amendment Materials and Their Equivalent Amendment Values (Ref. 1)



Tons of Amendment Material Equivalent to:

(100% Basis)

Chemical Formula

1 Ton of Pure Gypsum

1 Ton of Soil Sulfur

Gypsum

CaSO4.2H2O

1.0

5.38

Soil Sulfur

S

0.19

1.00

Sulfuric Acid

H2SO4

0.61

3.2

Ferric Sulfate

Fe2(SO4)3.9H2O

1.09

5.85

Lime Sulfur

9% Ca + 24% S

0.78

4.17

Calcium Chloride

CaCl2.2H2O

0.86

--

Calcium Nitrate

Ca(NO3)2.2H2O

1.065

--

Aluminum Sulfate

Al2(SO4)3

--

6.34

The percent purity is generally given on the bag.

Types of Amendments

Calcium containing amendments such as gypsum react in the soil as follows:

GYPSUM + SODIUM-SOIL

CALCIUM SOIL + SODIUM SULFATE

Leaching is then undertaken to wash out the sodium sulfate. Repeated applications are necessary in many cases. The amount of gypsum used is substantial, often 1.5 or more tons of material per hectare, because gypsum is not highly water soluble, and, in many cases, the reaction described above takes a long period of time. A more precise measurement of the "gypsum requirement" is available from most soil labs, assuming a material of 100% purity.

Acids such as sulfuric acid undergo a two step process:

1. SULFURIC ACID + SOIL LIME

GYPSUM + CO2 + WATER

2. GYPSUM + SODIUM-SOIL

CALCIUM SOIL + SODIUM SULFATE

Acids are dangerous and corrosive, so handling them can be a problem. The volume applied has to be controlled because of excessive frothing. Occasionally, cheap industrial sources are available but must be used with caution because of the potential for heavy metal contamination. An analysis of spent acids is recommended. Acids are much faster than other reclamation procedures because the reaction is instantaneous.

Acid forming materials such as sulfur are much slower because they undergo a three step process, the first step requiring microbial intervention in the oxidation reaction:

1. SULFUR + OXYGEN + WATER

SULFURIC ACID

2. SULFURIC ACID + SOIL LIME

GYPSUM + CO2 + WATER

3. GYPSUM + SODIUM-SOIL

CALCIUM SOIL + SODIUM SULFATE

These steps can take years.

Effectiveness and Amount of Amendments

In the absence of a soil analysis for gypsum requirement, a rule of thumb is that something is better than nothing. Gypsum is usually used in large quantities, so 0.5 to 2 metric ton applications per hectare are not unusual. To convert the gypsum requirement to an amount of some other amendment, Table 7.3 offers a simple guideline. Simply multiply the gypsum ton equivalent by the gypsum requirement.

If the material being considered is not 100% pure, a simple calculation will indicate the amount needed to be equivalent to 1 metric ton of pure material:

For example: If gypsum is 60 percent pure, the calculation would be 100/60 = 1.67 m tons. In other words, 1.67 tons of 60 percent pure gypsum is equivalent to 1 m ton of 100% material.

Sulfur presents an additional challenge since not only purity but the fineness of the granules must be accounted for. The finer the material, the faster microbial oxidation will occur. Coarse grade materials are highly insoluble and may take years to be active.

7.2.4 Management of saline and sodic soils

Often, it is too expensive or impractical to reclaim saline or sodic soils or even maintain them at low salinity levels. It may be impossible to adequately drain an area, amendments may not be available or may be too expensive, or the water used for irrigation may be of poor quality.

In these situations, there are various management practices that will aid in controlling or reducing the impact of salts or sodium:

1. Selection of crops or crop varieties that have higher tolerances for salt or sodium (see Table 7.2).

2. Use of special planting procedures that will minimize salt accumulation around the seed (see Figure 7.2).

3. Use of the appropriate irrigation method for the root characteristics of the crop (see Figure 7.3).

4. Use of sloping beds and other special land preparation procedures and tillage methods to provide a low salt environment (see Figure 7.4).

5. Use of irrigation water to maintain a high water content to dilute the salts or leach them for germination or from the root zone.

6. Use of physical amendments such as manure and compost for improving soil structure and filth. Conservation tillage to incorporate crop residues will help create drainage.

7. Deep ripping of soil to break up sodic and other hardpans or other impervious layers to provide internal drainage.

8. Use of chemical amendments as described.

9. Good, sound farming practices and careful fertilizer management.

Figure 7.2 Salt Accumulation Patterns (Ref. 1)


Sprinkling or surface flooding


Border check


Furrow irrigation


Localized irrigation (drip or trickle)

Figure 7.3 Bed Shapes and Salinity Effects (Ref. 1)


Sloping seedbeds


Sloping seedbeds used for salinity and temperature control


SOIL SALINITY AT PLANTING TIME (dS/m)

- The pattern of salt build-up depends on bed shape and irrigation method. Seeds sprout only when they are placed so as to ovoid excessive salt build-up around them.

Figure 7.4 Bed Shapes and Salinity Effects (Ref. 1)


Salinity control with sloping beds


Flat top beds and irrigation practice

<<TOC3>> 7.3 Irrigation water quality

An understanding of the quality of irrigation water is essential in any salinity or sodium control program. Often, poor quality water is the source of the salinity or sodium problem. Table 7.4 presents some quality guidelines for evaluating the riskiness of the water. If water is of poor quality, tactics such as dilution with other water sources or applications of larger leaching amounts can be implemented.

TABLE 7.4 Effect of Irrigation Water Quality on Soil Salinity, and Permeability, Toxicity (Ref. 12)


Impact of Irrigation on Water Quality


None

Moderate

Severe

Salinity

<0.75

0.75- 3.0

>3.0

ECw (mmhos/cm)

Permeability

ECw (mmhos/cm)

>0.50

0.50- 0.2

<0.2

adj. SAR:





Montmorillonite1

<6.00

6.00- 9.0

>9.0


Illite2

<8.00

8.00-16.0

>16.0


Kaolinite3

<16.00

16.00-24.0

<24.0

Toxicity (most tree crops)

Sodium (adj. SAR)4

<3.00

3.00- 9.0

>9.0

Chloride (meq/l)5

<4.00

4.00-10.0

>10.0

Boron (mg/l)

<0.75

0.75- 2.0

>2.0

Miscellaneous

Nitrogen (mg/l)6

<5.00

5.00-30.0

>30.0

Bicarbonate (HCO3)

<1.50

1.50- 8.5

>8.5

pH

Normal Range 6.5 - 8.4

1 Temperate clay soils, highly expandable, not suited for ceramics or clay tiles.
2 Temperate clay soils or tropical soils in low rainfall or wet/dry climates. Not highly expandable. Can be used for ceramics.
3 Tropical clay soils in high rainfall areas. Usually have a distinct red or yellow color.
4 For most field crops (Ref. 1).
5 Sprinkler irrigation may cause leaf burn when >3 meq/l.
6 Excess nitrogen causes excessive vegetative growth, lodging, and delayed crop maturity.

Salinity problems can occur due to saline water being used in irrigation. Decreased soil infiltration rates can be the result of irrigation water that is low in salts but high in sodium or water that has a high sodium to calcium ratio. If infiltration problems are due to high sodium water, the effect will be noticed in the surface few centimeters of the soil.

Other water quality problems to watch for include:

1. Water high in iron, bicarbonate, or gypsum, which can result in unsightly deposits on cash crops.

2. Highly acid (low pH) or corrosive water that can result in severe corrosion of irrigation hardware such as pipelines and wells.

3. Other pH abnormalities (high or low) that can result in encrustation or other effects on crops.

4. Risks from diseases such as Bilharzia (schistosomiasis), malaria, and lymphatic filariasis, or other diseases borne by vectors such as mosquitoes. Vector breeding can often originate in situations where there is low water infiltration rates, use of wastewater for irrigation, or poor drainage.

5. Sediments that can clog irrigation structures, build films on leafy cash crops, making them unacceptable for marketing, and seal soils by depositing structureless silt on soil surfaces.


<<TOC2>> Appendix A - Math skills and tool use

A.1 Conversion factors

Abbreviations

ac

=

acre

C

=

Celsius

cal

=

calories

cfs

=

cubic feet per second

cm

=

centimeter

cusec

=

1 ft3/sec

F

=

Fahrenheit

ft

=

foot

gal

=

gallon

gpm

=

gallons per minute

ha

=

hectare

Hg

=

Mercury

hp

=

horsepower

hr

=

hour

in

=

inch

kg

=

kilograms

km

=

kilometer

kw

=

kilowatt

L

=

liter

lb

=

pound

m

=

meter

mi

=

mile

mm

=

millimeters

psi

=

pounds per square inch

sec

=

second

Length

1 inch

=

2.54 cm





1 foot

=

30.48 cm

=

0.3048 m



1 meter

=

39.37 in

=

3.281 ft



1 mile

=

5280 ft

=

1609.3 m

=

1.61 km

1 kilometer

=

1000 m

=

0.62137 mi



Area

1 inch2

=

6.452 cm2





1 foot

=

0.093 m2





1 acre

=

4047 m2

=

0.4047 ha

=

43560 ft2

1 hectare

=

10000 m2

=

2.471 ac



1 meter2

=

10.76 ft2





Volume

1 US gallon

=

231 in3

=

0.1337 ft

=

3.785 L

1 foot

=

7.48 gal

=

28.32 L

=

0.0283 m3

1 acre-foot

=

43560 ft3

=

1233.5 m3



1 meter3

=

1000 L

=

264.2 gal

=

35.31 ft

1 liter

=

1000 cm3





Weight

1 kilogram

=

2.2 lbs

=

35.27 ounces

1 pound

=

0.453 kg

=

453.59 g

1 ounce

=

28.35 g



1 ton

=

907.18 kg

=

2000 lbs

1 metric ton

=

1000 kg

=

2204.6 lbs

Flow Rate

1 cfs

=

1 ft3/sec

=

1 cusec



1 gpm

=

0. 00223 cfs

=

0. 0631 L/sec

=

0.227 m3/hr

1 cfs

=

448.8 gpm

=

28.32 L/sec

=

101.9 m3/hr

1 m3/hr

=

0.2778 L/sec

=

0.00981 cfs

=

4.403 gpm

1 L/sec

=

15.85 gpm

=

0.0353 cfs



Pressure (density of water at 39.2 F. approximately 4° C)

1 psi

=

2.31 ft water

=

2.04 in Hg

=

0.0703 kg/cm2

1 ft water

=

0.433 psi

=

0.883 in Hg

=

0. 0304 kg/cm2

1 kg/cm2

=

10 m water

=

28.97 in Hg

=

736 mm Hg

1 kg/cm2

=

14.22 psi

=

32.8 ft water



1 m water

=

3.28 ft water

=

0.1 kg/cm2



Temperature

Degrees F

=

(Degrees C 1. 8) + 32

Degrees C

=

Properties of Water

Temperature

Vapor Pressure


°C

°F

mm Hg

psi

ft water

Specific Gravity

4

39

6.101

0.118

0.272

1.000

10

50

9.209

0.178

0.411

0.999

15

59

12.788

0.2468

0.570

0.999

20

68

17.535

0.338

0.782

0.998

Absolute Pressure at Various Altitudes

Altitude

Atmospheric Pressure

Feet

Meters

psi

Feet of Water

Meters of Water

0

0.0

14.7

33.9

10.33

500

152.4

14.4

33.3

10.14

1000

304.8

14.2

32.8

9.99

1500

457.2

13.9

32.1

9.78

2000

609.6

13.7

31.5

9.60

2500

762.0

13.4

31.0

9.44

3000

914.4

13.2

30.4

9.26

3500

1066.8

12.9

29.8

9.08

4000

1219.2

12.7

29.2

8.90

4500

1371.6

12.4

28.8

8.77

5000

1524.0

12.2

28.2

8.59

5500

1676.4

12.0

27.6

8.41

6000

1828.8

11.8

27.2

8.29

6500

1981.2

11.5

26.7

8.13

7000

2133.6

11.3

26.2

7.98

7500

2286.0

11.1

25.7

7.83

8000

2438.4

10.9

25.2

7.68

8500

2590.8

10.7

24.7

7.52

9000

2743.2

10.5

24.3

7.40

9500

2895.6

10.3

23.8

7.25

10000

3048.0

10.1

23.4

7.13

Work - Power

1 hp

=

0.746 kw



1 kw

=

1.34 hp



1 hphr

=

0.746 kwhr

=

2546.1 BTU

1 kwhr

=

1.341 hphr

=

3410.1 BTU

1 cm water evaporated requires 590 car/cm2 of energy or 590 langleys in a day.

<<TOC3>> A.2 Common formulas

Area of a rectangle

=

length width

Area of a triangle

=

base height 1/2

Area of a circle

=

3.142 radius2

Circumference of a circle

=

2 3.142 radius

Volume of a cube

=

base width height

Velocity

=

A.3 Trigonometric table

(All angles are given in degrees.)

ANGLE TANGENT

SINE

TANGENT

ANGLE

SINE

0.0

0

0

23

.3907

.4245

0.5

.0087

.0087

23.5

.3987

.4348

1.0

.0175

.0175

24.0

.4067

.4452

1.5

.0262

.0262

24.5

.4147

.4557

2.0

.0349

.0349

25.0

.4226

.4663

2.5

.0436

.0437

25.5

.4305

.4770

3.0

.0523

.0524

26.0

.4384

.4877

3.5

.0610

.0612

26.5

.4462

.4986

4.0

.0698

.0699

27.0

.4540

.5095

4.5

.0785

.0787

27.5

.4617

.5206

5.0

.0872

.0875

28.0

.4695

.5317

5.5

.0958

.0963

28.5

.4772

.5430

6.0

.1045

.1051

29.0

.4848

.5543

6.5

.1132

.1139

29.5

.4924

.5658

7.0

.1219

.1228

30.0

.5000

.5774

7.5

.1305

.1317

30.5

.5075

.5890

8.0

.1392

.1405

31.0

.5150

.6009

8.5

.1478

.1495

31.5

.5225

.6128

9.0

.1564

.1584

32.0

.5299

.6249

9.5

.1650

.1673

32.5

.5373

.6371

10.0

.1736

.1763

33.0

.5446

.6494

10.5

.1822

.1853

33.5

.5519

.6619

11.0

.1908

.1944

34.0

.5592

.6745

11.5

.1994

.2035

34.5

.5664

.6873

12.0

.2079

.2126

35.0

.5736

.7002

12.5

.2164

.2217

35.5

.5807

.7133

13.0

.2250

.2309

36.0

.5878

.7265

13.5

.2334

.2401

36.5

.5948

.7400

14.0

.2419

.2493

37.0

.6018

.7536

14.5

.2504

.2586

37.5

.6088

.7673

15.0

.2588

.2679

38.0

.6157

.7813

15.5

.2672

.2773

38.5

.6225

.7954

16.0

.2756

.2867

39.0

.6293

.8098

16.5

.2840

.2962

39.5

.6361

.8243

17.0

.2924

.3057

40.0

.6428

.8391

17.5

.3007

.3153

40.5

.6494

.8541

18.0

.3090

.3249

41.0

.6561

.8693

18.5

.3171

.3346

41.5

.6626

.8847

19.0

.3256

.3443

42.0

.6691

.9004

19.5

.3338

.3541

42.5

.6756

.9163

20.0

.3420

.3640

43.0

.6820

.9325

20.5

.3502

.3739

43.5

.6884

.9490

21.0

.3584

.3839

44.0

.6947

.9657

21.5

.3665

.3939

44.5

.7009

.9827

22.0

.3746

.4040

45.0

.7071

1.000

22.5

.3827

.4142




A.4 List of common tools

- 3-5 HP CENTRIFUGAL PUMP
- ABNEY LEVEL (HAND LEVEL)
- AX
- BUCKETS (5-GALLON)
- CALCULATOR, SCIENTIFIC
- CARPENTER'S LEVEL
- DIGGING BAR
- GRAPH PAPER
- HACK SAW
- HAMMER
- HOES
- HOSE LEVEL (30 m TRANSPARENT 1/2" (phi = diameter) TUBE)
- LUMBER
- MACHETE
- METAL SHEERS
- NAILS
- PICKS
- PIPE THREADER
- PIPE WRENCH
- PLASTIC ZIPLOCK BAGS
- PLIERS
- PRESSURE GAUGE
- PVC PIPE (1", 2", 3" (phi = diameter), 1 1/2 m LENGTH)
- RAIN BIRD SLIDE RULER CALCULATOR
- ROPE
- RUBBER INNER TUBE
- SAW (RIP, CROSS-CUT)
- SCREW DRIVERS
- SHEET METAL
- SHEETS OF PLASTIC (15-20 m2)
- SHOVELS
- SOIL AUGER
- STOP WATCH
- STRING
- SURVEY ROD
- TAPE MEASURE - LARGE (50 m); SMALL (3 m)
- TEFLON TAPE
- VICE GRIPS
- WOOD AND METAL FILES
- WRENCH


<<TOC2>> Appendix B - Community organization and development

B.1 Community situation analysis/needs assessment

When involved in rural development, problems in poor areas can appear staggering and overwhelming. A development worker or a development committee may want to solve all problems, but villagers may be able or willing to work on only one or a few. With so many problems to solve but limited time and resources, a complete and accurate assessment of needs is essential. A needs assessment identifies and prioritizes a community's problems so they can be solved efficiently. It focuses general needs into specific obtainable goals.

