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CLOSE THIS BOOKIrrigation Reference Manual (Peace Corps, 1994, 485 p.)
Chapter 1 - Introduction
VIEW THE DOCUMENT1.1 The role and purpose of irrigation
VIEW THE DOCUMENT1.2 Introduction to the irrigation reference manual
2.1 Watersheds
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENT2.1.1 Watershed hydrology
VIEW THE DOCUMENT2.1.2 Hydrologic processes
VIEW THE DOCUMENT2.1.3 Assessing watershed conditions
VIEW THE DOCUMENT2.1.4 Soil and water conservation practices
2.2 Water flow measurement
VIEW THE DOCUMENT2.2.1 Units of measurement
VIEW THE DOCUMENT2.2.2 Measuring devices in open channels
VIEW THE DOCUMENT2.2.3 Float method
VIEW THE DOCUMENT2.2.4 Weirs
VIEW THE DOCUMENT2.2.5 Siphon tubes
VIEW THE DOCUMENT2.2.6 Bucket and stopwatch method
VIEW THE DOCUMENT2.2.7 Orifices
2.3 Surveying
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENT2.3.1 Profiling
VIEW THE DOCUMENT2.3.2 Steps in making a topographic man
VIEW THE DOCUMENT2.3.3 Abney level surveying
VIEW THE DOCUMENT2.3.4 Simple levels for use in surveying contour lines
VIEW THE DOCUMENT2.3.5 Compass use
2.4 Soil-Plant-Water relationships
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENT2.4.1 Soil moisture storage and availability
VIEW THE DOCUMENT2.4.2 Estimating soil water characteristics on site
VIEW THE DOCUMENT2.4.3 Development of the soil water reservoir
VIEW THE DOCUMENT2.4.4 Soil water availability and crop use patterns
VIEW THE DOCUMENT2.4.5 Soil intake characteristics
VIEW THE DOCUMENT2.4.6 Soil chemistry and fertility
2.5 Conducting initial environmental evaluations of irrigation projects
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENT2.5.1 The role of environmental assessment
VIEW THE DOCUMENT2.5.2 Illustrative environmental review form for irrigation or water resource development projects
3.1 Diversions
VIEW THE DOCUMENT3.1.1 Types
VIEW THE DOCUMENT3.1.2 Types of construction materials
VIEW THE DOCUMENT3.1.3 Construction and maintenance factors
VIEW THE DOCUMENT3.2.1 Hand mixing
VIEW THE DOCUMENT3.3.1 Spring box designs
VIEW THE DOCUMENT3.4.1 Location of the pond
VIEW THE DOCUMENT3.4.