Identifying a community's needs is only half the equation in development program planning. The community's resources, physical and social, must also be identified to determine the best methods and technologies required to solve a problem. A good appraisal can focus development work on important and solvable problems using technology that is locally available, understandable and transferable. Too often development projects designed by outside experts have scratched an itch that wasn't there and have used an exotic and expensive technology.

A project that addresses a pressing community need will generate genuine enthusiasm. Using appropriate technology to address a problem will ensure that villagers can afford the technology, repair it when it breaks down, and adapt it to new problems and situations. Such a project can live on beyond the funding life. Community situational analysis and needs assessment is the first step in matching a technology to local conditions.

Most irrigation projects involve a number of people in a community. Irrigation projects involve the farmers who will use the water for irrigation, local and state officials who administer water rights, irrigation equipment suppliers, and a number of other individuals and organizations. Most irrigation projects can be developed and operated successfully only with community participation during all phases. From the beginning of the planning for an irrigation project, the community must be involved. The community must feel that irrigation is a priority need if it is to devote the time, energy, and other resources to the project. Thus a community needs assessment should be part of any project in its early phases. If the project's priority is shown to be improving agricultural production or stabilizing food supplies, the idea will be well received.

Along with the needs assessment should come evaluation of the resource base -- physical, social, legal, and possibly even religious. If the right conditions are not present the project may have very limited success. For example, a project cannot be built if resources are not available or if other needs require the investment of scarce resources. Physical availability of water or limits on its availability due to water rights considerations may be serious limitations. Rapid appraisal techniques are useful in developing the need and feasibility of irrigation. This section presents some basic concepts on community analysis and needs assessment as well as some rapid appraisal techniques. It also highlights the most important human and physical resources that must be evaluated when determining whether or not irrigation will be developed and what the benefits and limitations of irrigation might be.

<<TOC3>> B.2 Rapid rural appraisal

B.2.1 General Concepts

Rapid rural appraisal (RRA) was developed to enable rapid decision making in rural development projects. Techniques have evolved from the need to get good quality data fast while avoiding the expensive and time consuming traditional survey methods used by researchers or total immersion methods used by ethnographic studies. There are various methods that can be used in RRA. The purpose of the appraisal will determine which method can be used.

As an example to illustrate the general approach to RRA, consider a village where family income is too low to meet all expenses. Families must seek temporary employment elsewhere. The Volunteer involved in rural development in this village identifies this very general need. He or she then conducts an appraisal with community leaders and indigenous agricultural experts using direct observation and guided interviews. This appraisal generates a list of more specific problems related to low income levels. A meeting with community members results in the analysis, prioritization, and development of a specific program goal.

To accomplish this goal, a situational analysis is conducted to identify community and outside resources that can be applied to solving problems and constraints. The situational analysis will also identify potential problems that need to be considered in the project design. More meetings and data collection follow to adapt and verify the program using other appraisal methods. This example illustrates that community needs assessment/situational analysis is an iterative process continually evolving as programs grow.

Quick-and-simple investigations require some general principles to avoid incomplete data collection or inappropriate and misconstrued information. These are:

1. Take sufficient time. Rushing can result in incomplete or inaccurate results.

2. Use a participatory approach. Local people's knowledge of soils, seasons, plants, domestic and wild animals, farming practices, diet, cooking practices, child care, as well as social customs, relations, and organizations is very important to consider. Ask rural people to identify the problems and resources.

3. Select community representatives carefully to avoid incomplete data or personal biases and hidden agendas. In many traditional cultures, there are hidden leaders who aren't obvious to an outsider but are instrumental in decision making in a community. A useful method for identifying community power actors and creating a committee of local participants identifies the following four power roles (Ref. 49):

· Positional Power: easily identifiable leaders, such as government officials and teachers.

· Reputational Power: leaders typically known by their reputation but their identity may be less obvious. Examples include practitioners of indigenous traditions, successful farmers, and religious leaders.

· Decision Making: instrumental leaders in the resolution of community issues who may not be represented above. Examples are elders and ax-official leaders.

· Social Participation: active leaders in community voluntary associations. Examples include religious leaders and club leaders.

4. Use key indicators that integrate several variables. Investigating, calibrating and measuring these parameters can save time.

Some examples include:

· soil color and characteristics,
· plant species present and appearance of growth,
· farm size and condition,
· nutritional condition of household members, and
· soil cover and erosion.

5. Use an iterative and continuing approach to identify trends instead of making a snapshot assessment. With experience, participants in rapid rural appraisals will improve their accuracy and completeness in identifying community needs and resources.

B.2.2 Methods of Rapid Rural Appraisal

There are many methods for rapid rural appraisal. Any one method should not be relied on alone, and a combination of methods is recommended. Local conditions and the abilities of local participants should determine the direction of the appraisal. Local participation is essential in a rural appraisal.

1. Guided interview. There is no formal questionnaire but a simple checklist of questions that the interviewer uses as a flexible guide. Not all questions are asked of all interviewees, but a composite is developed. Casual conversations result in more valuable information and encounter less distrust and resistance than formal questionnaires. Women, children, and other development projects personnel should be interviewed along with village men.

The question guide should be reviewed before direct conversation is undertaken. Notes and answers are jotted down after the interview to avoid discomfort and suspicion. The questions asked will vary with the purpose of the appraisal. They can be as varied as, for example, what disease problems occur in a certain crop; how are family economic decisions made and who makes them; what are your fertilizer sources, costs of production; and what is a typical diet.

2. Direct observation. This method will enable one to avoid being misled by myth. The importance of walking, seeing, and asking is stressed. Biases are left behind. Even the less experienced eye can identify important facts, such as the kinds of crops and livestock raised, water resources, topography, availability of transportation, and marketing possibilities. Those with more experience will collect more information.

An example of this method is walking with village leaders through the village and observing how many villagers are planting a certain crop or pruning their fruit trees. A very effective, though time consuming method, is learning by doing. For example, by hiring oneself out as a laborer to farmers, one learns local farming practices.

3. Informal transects. This method involves walking away from the road at right angles periodically and observing soil, crop, or other conditions, depending on the purpose of the appraisal. This method is best conducted after some experience has been gained as an observer or in the company of a village expert or leader. It is a useful method for gathering baseline data. Example topics for baseline information include the incidence of a crop disease or weed species, soil conditions, and watershed conditions.

4. Local researchers. Making use of ad hoc research by local students or by national university students can be a quick source of excellent and practical information. Many agricultural schools require field study, but these students are not commonly available at the local level. A variant to this method is making use of local secondary students to conduct local studies or applied research. Their knowledge of local conditions and enthusiasm can result in excellent data. Collaboration with local instructors may even add this study to their curriculum.

5. Local experts can be used to inventory local needs and resources as long as cross-checking guards against biases and hidden agendas are made. A local development committee would be able to identify these experts or might even serve in this capacity. Again, it is important to include groups that might be excluded, such as women, the poorer families, and indigenous or traditional experts.

6. Reading and collecting of local information provides baseline data. Maps, histories, census data, anthropological ethnographies, and records of local cooperatives, health posts, marketing organizations, stores, distributors, feed mills, and governmental organizations are all valid sources. Aerial inspection for general appraisals and less accessible locations is useful for a big view of the topography, soils, and vegetation, if possible.

7. Meetings with good representation involve many people, allow people to bounce ideas off one another, and discuss disagreements, and limit the potential for biases. In some cultures, people may not be accustomed to participating freely in a meeting. Information may be limited because these individuals may not express opinions that are not in agreement with other villagers, village power actors, or leaders. A good moderator is required to bring all opinions into the open.

8. Brainstorming techniques involve a panel of community leaders or group participants who meet to develop a list of community needs and resources and prioritize them from the point of view of the group members. The advantage of this method is that it not only gathers good information but is a learning and empowering process.

The first step is a brainstorming period where ideas are presented without evaluation. Then the group eliminates items that do not belong in the list, such as needs that are not true needs. The next step is to arrange the problems or resources (depending on the purpose of the brainstorming session) into similar groups and eliminate repetitions. Finally, priorities are developed by the group from the list.

9. The Sondeo or a multidisciplinary team rapid survey method is both systematic and open. It can draw information that is not obvious but very important. The process is also participatory by design. It allows the problem to determine the direction of the process as opposed to the process determining the problem. The survey team can consist of engineers, agronomists, socio-economists, and local representatives. In its traditional form, it is a six-day procedure but variations should never take longer than 2 weeks. The following method is adapted from Ref. 23:

Day 1 - General reconnaissance of the area by the whole team as a unit. Interviews are general. After each interview the team meets to interpret results. In these discussions each discipline's interpretation of facts and view is critical.

Day 2 - Team members break up into pairs for reconnaissance interviews and discussions for a half day. Then the group meets to discuss findings and plan the direction of questions for the next day.

Day 3 - Repeat of Day 2, only pair members are switched. There should be a convergence of opinions and topics forming now. Interview/discussion cycles are important at this stage as opposed to just interviews. Interview topics and questions are more specific and are determined during the discussion periods. The interview guide is not a questionnaire but a checklist of topics (guided interviews). This checklist covers and divides topics into bite-size chunks that can easily be covered with a single individual or family. The results of the previous days will set the direction of the process.

Day 4 - Repeat of interview/discussion cycles, but before 5th day, team members are given a portion of the report to write. Members need to be close for these last days to share information. Many three-cornered discussions should be included between interviewee, social scientist, and agricultural scientist to identify problems and resources and propose ideas for appropriate technologies.

Day 5 - Report writing and return to the field to clarify points if necessary.

Day 6 - Report sections are read, conclusions and recommendations are drawn. A single report is compiled by the team.

B.2.3 Information to Gather on an Area or Group

Human Resources

Economic: Sources of income, distribution of income in the community or group, alternative sources of income, who are the poorest people, how do their income sources compare to richer members, how do people save, sources of possible loans and credit, interest rates, collateral and repayment of loans, who makes decisions and how are family economic decisions made, divisions of labor in families and among community members, problems with economic exploitation, opinions about economic future. How important is irrigation to the economic well being of the community or how important could it be? If possible, compare the economic condition of farmers who have irrigation available to those who don't.

Social: How are families structured, inter-familiar relationships, do the people work well in groups, what are the rights and responsibilities of group labor, forms of group labor, what are the barriers to cooperation, examples of successes, what is respected in the village, how are innovators looked upon, who are the community leaders and why, are there seasonal migrations, is there faith in development, what development organizations exist, problems with paternalism, literacy. Are there existing water users organizations? Are they effective in distributing water equitably and efficiently? Are they capable of maintaining the irrigation systems and collecting fees for the purpose? What can be done to improve existing organizations? If there are no existing water users organizations, could one be formed with an existing organization as a basis?

Political: What government programs affect the village, government's policy towards private development, local government officials, political history, strength of local government and responsiveness to local needs, official regulations regarding water rights and land tillage. Is irrigation a political priority? If irrigation is a priority, then it is generally much easier to obtain support for new projects in terms of resources.

Legal - Water Rights: What are the water laws of the country or state? How is a water right established, transferred, or taken away? What is the water rights situation? Can new water rights be established, or is the water supply too limited to accommodate new rights? If a water right is established, how secure is it? Can other, more powerful individuals deprive the user of this water in the future?

Health: Diet, nutritional state of villagers both poor and rich, seasonal food problems, sources of food, common diseases, health care available on the local level. Could nutrition and health be improved through the use of irrigation to grow vegetables or other crops needed to improve the food supply?

Physical Resources

Area: Topography, water resources, local soils, climate, rainfall, ecological dangers, homogeneity of area, forestry resources, existing irrigation infrastructure and condition.

Agriculture: Crops grown, cropping practices, alternative crops tried in the past, fertilizer used, pest and disease problems, irrigation present and potential, size of farms in the area and distribution, tenancy relationships, price of land and market, possible land expansion, source of seed and reliability of seed sources, crop rotations, storage methods, costs of production, cash crops and subsistence crops, potential new crops, local seed selection practices, poisonous plants in the area, draft animals used, livestock present and distribution in the community, feed and fodder for livestock, vaccinations, small animals raised, local breeding practices, limiting resources of crop and livestock production.

Markets: Where are they and how do they operate, are they free, government regulation and promotion, black or informal market present, taxation, co-ops present and effect on markets, transportation used and reliability, local processing done, availability of price and marketing information, marketing bottlenecks, purchasing contracts available, monopolies present, seasonal problems in transportation, seasonal price trends.

It is often useful to view and analyze needs in terms of one of the four need dimension categories listed below.

Felt needs:

a wish list, unrestrained by cost reality and priorities.

Expressed needs:

needs of a community expressed by their activities.

Normative needs:

needs from the perspective of experts in the field or public policy.

Comparative needs:

needs resulting from inequalities and services.

This will improve the focus of a needs assessment.

<<TOC3>> B.3 Water users associations

(Adapted from Refs. 29 and 37)

Irrigated agriculture, by its very nature, is a joint enterprise. It requires cooperation between all users involved in the operation, maintenance, and improvement of the irrigation system. Experience has shown that without active participation of farmers, irrigation systems are rarely very efficient or cost effective (Ref. 37). A water users association is the connection between the physical irrigation system and the surrounding social systems. This section will attempt to address the role, formation, and structure of a water users association.

A water users association is a collaborative effort between individuals served by a common source of water to allocate, distribute, and manage water in an efficient manner for the benefit of all users. Associations vary in structure and can have important roles from early in the design of an irrigation system to daily management of an established system. Some important aspects and benefits from the formation of these organizations include:

1) A water users association can be essential to the securement and protection of water rights.

2) A water users association can get farmers involved in local decision making and give them a sense of ownership. Local ownership will tend to keep an irrigation system responsive to local needs. It will encourage users to be more involved in the management and maintenance of the system.

3) Farmer cooperation can result in less time required for system operation. In the case of a government-run system, there is less administrative time spent coordinating between a government agency and farmers on system management and maintenance. The farmers themselves are running these activities. In the case of an undefined management structure for a system, there is less disorganization and time spent trying to make management decisions, as well as less time spent repairing systems that didn't receive proper management and maintenance.

4) In the design or improvement of a system, local wisdom and experience of farmers should be applied to improve system designs. This will assure a system is adapted to local social systems, land tenure patterns, topography, and soil conditions.

5) A water users association can be an important liaison between the local system and outside government and non-government agencies, including:

· extension agencies with alternative or new sources of seed, cultural practices, fertilizers, and agro-chemicals;

· government water authorities;

· engineers and other professionals; and

· funding agencies or entities.

6) The association provides local administration of the irrigation system through operation, maintenance, and management of physical facilities. This can involve hiring personnel to maintain a system or organizing group labor for the management of a system. It is more responsive to local needs than a distant government agency if users are properly trained.

7) The association, if properly organized, will provide equitable distribution of water to all users. This can involve hiring a ditch walker to supervise water distribution or organizing irrigation schedules among members. Proper training is required to assure a water users association can effectively and efficiently manage the distribution of project water, so tail end users don't get shorted.

8) A water users association may act as a third party, legal entity, or court to resolve water disputes among members.

9) The association can also collect water fees from members to pay for the upkeep and improvement of facilities. Pricing of water to its real value encourages farmers to use it wisely and provides funds for system operation. These funds can then be applied to pay for:

· salary of a ditch walker or other personnel to manage and maintain a system,
· replacing structures,
· purchase of equipment and tools,
· routine maintenance, and
· system improvement.

A water users association can have many formats to perform some or all of the functions listed above. No blueprint exists for gaining effective farmer involvement. In forming a water users association, some guidelines include:

1) Initiate an association where a predictable water supply is assured. This is especially important in regards to collection of fees and participation in group activities.

2) Start with local organizations. Build upon and strengthen these. Local informal leadership works within the local cultural context. An outside change agent should use much caution before introducing new organizational forms.

3) Do not bypass group leaders and leaders of factions. For example, equal representation is needed by head and tail water users.

4) The institutional and physical environments are important determinants in the structure of a water users association. Governmental agencies may require various levels of control of a project. The first step in building an effective water users association may be getting the water authority to release some control to the local level.

Water user associations vary in structure according to factors presented above but important and common structural components to be considered include the following:

1) Establishment

a) Voluntary - small water projects developed without extensive government assistance often form water users organizations to insure that the system is operated fairly and efficiently. These voluntary organizations may have only two water users or they may have several. Their rules and regulations are generally written within a legal framework to assure a common understanding for present and future users. The rules and regulations may be as simple as indicating when meetings will be held, how water deliveries will be scheduled, and how maintenance activities will be organized and paid for.

b) Compulsory - either by government or by a majority of the water users electing to do so. Government sponsored projects that serve a number of water users generally require that a water users organization be formed. The government may provide assistance in setting up the organization.

2) Basis for Organization

a) Water Laws and Water Rights - Water laws are developed to protect the water rights of an individual or group and provide the legal framework for establishing, transferring, and otherwise administering water rights. In some countries all water users along a stream or canal may have equal rights to water, but when water is in short supply the state may impose certain rules and regulations. These rules or laws may establish that a farmer's water right is based on landholding or antiquity of usage. The farmer's water rights are often expressed in terms of flow rate, volume of water, or time of availability. Once a water right is established the laws provide mechanisms for safeguarding these rights. Generally, to establish a water right the user must insure that he/she will not injure other users through exercise of this right (i.e. he/she will not hinder their ability to obtain water). Water rights and water laws must be clearly understood before the planning for an irrigation system begins.

b) Customary Basis for Organization - A water users organization may, in some cases, be a very informal arrangement based on culture, economics, or traditions. The organization may have few written laws but may be quite functional. If such organizations do exist and operate successfully, they will probably continue to do so for a long time.

c) Association's Title - The organization may be called a canal company, a council, a water users group, a district, a cooperative, or a number of other things.

d) Bylaws, Rules, and Regulations - The majority of organizations with more than just a few members develop bylaws, rules, and regulations within the context of local, state, and national laws. A water users organization will generally function effectively if a good set of bylaws, rules, and regulations are adopted. Some of the components that should be addressed are:

· Organizational set-up - elections and responsibilities of elected or hired personnel.

· How water will be distributed - How often will a user get water and how much? How will the water be measured? What structures will convey and distribute the water? Who will distribute the water (e.g. water commissioner, ditch rider)?

· Maintenance - What maintenance is expected of water users themselves and what maintenance will be conducted by the organization? A good maintenance plan is an essential element of any association.

· Water charges - How will water taxes or contributions be determined (e.g. on the basis of size, equally)? What will the water charges or taxes be used for (e.g. maintenance, operations, administration)? How will the water charges be collected? What will the penalties be for non-payment?

· Enforcement of bylaws, rules, and regulations -How will these be enforced in terms of procedures and penalties?

· Settlement of disputes - How will disputes be settled with a minimum of conflict? What are the procedures, and how are settlement agreements enforced?

3) Organizational Powers

The association has a mandate to distribute water according to bylaws set up by the users. To enforce these rules, an organization may have the following internal sectors:

a) A general assembly made up of all the users and having the highest authority. The general assembly decides major activities of association, approves budget and fees, decides on construction activities, and ratifies rules, regulations, and sanctions of violators. It elects representatives with duties to represent the water users.

b) A board of directors or an executive branch of the association chosen by the general assembly. The size of the board and whether members are paid depends on the local situation. Term of service of board members and reelection procedures must be set up in the association bylaws. The board of directors manages the organization, keeps records, collects fees, maintains the system, organizes rehabilitation, manages equitable distribution of water, and represents the association with outside agencies. It will hire or select water commissioners, ditch riders, and administrative, technical, or other personnel who will carry out the daily tasks associated with water distribution or maintenance.

c) A judicial sector. Water problems and conflicts among farmers are often deeply rooted and need to be routinely addressed. This can be done by:

· a judicial tribunal selected by the general assembly,
· the board of directors,
· a group of outside individuals and/or members respected by the users, or
· traditional local courts or leaders.

d) Specific people may be selected or employed by the association to carry out specific functions under the direction of the board of directors and the general assembly:

· ditch walker to supervise daily delivery of water, and
· maintenance supervisor.

Flexibility must be programmed into the bylaws and association for adaptability to membership changes, physical and governmental changes, and local social and economic changes. Flexibility will also help a water users association adapt as members gain more experience.

<<TOC3>> B.4 Formal and non-formal communication techniques

An outside development agent needs to be able to communicate well to be an effective agent of change. Communication comes in many guises, from lectures to informal chats on the street. A Volunteer who comes from a culture a high literacy rate and with a background of formal schooling needs to adapt communication techniques to the local site. Oral tradition is often very strong and literacy rates very low. Formal communication techniques that the Volunteer may be accustomed to will not be effective in this environment. Traditional communication methods can be adapted, however, and used to transmit modern messages. An example of very successful adaptations includes the use of folk theater in Asia to spread new ideas. Asia has a centuries-old tradition in folk theater. A Volunteer can use this tool to motivate villagers or introduce them to new concepts.

The old stand-by method for selecting a means of communication still works. Observe and listen to how ideas and opinions are exchanged before attempting to speak. Once the methods are understood, use them to exchange ideas. Often, the weakest link in communication is listening.

B.4.1 Review of Communication Techniques

The following section reviews the strengths, limitations, and materials needed for a wide range of communication techniques.

Material/Media

Strengths

Limitations

Local circulars, pamphlets

Low cost, ease of proliferation, self-proliferating

Easy to ignore, little feedback

Filmstrips

Portable, low-cost, adaptable, compatible

Requires darkness, electricity

Folk theater,

Entertains, local interaction encourages interaction

Requires talented, motivated, facilitators

Epic narratives

Reliance on local culture, entertains

Only available in some areas, limited

Slide/tape presentations

Entertains, motivates, complementary with other materials

Cost and time in production, requires electricity

Radio

Low cost to reach wide audience

Problems in encouraging feedback

Radio forum

Encourages feedback, wide audience

Problems in feedback, interaction

Television

Wide audience, very persuasive media

Cost, little feedback, local availability problems

Audio tape

Permits review, self-directedness

Cost, requires some training

Video tape

Adaptable for many uses, can involve local interactive production, permits review

Cost and time in production, requires training in use, availability problems

Computer software

Interactive, learning, permits self-pacing

Cost, computer phobia, requires skilled producer

Dramatic skits

Entertains, can use with large groups

Problems in reproduction

Role play

Interactive, encourages feedback, relevancy of ideas, encourages empathy

Requires good facilitator, willing trainees

Brainstorming

Interactive, spawns many ideas, self and group introspection

Requires respect among group members

Games

Interactive, action oriented, encourage feedback

Difficult to produce and adapt to local conditions

Simulations

Adaptable, action oriented, encourage feedback

Requires good facilitator, willing trainees

Group discussion

Encourages feedback, self-proliferating, active

Requires good facilitator

Lectures

Can present many ideas, complementary

Formality, requires good instructor

Debates

Different views, motivates analysis

Requires good moderator

Demonstrations

Action oriented, seeing is believing

Requires good timing and planning to be relevant

Posters, photos, flip charts

Graphic impact, wide appeal, low cost, portable

Static presentation

Field trips, exhibits

First hand experience, seeing is believing

Requires good planning and timing

Circular response

Equal presentation of views, interactive

Requires willing participants and facilitator

Case studies

Permits later review, self-explanatory

Problems in relevancy

Photo storybooks

Entertaining, self- proliferating

Little feedback, requires skilled producer

The method selected will depend on the situation in which it will be used. Some questions to ask before selecting a method or medium include:

- Is action orientation or static orientation required in respect to topic (to solve problems, improve skills) ?

- Is it cost effective to procure, reproduce, distribute, and store?
- Will it result in indirect versus direct interaction?

- Is it easy to reproduce and can it be used widely? Is it easy to reproduce by Volunteers or trainees in their home setting?

- Does it encourage feedback?
- Does it motivate? Does material or medium inspire people to change?
- Is it durable/repairable?
- Is it immediately relevant to the field or local culture?

- Is it adaptable material and media to a development message and to local conditions and constraints?

- Is the material self-proliferating?
- Is it complementary with other medias in mixed presentation?
- Is it self-explanatory, or does it always require a trainer?
- Is it portable?
- Is it produced from locally-produced materials?
- Is there a linkage of materials/medial with learning styles?
- Does it permit later review?
- Does it allow learning to set its own pace?
- Does it have entertainment value while still fulfilling its objective?
- Does material or media adapt to time requirements or restrictions?
- What amount of training/skill is needed for trainees to utilize?

<<TOC3>> B.5 Problem solving

B.5.1 A Problem Solving Method

This exercise is designed to help the Volunteer analyze problems when people work or live together. The exercise is programmed. That is, it is presented in a series of separate steps or "frames,'" each of which contains a complete and separate idea, question, or instruction.

Be sure to fully understand and have completed each frame before going on to the next.

1. The first step in this process of analysis is to identify the problem to be worked on. Describe the problem as you now see it.

2. Most problem statements can be rephrased so that they describe two things:

a. the situation as it is now.
b. the situation as you would like it to be (the ideal).

Restate your problem situation in these terms.

3. Most problem situations can be understood in terms of the forces that push toward improvement and the forces that resist improvement, in other words, driving forces and restraining forces.


4. It is useful to analyze a problem by making lists of the driving and restraining forces affecting a situation. Think about these now and list them below. Be sure to list as many as you can, not worrying at this point about how important each one is.

5. Now review the two lists, and underline those forces that seem to be the most important right now, and which you think you might be able to affect constructively.

Depending on the problem, there may be one specific force which stands out, or there may be two or three driving forces and two or three restraining forces that are particularly important.

6. Now, for each restraining force you have underlined, list some possible action steps that you might be able to plan and carry out to reduce the effect of the force or eliminate it completely.

Brainstorm. List as many action steps as possible, without worrying about how effective or practical they would be. You will later have a chance to decide which are the most appropriate.

For example:

RESTRAINING FORCE A
Possible action steps to reduce this force:

RESTRAINING FORCE B
Possible action steps to reduce this force:

7. Now do the same with each driving force you underlined. List all the action steps that come to mind which would increase the effect of each driving force.

For Example:

DRIVING FORCE A
Possible action steps to increase this force;

DRIVING FORCE B
Possible action steps to increase this force:

8. You have now listed possible action steps to change the key forces affecting your problem situation. Review these possible action steps and underline those that seem promising.

9. List the steps you have underlined. Then for each action step, list the materials, people, and other resources that are available to you for carrying out the action.

For example:

Action Steps

Resources Available


10. Now review the list of action steps and resources in the previous frame and think about how they might fit into a comprehensive action plan. Eliminate those items that do not seem to fit into the over-all plan, add any new steps and resources that will round out the plan, and think about a possible sequence of action.

11. The final step in this problem-solving process is to plan a way of evaluating the effectiveness of your action program as it is implemented. Think about this now and list the evaluation procedures to be used.

12. Now you have a plan of action to deal with the problem situation. The next step is to implement it.


<<TOC3>> B.6 Project planning and proposal writing

Community or Group "Ownership" of a Project

It is very important for a group to feel ownership of a project. No matter how well a project is designed or built, if the people of a community do not feel they have a claim in the project, it may fail. A sense of project ownership will ensure that maintenance plans and project repairs are carried out in the future. A project that people have sacrificed for is not so much a "handout" but more of a community goal.

Community Need and Interest

While it is essential to determine community need and interest, actual measurement maybe difficult. Some projects will be easy to start, and a local committee will take the initiative. With others, only a few progressive people may recognize the potential for a project. In some cases there may be community interest in an idea, for example, an undeveloped water source, but the community may be unfamiliar with the technology to
improve it.

Means to encourage community interest include:

A) Project initiators are recognized leaders in the community. A community power representative with important positional, reputational, decision-making and/or social standing can make things happen and motivate people to organize and work together. The best situation would be a local committee comprised of several important community leaders.

1) It is important to learn from possible sources if the project initiators can provide the necessary assistance and organization to complete the project tasks.

2) A progressive project is usually driven by progressive people. Project initiators should exhibit a history of "progressive characteristics." For example, initiators might have planted non-traditional crops or been the first to try fertilizers or agro-chemicals; These progressive leaders should also be well accepted and respected by the larger community. They should not be isolated innovators ready to try anything new an outsider suggests.

3) It is important to assess the personal interest of the leaders or drivers of a project. They may have a hidden agenda and use a unsuspecting outside development agent to fulfill personal needs. Projects that appear to be motivated largely for personal gains by specific individuals will incur many organizational and administrative problems.

4) The overriding support of community leaders is essential, even if they aren't personally involved. In many cultures, it is very important that these leaders share some of the credit for the project's success.

B) Holding a community meeting is a very good way to identify and inspire community interest in a project. The number of people attending the meeting and their position in the community is a good indicator of a project's potential success or of organizational changes that may be needed in order to achieve success.

1) Attendance at meetings is highly important, as people who are not interested enough to go to meetings will often not participate in the project's implementation.

2) It is important to motivate participation by people that will be doing the work of implementing and managing a project.

C) Divisions in the community or group (political, ethnic, religious, or social) are difficult to overcome. Sometimes these divisions are hidden to an outsider. Attending social functions is often a good way to identify potential or existing divisions. If opposing factions exist, plans must be made early to deal with this problem.

D) Willingness to spend their own money for any part of the project can be an essential indicator of community or group support for a project. This sacrifice will help increase their ownership and perceived value of a project.

Community Participation and Organization

If there isn't an existing committee in the community that can organize a project, forming one early in a project's planning is important. Election of leaders to head meetings, organize work schedules, collect money for project expenses, help set the project direction, and set meeting agendas is important. This committee must be representative of the community. It is often useful to legalize the committee so they can solicit help from agencies and local government. A written record of meetings serves two purposes: 1) it provides a record of activities, and 2) it helps build a consensus. Remember to let the committee be the main movers behind the project. As a Volunteer, you can guide this committee and contribute to solving technical problems, but ultimately the project belongs to the community.

Outline Specific Objectives for Achieving the Goal

A clearly stated project goal is important to rally support, as long as the goal meets the community's need. Several specific objectives will clarify the steps to reaching the goal. Specify:

· Who will benefit from the project and what those benefits will be.
· Who will be working on or contributing to the project.
· Time frames for steps in the project.
· What the project will consist of and the steps needed to accomplish it.

Clear and measurable objectives are essential to proactive planning. It is also important to anticipate possible problems and their solution. A committee may, for example, have a general goal of increasing individual member income. A specific objective is to construct a ditch to irrigate specified lands with nearby river water so each member can have dry season crop production. From this take-off point, the group would make plans on how to design and construct the ditch. They might anticipate potential problems with water rights early and start working on avoiding or resolving the problem with the appropriate government agency.

Initial Technical Assessment

The next step is to complete a brief technical assessment to determine if the project is technically feasible and get an idea of the resources needed to complete the project. If the project is not feasible, the organizational steps completed up to this point can be used to develop other projects.

Identify Resources

To successfully complete the project, the development committee and the Volunteer will need to identify resources both locally and externally. These include:

· local supplies (Local contributions can ensure community participation in the proposal and are important to fully quantify in any funding request proposal.);

· local knowledge, skills, and labor availability;

· availability and cost of purchased materials;

· transport of materials;

· location, and if land is involved, proper titles;

· water rights (It is sometimes advisable to keep a low profile while establishing water rights and land titles to avoid conflict with non-members);

· outside agency technical support and possible assistance; and

· financial possibilities and community organizational support.

Project Design

Determine technologies to be used, develop specific technical designs and plans, and form a materials list. Design a project to meet anticipated problems identified in earlier planning stages. In water or irrigation projects, it is important to plan for future fluctuating water supplies. Sabotage, group divisions, children, and farm animals are of major importance in project design, along with the technical factors. For example, the effects of future fishing in a surface irrigation project's canals may change their design.

Funding of the Project

Research is required to fund a project. The community or group members may fully or partially fund a project. A loan or grant for part of a project may be necessary. The local committee must be integrally involved in financial arrangements.

· Care must be taken to assure there is equity in contributions and payments.

· In case of a loan, the funding agency should fully explain its accounting. The local committee should be instrumental in developing financing and repayment schedules.

· Accurately price the materials and alternatives.

· Calculate the costs per member.

· Decide how the community or group will pay for operational and maintenance costs.

· Decide on how and when fees and contributions will be collected.

· Determine wages for paid construction workers.

· Conduct economic and financial analysis of the project and alternatives.

Proposal Writing

The most important thing to remember in proposal writing is you are writing the proposal for the funding agency and you must address their needs. Find out what the funding agency wants and tailor the proposal for them. Lending agencies need to be assured the loan will be paid back. Some agencies want to see community participation. Other agencies want to fund projects that will address a great need. Finally, some fund only certain types of projects.

Often, the success of a project is integrally tied to the ability of the Volunteer or the committee to procure funding. Irrigation projects are often expensive, but their benefits are long-term. The willingness of funding agencies to donate or

lend money for projects is often tied to the presentation of a clear and acceptable proposal. Many agencies have standard formats for proposals or application forms. The individual agency requirements must be thoroughly investigated prior to application. The following is a general outline of recommended material that should be included in a proposal.

INTRODUCTION

1. Summary: Briefly describe the subject of the proposal, the applicant, and the community.

2. Statement of the Problem: This section should attract the reader's attention and make them interested in the problem. Make sure that you define the problem in reasonable terms; that there is a clear relationship between the problem and the proposed project (in other words, the problem can be made better or resolved by the project); and that you support your statements with evidence, including statistics.

3. Community Background: Concisely describe the community in relation to the type of work you plan to do. Include topographical information, social institutions, socioeconomic data, and population information. Describe past development work in the area. This background should give the community and project credibility.

4. Personal Background: Describe yourself and your background, including your experience and intentions in the community.

5. Goals of the Project: The goals should state what the community wants to accomplish with the project. They should be somewhat general, long-term, and attainable by completion of the project.

6. Objectives of the Project: Objectives are the individual activities involved in accomplishing the goals of the project. They should be clear, specific, and measurable. They should state what and how much will be done, who will do it, and when it will be done.

PROJECT OUTLINE

1. Description of the Project: This section contains the nuts and bolts of the plan. It should clearly state specific activities, construction methods, administrative procedures, and community mobilization strategies. Also, it should include schedules, time lines, a simple design layout, and staffing needs. It is important to be realistic about the scope of your activities, the resources available, and time needed to complete the project.

2. Total Cost of the Project: This should be a lump sum figure, including cash needs and in-kind contributions. Keep it feasible.

3. Budget Breakdown: This section should be a reasonably detailed listing of expense items. Break it into categories:

· materials and tools,

· labor and personnel costs (this may be converted into dollar amounts if it is in-kind contributions), and

· incidentals such as transportation or administrative costs.

Add 10-15 percent to the total for inflation and contingency. Also, make sure that your prices are correct, the funding source may check them. Review your project and make sure that it contains all the budget information that the funding source requires. If the budget is extremely long, you may want to write a short budget summary after the detailed budget listing.

4. Funding Request: This is the amount of money that you want from the funding source. At this point in the proposal, the funding agency knows exactly what you plan to do, how much the project will cost, their expected contribution, and donations from other sources. Make sure you include all the community contributions including value of items like rocks and boards used in construction, donated land, labor, and organizational and managerial time.

PROJECT ASSESSMENT

1. Evaluation and Documentation (Accountability): This section explains who will evaluate and document the project, and how and when it will be done. You should focus on both the outcome (results) of the project and the process used for implementation. Define the criteria that will be used in the evaluation, list information gathering techniques, explain how the evaluation will be used to improve the project, and describe the reporting procedures.

2. Future of the Project: Describe the project after implementation, focusing on your program for operations and maintenance. If funds will be required, make sure that you explain how the community plans to meet their operating costs.

3. Environmental Effects of Project: Describe the projects impact on the local environment such as changes in cropping patterns, effect on watershed, effects on soil and effects on water erosion. Include environmental assessments made (EA) as described in Chapter 2.

4. Benefits of the Project: This section relates directly to the goals of the project. It is an important section that acts as a conclusion to the proposal. The long and short term benefits to the community should be stated, such as any health improvements, transfer of skills, benefits to the environment, and community mobilization.

PROPOSAL REVIEW

Make sure that your proposal is reviewed and edited by a knowledgeable person before you submit it to any funding source. Here are some suggested review guidelines:

1. Are project activities well planned and the approach to implementation clearly defined?
2. Are the costs reasonable and related to the work to be done?
3. Is there strong local support and participation by the community?

4. Will community members learn valuable new skills through their involvement with the project?

5. Are the goals and objectives clearly stated, reasonable, and attainable?
6. Is the time allowed for implementation adequate?
7. Is there an evaluation process built into the project and is it ongoing?
8. Is the proposal clearly written and free of grammatical mistakes?

9. Does the proposal give the impression that the project is well organized and will be successful?

Evaluation

An evaluation after proposal presentation is very important, but evaluations should occur during all stages of project development. Evaluations should be an ongoing participatory action. Evaluations are not only designed to test a project's success but also to determine a project's future. Using a formative, instead of just a curative evaluation, can provide useful feedback to keep a project on track, meeting the objectives set by the group. All parties involved in the project, especially the recipients, will have to be included in the evaluation process.

Project Construction

Some ideas that may help during this phase of the project to save time and headaches:

· Procure outside materials and collection of local materials:

- shop around for the best price;
- get cement and other perishable materials last;
- set up controls so materials can't be stolen; and
- determine if the local government can get any supplies at a reduced or tax-free price.

· Choose or elect a construction supervisor and equipment manager.
· Form work crews and setup work schedules.
· Plan for safe storage of materials and tools.

Formation of Maintenance and Operational Plans

These plans should be discussed and finalized earlier in planning, then put into writing before the project is inaugurated. This is a very important part of the project. Factors to include in the operational plans include:

· project rules and responsibilities,
· water users organization,
· fees for maintenance and improvement,
· who executes and when are routine maintenance procedures done,
· project maintenance materials and tools -- who controls them and their location,
· salaries of maintenance personnel,
· penalties for non-compliance with project rules and responsibilities,
· procedures for changing members, and
· settlement of conflicts.

<<TOC3>> B.7 Economic analysis

Financial and economic analyses of a project are decision-making and planning tools that are often a requirement for presenting information to a governmental agency, funding agency, or the potential water users themselves. These analyses can often determine the fate of a project. The information is important to determine the annual costs, total ownership costs, and operating and maintenance costs of a project. These analyses also will determine if a project is viable and profitable and are important for keeping costs in line. It is important to evaluate the with and without effects of the proposed project rather than conducting a before and after comparison (a common error).

An economic analysis evaluates the costs and benefits of a project in more than money terms. It attempts to quantify benefits such as health, education, and cultural preservation. It predicts whether an investment is wise and worth undertaking. It is a useful tool for project evaluation.

A financial analysis, on the other hand, concentrates on the money aspects of a project and determines if it is an affordable and creditworthy option. Can the project's cash flow allow the project to continue? Financial analysis tends to play a more important role in the implementation of a project.

Economic Analysis

An economic analysis can be conducted in many ways. It is most commonly used to prove the economic feasibility of a project to a funding agency or governmental agency. Many funding agencies require this type of analysis to be presented as part of the project proposal. The method used to analyze a project is often determined by the agency involved. This summary only describes some of the more common methods of analysis. The agency in question can often help with the analysis.

Economic analysis weighs up the costs against the benefits of a project. It can also analyze a project's risk to its members. A lending institution will be interested in both, a grant funding agency may be interested only in the balance of costs and benefits. Subsistence farmers will be motivated by minimizing risk since almost any cost they incur will take food from their mouth or affect the well-being of their household.

The starting point for any analysis involves gathering the following information:

· construction costs;

· annual maintenance costs;

· fixed costs - costs of operating the irrigation system that don't vary whatever the intensity of use of the system, i.e. land costs, irrigation membership, water and maintenance costs, equipment costs of system;

· variable costs - costs that vary with crop grown and expected yield or production, i.e., fertilizer, seed, power costs, agro-chemicals, transport, labor, equipment and maintenance costs related to crop;

· cost of any borrowed money (interest) - for long-term (construction and equipment) or short-term (crop production or operating) loans;

· expected yields;

· average prices for produce at times when production is from irrigated fields; and

· a listing of other non-monetary benefits such as clean water sources, and reduced labor requirements, and an approximation of what these benefits are worth. Money values can be given to these benefits by keeping in mind the concept of opportunity cost. The opportunity cost is the value of the alternative or other opportunity that has been given up to achieve the present alternative. For example, the opportunity cost of a water user spending two hours repairing a canal might be quantified by allocating the value of the pay he might have received working for a local landowner.

Some of this data is developed from projections and can result in optimistic or pessimistic estimates. Care must be taken to stay as far away as possible from these extremes and present unbiased estimates. It may be a good idea to test both the optimistic and the pessimistic options (sometimes called "sensitivity analysis'.) as this will be a good measure of the riskiness of a project.

Remember an important fact: the timing of benefits and costs is critical to everyone involved in a project. The sooner benefits are seen, the better for everyone. Why? Because these benefits can be used productively sooner; most individuals prefer to spend now rather than tighten their belts for later; and at inflation rates existent worldwide, monetary benefits are more valuable now than later.

Risk Analysis

Some quick indicators of the riskiness of a project that can be tested early in planning are:

1) Cost/ha: Construction costs/number of hectares in the project. This can also read cost/unit of land.

This relationship is useful for quantifying the amount of money per unit of land that will be needed above and beyond what is already being spent. It can be used to determine how much money must be borrowed as compared to how much money water users can put forward based on the average farm size.

2)

Useful to estimate the financial risk each member will assume if land acreage were uniform.

3)

Project is profitable if break even point is one or greater.

4)

Calculates the time required for the project to recover the initial system investment (cost).

Return Analysis

These analyses are a little more detailed. They are used to evaluate overall project economics. Some methods are:

1) Partial budgeting can be used to estimate net benefits of two alternatives, for example, a with or without irrigation project choice. The method is easier than complete budgeting because not all the costs and benefits need to be calculated or estimated, only those that vary or are different between the two options. Generally, budgets do not look at total costs and benefits but rather at per unit area (per hectare or acre) or per unit of production costs and benefits. These budgets are quick decision-making methods for rapid appraisals but often are not acceptable for funding and government agencies who look for greater detail.

Net Benefit costs. = Total change field benefit - total variable

Example:


Present Practice

With Irrigation

Benefits:

Net yield

20 kg/ha

60 kg/ha

Price to farmer*

$1.80/kg

$1.80/kg

Total gross field benefit

$36

$108

Variable Costs

Cost of water

$0

$5.00

Loan repayment

0

50.00

Canal Maintenance labor 2 days $2.00/day (opportunity cost)

0

4.00

Total variable costs

0

59.00

Net Benefit

$36.00

$49.00

*$ = undefined monetary units.

In this example, irrigation increased the net per hectare benefit by $13.00. Often quality rather than yield may improve because of irrigation, and the price of the product may increase.

Partial budgeting often compares the "do" option against the "do nothing" option but can also be used to compare different practices, different levels of inputs (such as fertilizer), and so forth.

2) Rate of return

Net income = earnings - expenses

This is a useful relationship to demonstrate profit and is used by World Bank and other lending institutions. Often, farmers on a subsistence level require a rate of return of over 50% because of their inability to carry risk. For a subsistence farmer to invest in an alternative technology, he or she has to take resources from a very limited pool. If choices are not fail-safe, the family or household doesn't have a reserve to draw upon and must cut down consumption of food, or expenses like clothing and schooling for its children. Also, returns from innovative technologies that are not monitored closely are not visually and financially obvious unless increases of over 30% are achieved.

3) Net present value (worth) = present value of a string of benefits - present value of a string of costs. The net present value analysis includes the cash flow of an investment (benefits - costs) and the time value of money and risk. The time value of money refers to the fact that money in your hand now is worth more than the same money promised at some time in the future.

The same principle applies to risk. The higher the potential risk, the more the future money must be discounted to be equal to present money's value. Money now is more secure than future money. When someone lends you money, the interest rate charged is basically the same as discounting the future money you will pay. With risk, this interest rate is higher because of a potential disaster.

The basic technique used in net present value analysis is to discount costs and benefits that have been projected into the future to the present time (one point in time). Agencies use this method to compare and rank options or projects.


NPV

=

Net present value.

P

=

Initial investment or starting point.

FN

=

Cash flow (benefits - costs), subscript is year of cash flow.

i

=

Interest rate for discounting future values or what the investment amount would earn each year if put somewhere else.

(1+i)-N

=

The discount factor, taking into account that each year the amount discounted is compounded by 1+i.

N

=

Years of the project.

An example of a four year project can illustrate this point.

Year

0

1

2

3

4

Earnings

$0

$600

$700

$800

$900

Operating costs

$1000

$100

$200

$300

$400

Cash flow

$-1000*

$500

$500

$500

$500

* Original investment or expense. The discount rate is i=10% since that is what savings accounts are paying.

NPV

=

-P + F1(1+i)-1 + F2(1+i)-2 + F3(1+i)-3 + F4(1+i)-4


=

-1000 + 500(1.10)-1 + 500(1.10)-2 + 500(1.10)-3 + 500(1.10)-4


=

-1000 + 500(0.909) + 500(0.826) + 500(0.751) + 500(0.683)


=

$585

A decision can be made of the worthiness of a project as follows: If NPV > 0, the project looks good. The investment will earn more than just putting it in the bank. If NPV = 0, a choice is difficult and needs further evaluation of intangible benefits. If NPV < 0, the project is rejected because the "do nothing. option is preferable; the investment will earn more by being place elsewhere (for example, in the bank). If there are multiple projects, a funding agency may choose projects with the highest net present value.

While operating costs and cash benefits may be easy to determine, other costs and benefits may be hard to quantify or may not be evident in some projects. Better nutrition of farm families eating some of the added production and labor saving projects that free children from farm labor to go to school are examples of intangible benefits that are more difficult to value.

4) Benefit - Cost Ratio is used by the United States government for evaluating projects. To use this method the benefits and costs must be distinguished rather than just using cash flows as in the preceding example.


S

=

Salvage value


=

Value of project at the end of project life.

Bn

=

Value of benefits in nth year.

Cn

=

Value of operating costs in nth year.

P

=

Investment.

I

=

Discount rate.

N

=

Year of project.


Using the same numbers as in the previous example and assuming the salvage value after the 4th year is 100, the benefit- cost ratio will be:

*Costs in the denominator are expressed as positive values.

To analyze the project using benefit-cost analysis, if B/C > 1 the project is acceptable and if B/C < 1, the project is rejected. Again, multiple projects can be evaluated by selecting the project with the highest B/C ratio or best return. As in NPV, care must be taken to choose a valid discount rate (i) and to value all the benefits correctly.

Sensitivity Analysis:

In many cases there is uncertainty about the future benefits and costs such as yields, price of future projects, and future interest rates of money. For good project analysis, if one of these is expected to vary, it is best to run multiple analysis using different values of this one variable. An analysis of how this affects the outcome of the project can then be made. Using sensitivity analysis one can evaluate, for example, the net benefits over time of a project should interest rates go up. It can also be used to evaluate gloomy versus optimistic projections.

<<TOC3>> B.8 Financial analysis

Just because an irrigation system is economically viable doesn't mean an irrigation group can afford to finance it. The preceding examples indicate how a government or financing agency would review a project. The following defines how a group of farmers would analyze a similar project. An irrigation system may last for 15 to 20 years, but a financing agency may require the investment to be paid off in 6 to 10 years. Even if the irrigation system increases returns, the extra profits from irrigation must cover the loan repayments plus the water users minimum profit during the first repayment years.

Example:

$25,000 borrowed at 10% for 10 years

$25,000
.16275* = $4070/year

* Amortization factor (See Table B.2)

If the extra profits from irrigation are only $3500/year, the loan cannot be paid off unless production is subsidized the first years.

Using long term economic analysis, this example may have been an acceptable option, but from a farmer's more immediate point of view it may be difficult to obtain the cash to pay bills during the loan repayment years. Because of this, an irrigation group must accurately determine total costs and returns. To do this they must accurately estimate the annual cost of ownership and annual operating costs and compare these to the expected increase in production from using the system. Accurate annual costs determination is important since the initial cost is often only 1/3 of the total cost of irrigation.

Determining the Annual Ownership Cost

This is determined from (1) initial cost minus trade-in value, (2) interest, (3) taxes and insurance, (4) any fixed charges, (5) loss of land taken out of production for water development and (6) life expectancy of system. Table B.1 will assist in determining the annual ownership cost.

Procedure

1) Determine the initial cost of the irrigation system. This could come from the proposal materials list. Put these values in column 2.

2) To determine the annual ownership costs, multiply the initial cost by the appropriate amortization factor found in Table B.2. The amortization factor combines depreciation and interest in one number. To determine the appropriate factor, you need to know the interest rate of financing the system. Next, find the intersection of the expected years of life and the interest rate to get the amortization factor value. Enter the value in column 4 of Table B.1 and multiply it with the value in column 1 to get annual ownership costs. Example: A $4000 pump has a 12 year expected life. Money was borrowed at 12%. The annual ownership cost is: $4000 0.1614 = 645.6. The cost of the pump is $645.60 per year.

3) Add up all annual ownership costs of system components.

4) Estimate annual cost of taxes and insurance and enter. In some countries, this may be nothing.

5) Enter fixed charges of the irrigation system such as system water charges.

6) If any land was taken out of production to build the system, multiply the area by the value of production of this land in the past without the irrigation.

7) Total amounts in lines 6-9 for total annual ownership cost.

Annual Operation and Maintenance Cost

Annual operation and maintenance expenses need to be determined. They include (1) power costs, (2) repair and maintenance of equipment, (3) reservoir and field maintenance, (4) additional seed, fertilizer, pesticides and harvesting costs for the expected increase in yield with irrigation, and (5) labor.

Procedure:

1) Power costs = the fuel and oil consumption to run a power unit, if there is one. Use the following formulas:

Fuel or oil costs = horse power required no. of annual hours of operation cost/unit fuel / BHP hours/unit fuel.

Calculations are done for fuel and oil. Use the following Tables for brake horse power - hours per unit of fuel or gallon of oil.

TABLE B.1 Coat and Return Form *

Item (1)

Initial Cost (2)

Expected years of life (3)

Amortization factor (4)

Annual Ownership cost (5)

Well

25




Reservoir

50




Pump


Turbine

15





Centrifugal

12




Power Unit


Electric

25





Gasoline

10





Diesel

15





LP gas

12




Water Pipe


Plastic (PVC)

40





Polyethylene

5-8





Steel, coated

20





Aluminum - sprinkler use

15





Water works class

40




Concrete structures

20




Concrete pipelines

20




Land grading

20




Ditches

20




Land drainage

20




Sprinkler heads

8




Sprinkler systems


Hand moved

15





Solid set

20





Center pivot

10




(6) Subtotal average annual ownership cost: (total column 5).
(7) Taxes and insurance.
(8) Fixed costs.

(9) Loss of income from land out of production: price/ha
#ha.
(10) Total Annual ownership cost: (6 + 7 + 8 + 9).

* Projects may have other materials not on this list. Estimate expected life.

Table B.2 Amortization (Capital Recovery) Factors

Life

Interest Rate

Years

8.0

9.0

10.0

11.0

12.0

13.0

14.0

15.0

16.0

17.0

18.0

19.0

20.0

1

1.0800

1.0900

1.1000

1.1100

1.1200

1.1300

1.1400

1.1500

1.1600

1.1700

1.1800

1.1900

1.2000

2

.5608

.5685

.5762

.5839

.5917

.5995

.6013

.6151

.6230

.6308

.6387

.6466

.6545

3

.3880

.3951

.4021

.4092

.4163

.4235

.4307

.4380

.4453

.4526

.4599

.4673

.4747

4

.3019

.3687

.3155

.3223

.3292

.3362

.3432

.3503

.3574

.3645

.3717

.3790

.3868

5

.2565

.2571

.2638

.2706

.2774

.2843

.2913

.2983

.3054

.3126

.3198

.3271

.3344

6

.2163

.2229

.2296

.2364

.2432

.2502

.2572

.2642

.2714

.2786

.2859

.2933

3007

7

.1921

.1987

.2054

.2122

.2191

.2261

.2332

.2404

.2476

.2549

.2624

.2699

.2774

8

.1740

.1807

.1874

.1943

.2013

.2034

.2156

.2229

.2302

.2377

.2452

.2529

.2606

9

.1601

.1668

.1756

.1806

.1877

.1949

.2022

.2096

.2171

.2247

.2324

.2402

.2481

10

.1490

.1558

.1627

.1698

.1770

.1843

.1917

.1993

.2069

.2147

.2225

.2305

.2385

11

.1401

.1469

.1540

.1611

.1684

.1758

.1834

.1911

.1989

.2068

.2148

.2229

.2311

12

.1327

.1397

.1468

.1540

.1614

.1690

.1767

.1845

.1924

.2005

.2086

.2169

.2253

13

.1265

.1336

.1408

.1482

.1557

.1634

.1712

.1791

.1872

.1954

.2037

.2121

.2206

14

.1213

.1284

.1357

.1432

.1509

.1587

.1666

.1747

.1829

.1912

.1997

.2082

.2169

15

.1168

.1241

.1315

.1391

.1468

.1547

.1628

.1710

.1794

.1878

.1964

.2051

.2189

16

.1130

.1203

.1270

.1355

.1434

.1514

.1596

.1679

.1764

.1850

.1937

.2025

.2114

17

.1096

.1170

.1247

.1325

.1405

.1486

.1569

.1654

.1740

.1827

.1915

.2004

.2094

18

.1067

.1142

.1219

.1298

.1379

.1462

.1546

.1632

.1719

.1807

.1896

.1987

.2078

19

.1041

.1117.

.1195

.1276

.1358

.1441

.1527

.1613

.1701

.1791

.1881

.1972

.2065

20

.1019

.1095

.1175

.1256

.1339

.1424

.1510

.1598

.1687

.1777

.1868

.1960

.2054

25

.0937

.1018

.1102

.1187

.1275

.1364

.1455

.1547

.1640

.1734

.1829

.1925

.2021

30

.0888

.0973

.1061

.1150

.1241

.1334

.1428

.1523

.1619

.1715

.1813

.1910

.2008

35

.0858

.0940

.1037

.1129

.1223

.1313

.1414

.1511

.1609

.1707

.0800

.1904

.2003

40

.0839

.0930

.1028

.1117

.1213

.1310

.1407

.1506

.1604

.1703

.1802

.1092

.2001

TABLE B. 3 Annual Fuel Consumption (Ref. 51)

Fuel or Power

BHP-Hours per Unit of Fuel


Metric

English

Electric

0.9 per KWH

0.9 per KWH

Gasoline

2.3 per L

8.7 per gallon

Diesel

2.9 per L

11.0 per gallon

Propane

1.8 per L

6.8 per gallon

Natural Gas

2.4 per m3

6.7 per 100 cubic feet

TABLE B.4 Annual Oil Consumption (Ref. 51)

Type of Engine and Drive

BHP-Hours per Volume of Oil Used


Metric (per L)

English (per gallon)

Electric

2400

9000

Gasoline

800

3000

Diesel

800

3000

Propane

1100

4000

Natural Gas

1100

4000

Right Angle Gear Drive

1300

5000

Example of fuel costs:

Diesel:

80 hp required 900 hrs operation $.30/liter diesel/2.9 BHP hours/liter diesel = $7448/year.

Oil:

80 hp
900 hrs $50/gallon oil / 3000 BHP hours/gallon oil = $1200/year.

2) Annual repair and maintenance cost of power unit uses a similar formula as fuel and oil costs.


Repair costs = hp required
annual hrs of operation cost per BHP/hour.

Horse power required and annual hours of operation are in the system Table B.5.


TABLE B.5 Annual Cost of Repair and Maintenance (Ref. 51)

Type of Power Unit

Cost Per BHP / Hour

Electric motor and controls

$0

Gasoline

$.0030

Diesel

$.0027

Propane

$.0020

Natural Gas

$.0020

Example:

Repair costs of power unit:

80 hp
900 hrs operation $.0027/BHP = $194.40/year

3) Repair and maintenance costs of irrigation equipment. An estimate of this is initial cost 0.5%.

4) Cost of field, reservoir, and canal maintenance. This can be determined by number of days labor number laborers value of a day of labor.

5) If you expect to spend more on agricultural inputs, estimate these additional costs and enter them.

6) Labor: estimated labor hours/ha/irrigation number irrigations area of irrigated land in ha cost of labor per hour.

7) Add up all these operational and maintenance costs as follows:

Annual Operation and Maintenance Costs

Item

Cost

1)

Fuel:



Oil:


2)

Repair and maintenance of power unit:


3)

Repair and maintenance of irrigation system:


4)

Reservoir, field and canal maintenance:


5)

Additional agricultural inputs with irrigation:


6)

Labor Costs


Total operational and maintenance costs:

Return on Investment

To determine return on investment, one compares the costs of operation and maintenance and ownership costs to expected increase in production with irrigation. First the increase in

production under irrigation must be estimated and multiplied by the expected price. In dry season cropping under irrigation this would be the whole production (if nothing was grown previously) the usually higher price for dry season produce. In supplemental irrigation the return would be the increase in production under irrigation the usual price. Then calculate the total annual costs of irrigation (ownership cost + operational and maintenance costs). Use the following:

1) Expected increase in earnings with irrigation:

(Increase in production
price/unit)

2) Total annual cost of irrigation:
(ownership cost + operational + maintenance costs)

3) Expected additional profit from irrigation:
(line 1 - line 2)

Since all the total irrigation costs are included, if the profit is positive, the project is acceptable. Again, a risk management factor should be included in this analysis if water users have very little cash asset reserves. The additional profit will have to be large to offer a safeguard against problems during loan repayment years.

In subsistence agriculture conditions, the additional profit must be over 15% of the value of the total annual irrigation costs.


<<TOC2>> Appendix C - Summary of international irrigation center (IIC) training modules

The International Irrigation Center1 (IIC) training sessions are short video tapes designed to introduce subjects. Many have excellent computer graphics to explain concepts visually. Many are adapted to conditions that will be encountered by Volunteers in developing countries.

1 International Irrigation Center, Department of Agriculture and Irrigation Engineering, Utah State University, Logan, Utah, USA.

Specific modules are referenced in training sessions. An annotated list of these modules follows. If a VCR, television, and the tapes are available, it is recommended that the trainer make use of this resource. The videos are available through the IIC and through ICE.

Module #1:

"Introduction to Hydro-Agriculture." 5 min.


Contains a very basic introduction to the role and purpose of irrigation in tropical agriculture. This module would be appropriate module to use in the first training session.

Module #2:

"Soil Water Storage and Availability." 6 min.


Provides description of soil texture, structure, and water-holding capacity of soils. Uses good graphics.

Module #3:

"Quantitative Determination of Soil Moisture." 5 min.


Technically describes how soil water content is measured. Contains useful graphics that show water content in soils.

Module #4:

"The Use of Water by Plants." 6 min.


Describes the physical and biological factors involved in evapotranspiration and water use by crops during their growth cycle. Provides guidance in the scheduling of water applications in irrigated fields.

Module #5:

"General Procedures and Estimation of Reference Crop Evapotranspiration." 7 min.


Reviews procedures for estimating evapotranspiration; too technical for Trainees with limited background in irrigation sciences.

Module #6:

"Evapotranspiration: Selection of the Crop Coefficient". 12 min.


Contains an overly complicated explanation of developing crop coefficients for Trainees with a limited background in irrigation sciences.

Module #7:

"Crop Water Requirements for Ecuador." 9 min.


Covers hot/dry climates, dry/wet climates, and hot/wet climates, with an emphasis on conditions in Ecuador. It would be easy to discuss this material in reference to other Latin American countries, and it may be possible to adapt the material to other similar tropical climatic regimes.

Module #8.

"Critical Water Requirement Period for Crops." 6 min.


Describes the critical moisture periods for corn, small grains, alfalfa, peas, bananas, vegetables, fruit crops, cotton, and sugar cane.

Module #9.

"Determining When and How Much to Irrigate." 14 min.


Introduces irrigation water scheduling for multiple crops. Contains some math and practical guides, and the information should be understandable for most Trainees.

Module #10.

"Methods for Measuring Soil Moisture." 9 min.


Provides a somewhat technical, but very useful, guide to the use of the tensiometer and the neutron probe, along with the use of the hand feel method for measuring soil moisture.

Module #11.

"Feeling the Soil to Determine When to Irrigate." 9 min.


Visually introduces the feel method for determining available soil moisture from soil texture, feel, and observation. Explains when to irrigate using available water and rooting depth.

Module #12:

"Corn Production at Various Levels of Irrigation and Fertilizer Application." 10 min.


Introduces the concept that increasing levels of nitrogen and irrigations will increase yield to only a certain point. Uses many graphs and is beyond the needs of most Volunteers.

Module #13:

"Crop Selection and Time of Planting as a Function of Rainfall and Irrigation Water Availability." 13 min.


Reviews the steps in crop planning by using rainfall data, a useful process in irrigation planning.

Module #14:

"Subsoil Conditions That Affect Root Development, Water Penetration, and Aeration." 6 min.


Presents waterlogging problems and the consequences of overwatering.

Module #15:

"The Effects of High Water Tables on Crop Production and the Need for Drainage." 7 min.


Details specific crop tolerances to excess water saturation in the root zone. Explains capillary action of water, how salts are drawn up by this action, and what to do about it.

Module #16:

"Soil Salinity's Causes, Effects on Crop Production, and Problem Solution." 9 min.


Presents the causes of salinity. Contains examples of actions that can be taken to remedy the problem.

Module #17:

"Leaching Requirements for Adequate Salt Balance in the Soil." 14 min.


Introduces leaching and water balance for salinity control. Contains some complicated math. Explains use of salinity tolerance tables.

Module #18:

"Irrigation Uniformity''. 9 min.


Reviews causes of, and solutions to, uniformity problems.

Module #19:

"Soil Erosion and Its Control." 7 min.


Introduces the causes and processes that result in soil erosion. Includes examples of on-farm practices that can be constructed or applied to avoid or control soil loss.

Module #20:

"Water Infiltration into Soils.'' 9 min.


Explains infiltration processes and the factors that influence infiltration rates.

Module #21:

"Controlling and Conveying Water from the Source to the Field." 11 min.


Presents good information on open and closed systems, pipeline placement, control structures, and siphon use.

Module #22:

"Irrigation Methods: An Overview." 7 min.


Describes the major methods of irrigation (surface, sprinkler, trickle, and sub-surface) and principal factors in their selection.

Module #23:

"Furrow and Corrugation Irrigation." 8 min.


Explains the different types of furrow irrigation, their applicability, advantages/disadvantages, and proper design and limitations.

Module #24:

"Basin and Border Irrigation." 10 min.


Discusses the applicability of basin and border irrigation, the proper design and limitations, suitable crops, and management.

Module #25:

"Sprinkler Irrigation." 10 min.


Overviews the different types of sprinkler irrigation, their suitability, limitations, and use. Probably not applicable to Peace Corps Trainees because the size of systems discussed is larger than most Volunteers will encounter.

Module #26:

"Trickle Irrigation". 10 min.


Contains a good description of trickle irrigation, including system components, applications, usage, operation, and maintenance. Provides a good introduction to what trickle irrigation is and an overview of good, efficient irrigation. Micro-irrigation methods are also introduced.

Module #27:

"Other Irrigation Methods." 9 min.


Describes less efficient methods of irrigation (wild flooding, contour ditch flooding, water spreading, subsurface irrigation) and their applications. Provides a good explanation of subsurface irrigation. This material will be very appropriate to the field conditions many Volunteers will encounter.

Module #28:

"Surge Flow Irrigation." 8 min.


Explains water surge flow as applied to furrow irrigation and its increased application efficiency and advantages of use.

Module #29:

"Basic Concepts for Irrigation System Evaluation." 10 min.


Clearly describes the basic concepts and useful terms for system modifications to improve efficiency (furrow, basin, border, sprinkler, and trickle), performance parameters to evaluate efficiency (a lot of equations), and uniformity of system. Trainees may have to disregard the complicated mathematics.

Module #30:

"Types, Purpose, and Terminology for System Evaluation." 7 min.


Contains performance evaluations (observed, measured, and simulated), good questions to address while evaluating a system, and a clear introduction into the terminology used in evaluations. Includes many well-prepared graphs.

Module #31:

"Water Conveyance Losses." 7 min.


Discusses controlling water losses well. The section on measurement of water losses is a bit technical, but the graphics used are good.

Module #32:

"Water Measurement." 14 min.


Details different methods of water measurement -mostly surface but some pipeline. Methodologies are not explained, however, and the module simply inventories methods.

Module #33:

"Furrow Irrigation Evaluation and Improvement." 10 min.


Contains a good session on techniques for improving the design and management of furrows, and estimating possible furrow problems. Some points are poorly explained and may require trainer's support to avoid confusion.

Module #34:

''Border and Basin Irrigation Evaluation and Improvement." 11 min.


Clearly explains techniques for evaluating and improving the construction and management of borders and furrows.

Module #35:

" Sprinkler Irrigation Evaluation and Improvement." 12 min.


Clearly explains techniques used to evaluate sprinkler irrigation and improvement.

Module #36:

"Drip Irrigation Evaluation and Improvement." 6 min.


Explains techniques used to evaluate drip irrigation and improvement on a somewhat technical level. May not be applicable to Trainees.

Module #37:

"Improving Efficiencies Through Use of Reservoirs, Refuse Systems, and Automation." 7 min.


Contains some good suggestions of ways to improve the efficiencies of surface systems. Most parts are too technical for the needs of Peace Corps Volunteers, and the material is impractical for developing countries.

Module #38:

"Determining Water Delivery Requirements." 9 min.


Requires good math skills to follow this presentation, some of which is easily understandable and applicable. Useful for large projects.

Module #39:

"Field Water Delivery Schedules." 9 min.


Aids in identifying potential problems concerning improper farm water management and water delivery in large systems.

Module #40:

"Irrigation Management Program and Managing Systems with Water Shortages or Excesses." 12 min.


Contains some long lists, but all include good points regarding procedures that should be considered in the management of water in water short locations.


<<TOC2>> Appendix D - Case studies

CASE STUDY

Pipe Size and Accessories

A farmer in Honduras grows cabbage during the dry season using a gravity flow sprinkler system. He uses two rolls of 3/4" polyethylene (100 m/roll) in his main line piping. He wants to increase the flow capacity to his field, so he buys two rolls of 1" polyethylene tubing. The water source is about 200 m from a small tank. The farmer uses the main line to fill the tank and from there the water is distributed to the field.

In the installation of the new system, the farmer decides to have two mainlines because he did not buy unions and because the 3/4" tube fit snugly into the 1" tube. Each main line consists of one roll of 3/4" and one roll of 1", and the two lines are used to fill the tank.

Is this efficient use of tubing? Could a main line of two rolls of 1" tubing with a union have a higher flow rate into the tank than the present system? Is the tank necessary?

CASE STUDY

Pump Sizing and Installation

A farmer in Bolivia buys a small centrifugal pump with the assistance of a special credit program offered by an international agency. The farmer installs the pump without any technical assistance, plants a hectare of potatoes, and begins irrigating the crop by surface irrigation in furrows.

Midway through the growing season, the farmer finds that he is spending too much money on fuel for the pump, so he lengthens the irrigation frequency from every 10 days to 3 weeks. After going through only one cycle of this schedule, it is obvious to the farmer that the potato plants are in need of more water and will soon die if nothing is done. He goes into town to look for technical assistance at the local agricultural extension office.

The extensionist accompanies the farmer to his field. They first go to the location of the pump, and the extensionist finds that the pump has been installed 5 m above the stream. She checks the pump and observes that it has a 3" inlet and outlet and also finds that both the suction line and main line are 2" pipe.

What recommendations would you give the farmer to save his crop this year? What recommendations would you give the farmer for future plantings?

CASE STUDY

Inlet Flows into Furrows

An international agency donates an 18 hp centrifugal pump with aluminum coupled piping to an orphanage in southern Bolivia. The orphanage is irrigating vegetables by furrows for its own consumption on small 20 m 20 m plots that total less than 1/2 hectare.

The water is pumped into a head ditch at a very high flow and is channeled past each plot so that it can supply water to them. There is no control structure, so the entire flow is diverted towards a plot when irrigation begins and then into a single furrow. The water rushes down the furrow and, because there is no tail ditch, overflows the furrow and floods the end of the plot. It is repeated for each furrow.

At the midpoint of the growing season, it is apparent that something is wrong with the crop. Plants at the beginning of the furrows are small and unhealthy, while the plants at the end look healthy and are of normal size.

What has occurred in this irrigated field? What can be changed to improve the water distribution? Is this the appropriate method of irrigation under these circumstances?

CASE STUDY

Community Organization

This project started when the Committee for Community Development approached the Volunteer with an interest in constructing an irrigation project.

The water source was measured and, although small, was adequate for the project. The village was tucked into a valley, and the water source was well above the fields, resulting in a good gravity-fed, pressurized system. The soils were well-drained sandy loams and loams. Some of the village members were starting to experiment with growing alternate crops with surface irrigation during the dry season by utilizing small springs below the village. Because of the topography of the area, water from the larger spring above was to be piped in.

The next village had a successful pressurized pipe hose drag irrigation system, which motivated the Committee to approach the Volunteer. The spring was on the community common land, so there was no problem with water rights.

The topographical study was done, and the system was designed. Community meetings were held regularly during this period. These meetings were tumultuous. There were disagreements but, as long as it was clear that everyone could participate, the project planning process continued. A funding source was obtained, and everything looked good.

One afternoon the Committee president approached the Volunteer to tell him the community had decided not to go through with the project. Another village meeting was held, but no agreement could be reached. The project was never completed.

Late in the Volunteer's service, he attended a big social event in the community. None of the Committee members were to be found at the celebration - they were at their own private celebration. The Volunteer then learned there were two religious sects in the village, and the Committee for Community Development was made up of only one of these religions. Ignoring one sect had doomed the project from the beginning. A blended committee of both sects may have been able to complete the system, but it was too late for that Volunteer.

CASE STUDY

Estimating Community Need

The Community Development Committee approached a Volunteer about a potable water project. The community had some limited small, private water systems, but no community wide system. The village was spread out, and most village members had to go a long way to collect water.

The Committee members showed the Volunteer the potential water source, which was 15 liters/sec. This is much more than a village of 45 families requires from a potable water system.

A meeting was called to discuss the water system and possible irrigation with overflow. The Committee, some women, and children attended the meeting. This discouraged the Volunteer, but she described the project, and the Committee said they'd drum up support. The next week the Committee presented the Volunteer with a list of families committed to the project. This list included 42 names. Since there was now a large amount of support, plans were drawn up for a community water system with potable water and a surface system with overflow, to be directed to the plots belonging to the 42 interested people.

Community meetings were sparsely attended, with most families being represented by old women and children. The men were too busy to attend.

The municipal government was funding similar projects and agreed to fund this one if the community paid 25% of the costs and supplied local labor and supplies. In this wet/dry climate, dry season vegetables and potatoes got a good price and paying off this 25% could be done within the first year. The Committee took this news back to the community and, after a week, they responded that they would do the project only if all the costs were paid by the municipality. They also wanted food as payment for their labor, as this was how similar projects had been done in their area. The project failed, and there was no interest in pursuing other sources.

CASE STUDY

Community Participation

Using small springs and surface irrigation, a mountain village in a wet/dry climate had been working with a Volunteer on alternate crop projects during the dry season. A large water source was located 6 km from the village. The Volunteer and village members did a topographical study and waterflow test and found the water source to be high enough and large enough for a good pressurized pipe irrigation system. The water source was close to another village and on its common land. There were numerous water sources in this area, but some were too low to be useful in this irrigation project.

When the paperwork for the water rights was being done, other village members came to the government offices and testified that they would be left without water if this project were completed. The water rights petition was denied. Not knowing what to do, the Volunteer met with the community and told them they would have to solve this problem. The community then decided to pay the water authority's fact-finding committee to visit the spring site. Both communities met, with almost every member of both communities in attendance. An agreement was reached regarding where some of the springs would go for the system. The topography, along with a 600 m drop along the system, made the design of the project difficult.

A government loan was secured, and the project began. The work crews were organized by the community, and the Volunteer visited the site weekly. The Irrigation System Committee and the Volunteer drew up the following rules and regulations:

1. Funds for repairs and maintenance would be taken from annual donations.

2. A list of the system's supplies and who would be responsible for them was composed, along with a statement of group ownership.

3. Volunteers to serve on the committee, along with a tools/repair supervisor, would be elected annually.

4. A process for changing members was created.

5. Fines would be imposed on those who wasted water or did not maintain their individual branches.

6. If no water was used by a member within a 2-year period, it could be sold to a new member by the group.

When the project was almost finished, the community took some of the extra cement and PVC tubes to a spring at the neighboring community. They captured one of the smaller springs with a simple spring box, and laid out a stand pipe potable water system as a gift to their neighbors in this community. The reason for this was that there had been tension between the communities, and the committee wanted to prevent possible future sabotage.

Five years later, the system is still functioning well. During the dry season slash and burn, one fire got away but, because the plastic tubes were buried 60-80 cm below the surface, the tubes were not damaged. Two members have moved away, and one member was removed forcibly by the irrigation group. This caused some tension, but this member had not used his water, refused to pay his dues, and had not replaced broken lines.

CASE STUDY

Project Description

A pressurized pipe hose drag system irrigating 25 equal parcels, totaling 2 ha.

Project Costs

Materials:


PVC tubes

$4,031.15


PVC accessories

629.00


Cement

831.00


Reinforcing bar

1,250.00


Sprinkler heads

350.00


Total for materials

$7,091.15

Labor - mason:


26 days @ $8.00/day

$ 208.00

Transport:


2 trips $90.00/trip

$ 180.00

Total Project Investment

$7,479.15

The government Agricultural Development Bank will finance the project with a 5-year loan at 10% interest. Loan repayment = $7,479.15 0.2638 (amortization factor) = $1,973/year.

Scenario #1

Project is being used to irrigate dry-season crops.

5-Year Projection

VARIABLE COSTS

Year 1

Year 2

Year 3

Year 4

Year 5

Loan payments

$1,973

$1,973

$1,973

$1,973

$1,973

Maintenance fund pmts.

125

125

125

125

125

Extra seed, fertilizer, agro chemicals

100

140

160

200

240

BENEFITS

Year 1

Year 2

Year 3

Year 4

Year 5

Production on dry-season plots (kg/ha)

200.00

250.00

300.00

310.00

320.00

Dry-season price (kg)

9.20

9.30

9.40

9.50

9.60

Return ($)

1840.00

2325.00

2820.00

2945.00

3072.00

Scenario #2

Project is being used to supplement insufficient rainfall.

5-Year Projection

VARIABLE COSTS

Year 1

Year 2

Year 3

Year 4

Year 5

Loan payments

$1,973

$1,973

$1,973

$1,973

$1,973

Maintenance fund pmts.

125

125

125

125

125

Extra fertilizer with irrigation

45

50

55

60

65

BENEFITS

Year 1

Year 2

Year 3

Year 4

Year 5

Yields under irrigation (kg/ha)

200.00

250.00

300.00

310.00

320.00

Price (kg)

3.00

3.10

3.20

3.30

3.40

Irrigated return

600.00

775.00

960.00

1023.00

1088.00

Yields without irrigation (kg/ha)

50.00

55.00

60.00

65.00

70.00

Price (kg)

3.00

3.10

3.20

3.30

3.40

Return w/o irrig.

($)150.00

170.50

192.00

214.50

238.00

CASE STUDY

Users Associations

In the Azua Region of the Dominican Republic, water is scarce. Irrigation projects built in the mid-1900s had resulted in dismal failures. Areas near to the supply canals received excess water whereas areas farther away received no water. Excess irrigation in some places had resulted in waterlogging and salinity problems. Less than half of the land that could be irrigated actually was. Canals and drains were choked with weeds, and maintenance was a shambles. Large farms received water while small farms received none. The lack of discipline in the system did not permit irrigation schedules to be observed, maintenance to be undertaken, or water charges to be collected.

The diagnostic analysis of the system in the early 1980s showed a number of problems, which were then addressed with success. Water users associations were formed so that farmers could cooperatively address water issues. Through the water users associations, and with the assistance of the on-farm water management program, a number of positive changes resulted. For example:

1. Discipline was greatly improved so that water supplies could be more equitably distributed.

2. Small parcels of land were consolidated so that they could be irrigated efficiently.

3. On-farm irrigation works were developed that allowed farmers to irrigate efficiently, and farmers were trained to manage these systems.

4. Waterlogging and salinity problems were addressed through better water management and construction of a few drains.

5. Technical assistance allowed farmers to address other problems, such as pest management and soil fertility.

6. Maintenance was taken over by the water users, and it greatly improved. The water users organization was able to collect water charges much more effectively than had been done previously.

CASE STUDY

Inappropriate Technology

In the mid-1900s, the government of Peru attempted to help small-scale farmers in the mountain regions to improve their agriculture through irrigation. In a project near the mountain village of Chicche, a hastily implemented irrigation program brought water down to the area that was to be irrigated in open canals. Some canals were lined, and some were not. The area to be irrigated had slopes up to, and sometimes in excess of, 20%. No means for getting water to the farms from the main system were implemented.

Tremendous erosion problems developed quickly, and the expensive water system was not used. Finally, in the late 1970s, another program to improve irrigation in the mountains was developed. This included social formation and technical assistance. The costs of putting in delivery works and on-farm improvements on steep hillsides, however, were excessive, and the financial support to small-scale farmers that would allow them to improve their farms was non-existent. Thus, progress in developing the area for irrigation was very slow. This and many other similar projects failed because the farmers were not provided the means of financing improvements at the farm level. Projects often fail because planners and implementers do not account for some of the physical, financial, social, institutional, and other constraints faced by the farmer him or herself.


<<TOC2>> Appendix E - Annotated bibliography

ANNOTATED BIBLIOGRAPHY

(1) Ayers, R. S., and Westcot, D. W. 1985. Water Quality for Agriculture. FAO Irrigation and Drainage Paper No. 29. Rev. 1. Food and Agriculture Organization of the United Nations, Rome.
A good guide for evaluation of irrigation water. Excellent tables for evaluation.

(2) Ayres, Q. C. 1936. Soil Erosion and Its Control. McGraw Hill, New York.

Rare old text with many valuable practices applicable to Volunteer situations.

(3) Barghouti, and Le Moigne, G. 1991. Irrigation and the Environmental Challenge. Finance and Development, June: 32-33.

This concise article explains well the need and value of including environmental assessments in irrigation projects.

(4) Bradshaw, J. 1974. The Concept of Social Need. Ekistics: 37 (No. 200).

This reference is hard to find. It contains good ideas on community needs.

(5) Brush, R. E. 1982. Wells Construction: Hand Dug and Hand Drilled. Peace Corps Information Collection and Exchange Manual M-9. Washington, D.C.

This is an excellent description of the theoretical principles and field practices required to design, construct, operate, and maintain simple wells. It is extremely practical and thorough. Every Volunteer working with irrigation or water resources should have a copy.

(6) Bunch, R. 1982. Two Ears of Corn: A Guide to People-Centered Agricultural Improvement. World Neighbors, Oklahoma City, Oklahoma.

This text combines basic principles of community organization and mobilization with case examples of actual projects. Any Volunteer working with agricultural projects in rural communities will benefit from this information.

(7) Chakroff, M. 1978. Freshwater Fish Pond Culture and Management. Action/Peace Corps. Program and Training Journal. Manual Series No. 1B.

A good reference on the construction of small ponds that can also be used for irrigation.

(8) Chambers, R. 1983. Rural Development - Putting the Last First. Longman, Scientific, and Technical, New York.

Excellent book on community-driven developments. Good ideas for an outside change agent to incorporate into designing projects.

(9) Crozier, C. 1986. Soil Conservation Techniques for Hillside Farms. Peace Corps Information Collection and Exchange. Reprint Series No. R-62.

Excellent graphics and supportive text demonstrating more than a dozen practical soil and water conservation techniques that can be constructed by Volunteers are included in this manual. A very useful manual for Volunteers working with agriculture projects in hilly areas.

(10) Doneen, L. D., and Wescot, D. W. 1984. Irrigation Practice and Water Management. FAO Irrigation and Drainage Paper. Food and Agriculture Organization of the United Nations, Rome.

A good general description of irrigation methods and water management practices. A good introduction to irrigation for beginners. Does not contain much technical information, however.

(11) Doorenbos, J., and Kassam, A.H. 1979. Yield Response to Water. FAO Irrigation and Drainage Paper 33. Food and Agriculture Organization of the United Nations.

This manual is a must for Volunteers working in irrigation. It presents the water management strategies and irrigation needs for a number of worldwide crops.

(12) Doorenbos, J., and Pruitt, W.O. 1977. Guidelines for Predicting Crop Water Requirements. FAO Irrigation and Drainage Paper No. 24. Food and Agriculture Organization of the United Nations.

An excellent guide with plenty of good tables and information on crop water requirements.

(13) Driscoll, F. G. 1986. Ground water and Wells. Johnson Division, St. Paul, Minnesota.

An excellent reference for those who might work extensively with deep wells, and who have a good math background. Except for its good explanations on ground water and its origins, and discussion on drilling techniques, however, it is inappropriate for the Volunteer.

(14) Dunne, T., and Leopold, L. B. 1978. Water in Environmental Planning. W. H. Freeman and Company, San Francisco.

This is a standard textbook in hydrology, watershed management, and soil conservation. The text describes basic concepts and principles of hydrology and watershed management and relates numerous case examples of problems and solutions. While some of the material will be too technical for the non-specialist, the writing style is basic enough to serve as a useful occasional reference for some Volunteers.

(15) Eberle, M., and Persons, J. L. Appropriate Well Drilling Technologies: A Manual for Developing Countries. National Water Well Association, Worthington, Ohio.

An excellent description on lower cost drilling technologies. The Peace Corps manual, "Wells Construction," is available through ICE and is more complete.

(16) FAO Staff. 1985. Guidelines: Land Evaluation for Irrigated Agriculture. FAO Soils Bulletin No. 55. Food and Agriculture Organization of the United Nations, Rome.

Somewhat technical for some Volunteers but good soils evaluation guide.

(17) Freire, P. 1973. Education for Critical Consciousness. Seabury Press, New York.

Good ideas and methodologies for making training and education responsive to local needs and an empowering process.

(18) Gil, N. 1985. Watershed Development with Special Reference to Soil and Water Conservation. FAO Soils Bulletin 44. Food and Agriculture Organization of the United Nations, Rome.

A good manual describing how integrated watershed development projects are organized and implemented. Only a few of the sections on data collection and soil conservation techniques will be relevant for most irrigation Volunteers working with small systems.

(19) Gittinger, J.P. 1984. Economic Analysis of Agricultural Projects. 2nd ed. John Hopkins University Press, Baltimore.

This is a comprehensive but technical description of procedures and techniques used to conduct financial and economic analyses of agricultural projects. Volunteers may want to reference some information occasionally, but they would not need this text at their site.

(20) Griffen, R. E., Hargreaves, G., et. al. 1983. Pumps and Water Lifters for Irrigation. Handbook No. 3. Water Management Synthesis Project. Engineering Research Center, Colorado State University, Fort Collins, Colorado.

This manual is an excellent reference on pumping plants and water lifters. Basic concepts of pump selection, installation, and maintenance are well explained. Some of the technology will not be appropriate for small pumping plants.

(21) Hanson, B. D. 1985. Water and Sanitation Technologies: A Trainers Manual. Peace Corps Information Collection and Exchange. Training Manual T-32. Washington, D.C.

This manual presents, in readily understandable format, the basic concepts of pipelines and topographic surveys that are useful in designing and constructing irrigation pipelines. Thus, short sections from the manual were extracted for use in this manual.

(22) Hargreaves, G. H., and Samani, Z. 1986. World Water for Agriculture Precipitation Management. International Irrigation Center, Department of Agriculture and Irrigation Engineering, Utah State University, Logan, Utah.

Access to this manual is a must for trainers and Volunteers. It contains worldwide data on precipitation, temperature, and reference crop water use. This local data is sometimes hard to get, and here is an excellent summary. Trainers should still try to obtain local data, but this manual has some data for every country.

(23) Hildebrand, P. 1981. Combining Disciplines in Rapid Rural Appraisal: The Sondeo Approach. Agricultural Administration, Vol. 8.

Excellent method for community assessment. Used by ICTA in Guatemala.

(24) Hudson, N. 1981. Soil Conservation. Cornell University Press, Ithaca, New York.

A standard textbook with fair graphics and good explanations of principles, concepts, and practices in soil conservation. Much of the material relates to the author's extensive experience working in rural Africa.

(25) Ingalls, J. D. 1984. A Trainers Guide to Andragogy. Data Education, Incorporated. Peace Corps Information Collection and Exchange, Washington, D.C.

A theoretical but very practical explanation of adult learning concepts and techniques. Trainers and extension workers will benefit from the skill development exercises.

(26) Jackson, G. R. 1973. Lockwood-Ames Irrigation Handbook for Irrigation Engineers. W. R. Ames Company, Gering, Nebraska.

This handbook contains basic information needed to design a sprinkler irrigation system. The information is presented in many tables and graphs and is a very useful reference.

(27) Jordan, T. D. 1984. A Handbook of Gravity-Flow Water Systems. Intermediate Technology Publications, London, U.K.

This is an excellent handbook in the basic principles and field practices required to design and construct gravity-flow drinking water systems. It is presented in a non-technical manner and with good graphics. It contains good information on pressurized pipeline design, construction, and installation.

(28) Kennedy, W. K., and Rogers, T. A. 1985. Human and Animal-Powered Water Lifting Devices. Intermediate Technology Publications.

An excellent description of hand and animal-powered pumps in use worldwide. Includes expected outputs. Few details on design are included in this manual.

(29) Layton, J., Radosevich, G., Skogerboe, G. et. al. 1980. Improving On-Farm Water Management Through Irrigation Associations. Water Management Research Project, Colorado State University, Fort Collins, Colorado. 31 P.

Illustrated guide on water users associations structure and functions. Contact U.S. AID or International Irrigation Center, Utah State University, Logan, Utah, for copies.

(30) Leonard, D. 1969. Soils, Crops, and Fertilizer Use: A Guide for Peace Corps Volunteers. Program and Training Journal, No. 8.

This guide gives a clear, concise overview of everything a Volunteer will need to know about soils and fertilizer. There is good material on acid soils, but very little on salinity problems.

(31) Leonard, D., et. al. 1985. Agricultural Development Workers Training Manual. Volume III: Crops. Peace Corps Information Collection and Exchange, Washington, D.C.

This manual can supplement the Irrigation Training Manual for trainers wanting to include sessions on crop production. The manual includes session plans and some technical reference materials.

(32) Lynton, R., and Pareek, V. 1978. Training for Development. Kumarian Press, Connecticut.

Basic training theory and methodology for development in developing countries.

(33) Menard, P., et. al. 1985. Agricultural Development Workers Training Manual. Volume II: Extension Skills. Peace Corps Information Collection and Exchange, Washington, D.C.

This manual can supplement the Irrigation Training Manual for trainers wanting to include sessions on extension practices. The manual includes session plans and some technical reference materials.

(34) Merriam, J. L., and Keller, J. 1978. Farm Irrigation System Evaluation: A Guide for Management. Department of Agriculture and Irrigation Engineering, Utah State University, Logan, Utah.

A detailed guide on how to evaluate farm irrigation systems. It is too detailed for the type of work that most Volunteers will undertake, unless they are assigned to large irrigation projects.

(35) Michalak, D. F. and Yager, E.G. 1979. Making the Training Process Work. Harper and Row, New York.

The text describes principles and procedures which can be applied to plan training needs assessments, develop training sessions and curriculum, and carry out experiential training activities for adults. This is an important book for trainers to review and for potential extension or education Volunteers to reference.

(36) Nadler, L. 1982. Designing Training Programs: The Critical Events Model. Riddison-Wesley.

Good ideas and methodology to identify and design a training program that addresses local needs.

(37) Nobe, K. C. and Sampath, R. K. 1984. Irrigation Management in Developing Countries: Current Issues and Approaches. Studies in Water Policy and Management, No. 8. Westview Press.

Some ideas regarding the role of water users associations.

(38) Peace Corps. 1987. A Trainer's Resource Guide. Peace Corps Information Collection and Exchange Training Manual No. T-12. Washington, D.C.

A collection of materials that describe adult learning principles emphasized in typical Peace Corps training situations.

(39) Peace Corps. 1984. Rural Water/Sanitation Projects. Selected Technical Fact Sheets from U. S. AID Water for the World. Peace Corps Information Collection and Exchange.

The descriptions and diagrams for spring boxes, surface and ground water development, pump selection and maintenance, and storage tank construction will be valuable for irrigation Volunteers.

(40) Perrin, R., Winkelman, D., et. al. 1979. From Agronomic Data to Farmer Recommendations: An Economics Training Manual. Centro Internacional de Mejoramiento de Maiz y Trigo (CIMMYT), Mexico.

Good, concise guide to analysis of agricultural experiments and projects.

(41) Robinson, A. R. 1983. Farm Irrigation Structures. Handbook No. 2. Water Management Synthesis Project, University Services Center, Colorado State University, Fort Collins, Colorado.

An excellent reference for Volunteers working with many on-farm irrigation structures. Some of the structures are more complicated, however, and are oversized for the type of small projects that Volunteers will use.

(42) Salazar, L. 1983. Water Management in Small Farms. A Training Manual for Farmers in Hill Areas. Water Management Synthesis Project, University Services Center, Colorado State University, Fort Collins, Colorado.

This is a practical guide that has been used in many countries to teach farmers and technicians to manage water properly on the farm. It is extensively illustrated and is a must for the Volunteer. It is available through ICE. Pertinent sections have been included in Appendix G.

(43) Salazar, L. 1983. Water Management on Small Farms. Instructor's Guide. Water Management Synthesis Project, University Services Center, Colorado State University, Fort Collins, Colorado.

This is a companion to the training manual and contains useful experiences and demonstrations that the instructor can use as he or she trains farmers and technicians in the basic concepts of water management.

(44) Salazar, L., Hargreaves, G., and Stutler, K. 1987. Irrigation Scheduling Manual. The International Irrigation Center, Utah State University, Logan, Utah.

A manual that describes soil-plant-water relationships and irrigation scheduling practices. It has been used internationally for training irrigationists. It should be considered for all irrigation Volunteers and is used extensively as a reference in this manual.

(45) Schwab, G. O., Frevert, R. K., et. al. 1981. Soil and Water Conservation Engineering. John Wiley and Son, Inc., New York, New York.

This is a standard textbook in erosion control structures and practices, water conveyance systems, drainage, and irrigation. While some of the material may not be applicable to irrigation, it can serve as a good reference to water conservation. The text contains information on a wide range of materials, although it does not go into great theoretical detail.

(46) Shaner, W.W. 1979. Project Planning for Developing Economies. Praeger Scientific, New York.

Clear guide to steps of project planning and economic and financial analysis in developing countries (includes examples). Useful when writing project proposals for large agencies.

(47) Stern, P. H. 1985. Small scale Irrigation. Intermediate Technology Publications Ltd., London, U.K., and International Irrigation Information Center, Bet Dagan, Israel.

This handbook is a good source of information on irrigation practices in developing countries. Contains good tables and graphs that give general values for specific subjects. A good handbook for irrigation Volunteers to have.

(48) Stern, P. H., et. al. 1985. Field Engineering. Intermediate Technology Publications Ltd. London, U.K.

This is a good introductory reference for construction activities in rural areas. It covers many subjects and gives brief descriptions and general design criteria. A good reference for irrigation Volunteers to have.

(49) Tait, J. Identifying Community Power Actors: A Guide for Change Agents. North Central Regional Extension Publication No. 59. North Dakota Extension Service, North Dakota.

Hard-to-find reference. Important ideas in Appendix B regarding community leaders.

(50) Turner, J. H., and Anderson, C. L. 1980. Planning for an Irrigation System. American Association for Vocational Instructional Materials, Athens, Georgia.

This manual describes the selection and planning of different irrigation systems. It describes the different components in each type of system with graphics. The manual is easy to read and gives a good overview of irrigation practices.

(51) U.S. Department of Agriculture. Colorado Irrigation Guide. USDA Soil Conservation Service, Denver, Colorado.

A good reference for large farms but applicability to small farms is limited. One table from the guide was incorporated into this manual.

(52) U.S. Department of Agriculture. 1971. Ponds for Water Supply and Recreation. Agricultural Handbook No. 387, Soil Conservation Service.

A good reference on the design of ponds and small reservoirs. Description of materials and methods used in pond construction, and conditions to consider, are excellent. Design procedures are largely applicable to bigger ponds, however, than the Volunteer will work with.

(53) U.S. Salinity Laboratory Staff. 1954. Diagnosis and Improvement of Saline and Alkali Soils. USDA Handbook No. 60.

The bible on soil salinity and alkalinity. There is no new material on this difficult subject that is as good.

(54) Utah Power and Light Company. Energy Efficient Pumping Standards.

This pamphlet is not easy to come by and was used for its sketches, which are included in this manual.

(55) Watts, S. B. A Manual of Information on the Automatic Ram for Pumping Water. Intermediate Technology Development, Intermediate Technology Publications Ltd., London, U.K.

A good manual that explains in simple terminology the specifications, construction, and installation of an automatic ram. Gives details of all the components so that this water lifting device could be constructed at a local mechanic's shop. A good manual for irrigation Volunteers to have.

(56) Weidelt, H.J. (ed.) 1984. Manual of Reforestation and Erosion Control for the Philippines. German Agency for Technical Cooperation. Reprinted by Peace Corps Information Collection and Exchange, Washington, D.C. pp. 393-491.

The section on Erosion Control Techniques includes good descriptions and graphics of practical soil conservation and watershed restoration measures that can be constructed using very simple tools and skills.

(57) Westfall, D. G. 1980. Training Manual for Agricultural Water Management Specialists. Water Management Research Project, Colorado State University, Fort Collins, Colorado.

This is a training manual developed in Pakistan to train farmers in surface irrigation practices. It contains lesson plans and outlines the course of study in the training. This is a good reference manual that has a lot of good information on a wide range of subjects, including irrigation and drainage, rural sociology, farm management, agricultural extension, and soil sciences.

(58) Wood, A. D. 1976. Water Lifters and Pumps for the Developing World. Colorado State University, Fort Collins, Colorado.

(59) Wood, A. D., Ruff, J. F., and Richardson, E. V. 1977. Pumps and Water Lifters for Rural Development. Colorado State University.

Both of the above publications provide good descriptions of the types of pumps and water lifters used worldwide, along with some appropriate modifications. These references may be hard to come by except through the authors or university library.


<<TOC2>> Appendix F - Glossary of terms

GLOSSARY OF TERMS

ALKALINE: pH greater than 7.

ALLOWABLE SOIL DEPLETION, p AW: for given soil and climate, depth of soil water in the root zone readily available to the crop allowing unrestricted evapotranspiration; the fraction p of available soil water (see below); mm/m or inches/ft of soil depth.

AMENDMENT: chemicals added to soil or water in order to improve certain soil water properties such as infiltration rate or soil chemistry.

AMORTIZATION: gradual repayment of an amount or debt through regular payments over time. Depreciation is a form of amortization used to estimate the value of an asset over time at a given interest rate. Debt payments are often amortized over time. The capital recovery factor (see below) consists of an interest and an amortization component.

ANAEROBIC: the absence of oxygen.

ANNUAL EQUIVALENT: a series of equal annual amounts for a determined number of years that, when discounted at an appropriate interest rate, will sum to a specific present worth. The annual amount is calculated by multiplying the present value by the capital recovery factor (see below) for the appropriate interest rate and length of time.

AQUIFER: a water-bearing layer (stratum) of permeable rock, sand or gravel.

ARTESIAN WELL: a well that reaches water that, from internal pressure, flows up like a fountain.

ASSET: a business accounting term. Everything an individual or a company owns and that has a monetary value, such as cash, machinery, buildings, and land.

AVAILABLE HEAD: difference between the elevation of an upper water surface and a lower surface, such as a field or water surface.

AVAILABLE SOIL WATER, AW: depth of water stored in the root zone between field capacity and crop wilting point; mm/m or inches/ft of soil depth.

BASIC INTAKE RATE: rate at which water will enter soil when, after the initial wetting of the soil, the rate becomes essentially constant; mm/hr or in/hr.

BENEFIT/COST RATIO (B/C Ratio): selection criterion used in evaluating projects. The present worth of project's benefits is weighed against the present worth of its costs.

BENEFIT: in project analysis, any good or service produced by a project that benefits those for whom the project is being undertaken.

BHP: see Horsepower, Brake Horsepower.

BIT: a piece that operates at the bottom end of the tool string to loosen rock or soil in order to deepen a well being drilled.

BOTTOM SECTION: the part of a well that extends below the water table.

CAPACITY OR DISCHARGE: the rate of flow of liquid per unit time, as gpm or L/sec.

CAPITAL RECOVERY FACTOR: a factor used to calculate the annual value of an amount, machinery, or other asset over its expected life. It is used to calculate the equal installments necessary to repay (amortize) a loan over a period of time. The factor includes compound interest.

CASING: metal or plastic pipe used to keep open the drilled or excavated hole in a well.

CATION EXCHANGE: the interchange between one cation (positive charged ion) in solution and another on any negatively charged surface, such as clay or organic colloids.

CAVITATION: the vaporization of a pumped fluid as it goes through the pump impeller. The formation and collapse of vapor pockets as the liquid goes through the pump.

CEC (CATION EXCHANGE CAPACITY): the sum of all exchangeable cations that a soil can absorb; meq/100 gm.

CENTRIFUGAL PUMP: a pump in which water enters the center of the impeller and proceeds radially outward through the impeller.

CHECK DROP: check structure combined with a drop (see below) for dual-purpose function.

CHECK, CHECK STRUCTURE: structure built or placed across a channel at suitable points to control water levels and regulate water supply. Stop logs and check panels are the moveable sections placed in slots to control depths.

CHLOROSIS: general yellowing of plant tissue (to various degrees) caused by absence of chlorophyll. Can be due to absence of essential nutrient or other damage to plants.

CHUTE: an inclined drop or fall in which the lowering of the water surface is achieved over a relatively short length of channel.

COLLOID: matter having very small particle size and large specific surface (surface area per unit mass of material). In soils, these can be clays or organic matter.

COMPOUNDING: calculating the future value of a present amount that is growing at a given interest rate.

CONSUMPTIVE USE: the total amount of water taken up by vegetation for transpiration or building of plant tissue, plus the unavoidable evaporation of soil moisture, snow and intercepted precipitation associated with vegetation (also see evapotranspiration).

CONVEYANCE EFFICIENCY, Ec: ratio between irrigation water received at the inlet to a block of fields and that released at the project's headworks; a fraction.

CRITICAL PERIOD: periods during crop growth when soil water stress will have a lasting effect on crop growth and yields.

CROP COEFFICIENT, kc: ratio between crop evapotranspiration (ETcrop) and the reference crop evapotranspiration (ETo) when crop is grown in large fields under optimum growing conditions; ETcrop = kc ETo; fraction.

CROP EVAPOTRANSPIRATION, ETcrop: rate of evapotranspiration of a disease-free crop growing in a large field (one or more ha) under optimal soil conditions, including sufficient water and fertilizer, and achieving full production potential of that crop under the given growing environment; includes water loss through transpiration of the vegetation and evaporation from the soil surface and wet leaves; mm/day or in/day.

CROP WATER REQUIREMENTS: depth of water required by a crop or a group of crops for evapotranspiration (ETcrop) during a given period.

CROPPING INTENSITY: total cultivated area on a farm divided by total cropland. With multiple cropping, this value can be greater than 1.

DEEP PERCOLATION: the drainage of soil water by gravity below the maximum effective depth of the root zone.

DEFLOCCULATE: to disperse particles, such as clay particles in soil, by chemical or physical means.

DEPRECIATION: the reduction in value of an asset through wear and tear over time. Because actual depreciation cannot be measured until the end of the life of an asset, estimates are made using various accepted methods, including the "straight line method" and the "reducing-balance method."

DEPTH OF IRRIGATION, d: depth of irrigation, including application losses, applied to the soil in one irrigation application and that is needed to bring the soil water content of the root zone to field capacity; mm or inches.

DESILTING BOXES, SAND TRAPS: structures that reduce flow velocities so that sand and silt can settle and be removed.

DEVELOPMENT STAGE: for a given crop, the period between end of initial (emergence) stage and full ground cover, or when the ground cover is between 10% and 80%; days.

DISCOUNTING: the process of finding the present worth of a future amount.

DISTRIBUTION EFFICIENCY, Ed: ratio of water made directly available to the crop and that released at the inlet of a block of fields; Ed = Eb Ea; fraction.

DRAINAGE STRUCTURES: structures used for removing excess water away from irrigated areas into a drainage system.

DRAWDOWN: the elevation difference between the static water level and the pumping level of a liquid.

DROP PIPE: that section of pipe in a deep well pump assembly that extends between the pump cylinder and the pump body.

DROP STRUCTURE: a structure designed to lower the water surface in a channel in a short distance with safe dissipation of energy.

DROP: a farm structure built to mitigate excess grade when the slope of a ditch is greater than the grade that should be used for the ditch. Erosive velocities are reduced upstream.

EFFECTIVE FULL GROUND COVER: percentage of ground cover (specific to crop) when ETcrop is approaching maximum generally 70 to 80% of surface area; percentage.

EFFECTIVE RAINFALL, or EFFECTIVE PRECIPITATION, Pe: rainfall useful for meeting crop water requirements; it excludes deep percolation, surface runoff, and interception; mm/period or in/period.

EFFECTIVE ROOTING DEPTH, D: soil depth from which the full grown crop extracts most of the water needed for evapotranspiration; m or ft.

EFFICIENCY, PUMP EFFICIENCY: the ratio in a pumping plant between power output (Water Horsepower -- WHP) and power input (Brake Horsepower -- BHP); percent.

EFFICIENCY, MOTOR OR ENGINE

EFFICIENCY, OVERALL PUMPING PLANT = output Water Horsepower (WHP)/Input Horsepower to motor.

EFFICIENCY, TRANSMISSION: efficiency of the gearhead, belt drivers, and other components of the pump.

ELECTRICAL CONDUCTIVITY, EC: the property of a substance to transfer an electric charge (reciprocal of resistance); used as a measure of the level of salinity.

ELECTRICAL CONDUCTIVITY, IRRIGATION WATER, ECw: is used as a measure of the salt content of irrigation water; mmhos/cm or dS/m.

ELECTRICAL CONDUCTIVITY, SATURATION EXTRACT, ECe: is used as a measure of the salt content of an extract from soil that has been saturated with water; mmhos/cm or dS/m.

EVAPORATION, E: rate of water loss from liquid to vapor phase from open water or wet soil surface by physical processes; mm/day or in/day.

EVAPOTRANSPIRATION, ET: rate of water loss through transpiration from vegetation plus evaporation from the soil; mm/day or in/day.

EXCHANGEABLE SODIUM PERCENTAGE, ESP: the percent of the total Cation Exchange Capacity (CEC) of a soil occupied by sodium, i.e. the percent of the soil exchange sites occupied by exchangeable sodium.

EXTRA-TERRESTRIAL RADIATION, Ra: amount of solar radiation received on a horizontal at the top of the atmosphere; equivalent evaporation mm/day.

FARM GATE: the boundary of a farm; used in economics to delineate a boundary such as farm-gate price.

FIELD APPLICATION EFFICIENCY, Ea: ratio of water made directly available to the crop and that received at the field inlet.

FIELD CAPACITY, S: depth of water held in the soil after ample irrigation or heavy rain when the rate of downward movement has substantially decreased, usually 1 to 3 days after irrigation or rain.

FIELD SUPPLY SCHEDULE: stream size, duration, and interval of water supply to the individual field or farm.

FLOW RATE: the amount of water per unit time flowing past a point; L/sec or ft/sec.

FULL GROUND COVER: soil covered by crops approaching 100% when looking downwards from above.

GLEY: soil developed under conditions of poor drainage resulting in the reduction of metal elements and in a grey color with mottles at interfaces with better aerated soils.

GROUND COVER: percentage of soil surface shaded by the crop if the sun were directly overhead; percentage.

GROUND WATER TABLE: upper boundary of ground water where water pressure is equal to atmosphere, i.e. depth of water level in borehole when ground water can freely enter the borehole.

GROWING SEASON: for a given crop the time between planting or sowing and harvest; days.

HARDPAN: hardened soil layer caused by the cementing of soil particles due to physical processes such as compaction or chemical processes, for example, by sodium. The hardness does not change appreciably with changes in moisture content.

HEAD: the height of a liquid column above a specific point or the equivalent height for a given pressure.

HEAD, AVAILABLE HEAD: difference between the elevation of an upper water surface and a lower surface, such as a field or water surface.

HEAD, DISCHARGE HEAD: the head at the discharge of the pump. The pressure reading of a pressure gauge is converted to elevation of the liquid and velocity head (see below) at the point of gauge attachment.

HEAD, ELEVATION HEAD: the difference in elevation between two points in the system.

HEAD, FRICTION HEAD: the energy losses due to friction (resistance to water flow) between two points in the distribution system.

HEAD, HYDRAULIC HEAD: depth of water as referenced to a lower elevation. Height that water will stand in a tube. Energy available.

HEAD, LOSS: energy lost as a result of friction, impact or turbulence. Simply, the difference in head of two water surfaces connected by pipes or channels.

HEAD, NET POSITIVE SUCTION HEAD REQUIRED: the net positive suction head required to prevent cavitation.

HEAD, NET POSITIVE SUCTION HEAD: the total head at the suction flange of the pump less the vapor pressure of the liquid in the same units.

HEAD, PRESSURE: the pressure at a point expressed as an equivalent head of water, e.g. 10 psi = 23.1 ft of water or 1 kg/cm = 10 meters of water.

HEAD, STATIC: the elevation difference between a reference point on the system and the highest point on the system. The total static head is the difference between the pumping level (free water surface) and the highest point in the system.

HEAD, TOTAL DYNAMIC HEAD (TDH): the total head (energy) supplied by the pump to the liquid. It is the total discharge head at the discharge flange (including velocity head).

HEAD, VELOCITY: the kinetic energy of the flowing liquid in a pipeline.

HEADGATE: structure at the head of a watercourse, farm lateral, or field lateral that connects with the distributing channel. The turnout may be placed through the banks of the tertiary and quaternary canals for water delivery to fields.

HORSEPOWER, BRAKE HORSEPOWER: power required to drive a specific mechanical component.

HORSEPOWER, INPUT HORSEPOWER: the horsepower supplied to the prime mover (the power unit) of the pumping plant (may be electrical or other type of fuel).

HORSEPOWER, WATER HORSEPOWER: the horsepower that the pump imparts to the liquid.

HORSEPOWER: energy per unit time; 1 HP = 550 ft lb/see or 1 HP = 0.746 kw.

HYDRAULIC GRADE LINE: in an open channel, the water surface is the hydraulic grade line; in a closed pipe, the line joining the elevations to which water would stand in open gage tubes.

HYDRAULIC GRADIENT: slope of the hydraulic grade line.

HYDRAULIC HEAD: depth of water referenced to a lower elevation. Height that water will stand in a tube. Energy available.

HYDRAULIC RADIUS: area of the flowing water divided by the wetted perimeter. For pipes flowing full, this is equal to the diameter divided by four.

IMPELLER: the rotating components of the pump that impart energy to the liquid. Water enters the eye of the impeller and gains energy as it moves radially outward.

INITIAL DEVELOPMENT STAGE: for a given crop, the time between germination and early growth, when ground cover is less than 10%; days.

IRRIGATION EFFICIENCY: the ratio of the volume of water required for a specific beneficial use as compared to the volume of water delivered or actually used for this purpose. Commonly interpreted as the volume of water stored in the soil for evapotranspiration compared to the volume of water delivered for this purpose, but may be defined and used in different ways.

IRRIGATION INTERVAL, i: time between the start of successive field irrigation applications on the same field; days.

IRRIGATION REQUIREMENTS, LR: depth of water required for meeting evapotranspiration minus contribution by effective rainfall, ground water, and stored soil water; depth of water required for normal crop production plus leaching requirement, water losses, and operational wastes; sometimes called gross irrigation requirements. (See Net Irrigation Requirement.)

IRRIGATION SCHEDULING: the process of determining the amount and timing of water application or delivery to a farm or group of farms.

LATE SEASON STAGE: time between the end of the mid-season stage and harvest or maturity; days.

LEACHING REQUIREMENT: the fraction of water entering the soil that must pass through the root zone in order to prevent soil salinity from exceeding a specific value.

LEVEL: (adjective) perfectly horizontal; (noun) a device used to establish a perfectly horizontal line.

MARGINAL ANALYSIS: analysis of the effect of changing one variable upon another variable, other variables held constant. For example, varying the rate of fertilizer has an effect on yield; the additional cost of the fertilizer can be evaluated holding other costs constant. Marginal analysis is an important concept in economic analysis.

MEASURING STRUCTURES: weirs and other structures used to determine depth-discharge relationship.

NET BENEFIT: in project analysis, the amount remaining after all outputs are subtracted from all inputs, for example, the net cash flow.

NET IRRIGATION REQUIREMENT, In: depth of water required for meeting evapotranspiration minus contribution from precipitation, ground water, and stored soil water; does not include operational losses and leaching requirements.

NET PRESENT WORTH: in project analysis, a discounted measure of project worth, or the present worth of a stream of benefits minus the present worth of the stream of costs. Can be used as a selection criterion.

OPPORTUNITY COST: the benefit foregone by using a scarce resource for one purpose instead of for its next best alternative use.

OSMOTIC EFFECT: the force a plant must exert to extract water from the soil. The presence of salt in the soil water increases the force a plant must exert to withdraw water from soil.

PERCUSSION: a method of drilling a well by repeatedly dropping a bit.

PERMEABILITY: a measure of the speed at which water can move through a certain type of rock or soil. For example, sand is more permeable than clay because water moves faster through sand.

pH: a measure of acidity or alkalinity ranging from 1-14. It is the negative logarithm of the hydrogen ion activity.

PRECIPITATION: total amount of precipitation (rain, drizzle, snow, hail, fog, condensation, hoarfrost, frost, and rime), expressed in depth of water, that would cover a horizontal plane if there were no runoff, infiltration, or evapotranspiration.

PRESENT WORTH: the present value of an amount to be paid or received at some future date.

PRIMING: prefilling a structure, such as a suction tube or a centrifugal pump, with water before operation.

PROJECT CYCLE: the series of analytical phases through which a project passes, such as identification, planning, implementation, evaluation, and appraisal.

PROJECT: an investment activity upon which resources (costs) are expended in order to create assets that will produce benefits over an extended period of time.

PUMP: a device used to lift water or to provide pressure to water.

PUMPING LEVEL: the vertical distance from the centerline of the pump discharge to the free water surface from which the water is being drawn.

QUATERNARY CANALS (field laterals): canals branching from the minors and supplying water to outlets or turnouts. (head ditch, (USA); marwa (Egypt); watercourse (India and Pakistan)).

RATE OF RETURN: payment on an investment as a proportion or percentage of that investment.

REFERENCE CROP EVAPOTRANSPIRATION, ETo: rate of evapotranspiration from an extended surface of an 8 to 15 cm tall, green grass cover of uniform height, actively growing, completely shading the ground, and not short of water; mm/day or in/day.

RISK ANALYSIS: an analytical technique in which the probabilities of possible scenarios for all critical elements of a project are computed or evaluated.

ROTARY: a method of drilling a well by rotating a bit in a hole as the well is drilled.

SALINE SOIL: a non-alkali soil containing soluble salts in such quantities that they interfere with the growth of most plants.

SECONDARY CANALS, DISTRIBUTARY CANALS: canals that branch from main canals or branches and supply water to minors, outlets, and turnouts.

SENSITIVITY ANALYSIS: a technique by which a systematic analysis of the impact of different circumstances on the earning capacity of a project is undertaken. For example, uncertainty about future interest rates on loans would involve an analysis of a project using different interest rates.

SIPHON TUBES: pipes shaped in such a manner that they can be lain across a ditch bank and used to siphon water from that ditch into a field or furrow.

SODIC SOIL: an alkali soil containing exchangeable sodium in such quantities that it interferes appreciably with soil infiltration and structure and affects the growth of most plants.

SODIUM ABSORPTION RATIO, SAR: a ratio for soil extracts and irrigation water used to express the relative activity of sodium ions in exchange reaction with the soil; me/l.

SOIL AMENDMENTS: a substance or material that improves soil by modifying its physical properties rather than by adding appreciable quantities of plant nutrients.

SOIL INTAKE (INFILTRATION) RATE: instantaneous rate at which water will enter the soil.

SOIL STRUCTURE: arrangement of soil particles into aggregates that occur in a variety of recognized shapes, sizes, and strengths.

SOIL TEXTURE: characterization of soil in respect to its particle size and distribution.

SOIL WATER CONTENT: depth of water held in the soil; mm/m soil depth or in/ft soil depth.

STATIC LEVEL: the vertical distance from the centerline of the pump discharge flange to the free water surface while no water is being pumped.

STORED SOIL WATER, Wb: depth of water stored in the root zone from earlier rains, snow, or irrigation applications that partly or fully meets crop water requirements in the following periods; mm or in.

SUPPLY SCHEDULE: stream size, supply duration, and supply interval of irrigation water supply to field or irrigation block during the growing season.

TERTIARY CANALS OR FARM LATERALS: canals branching from secondary distributaries and supplying water to sub-minors, outlets or turnouts (laterals, (USA); meska (Egypt); minors (India and Pakistan)).

TRANSPIRATION: rate of water loss through the plant that is regulated by physical and physiological processes.

TURBINE PUMPS: a centrifugal pump designed for installation in a well. The bowls are usually set down in the water. Multistage assemblies may be set down at successive depths.

TURNOUT: structure that releases the water from a head ditch. Can be used to allow the water to pass through the banks of the head ditch onto a field, thus acting as a check gate and a headgate at the same time.

UNIFORMITY: the evenness with which a crop grows, water is applied, or water penetrates into the soil after an irrigation.

VOLUTE CASE: the case of a centrifugal pump in which the high velocity water coming through the impellers is converted to pressure head.

WATER CONTROL STRUCTURES: canals, flow measuring devices, check dams, diversion structures, and any structural methods employed in controlling the amount, direction, depth, or volume of water.

WATER SOURCE: any place where water can be obtained; for example a well, spring, river, lake, reservoir, tap, and faucet.

WATER TABLE: the upper limit of that portion of the ground that is wholly saturated with water.

WATER USER ORGANIZATION: an association of irrigators formed for the purpose of administering and operating an irrigation system.

WATERLOGGING: maintenance of saturated or near-saturated soil conditions in the root zone for an extended period of time.

WELL DEVELOPMENT: the process of re-arranging the soil particles around the intake section of a well to permit easier and better water flow into the well.

WELL: a hole in the ground that reaches below the water table and that is used as a source of water.

WHP: see Horsepower, Water Horsepower.

TO PREVIOUS SECTION OF BOOK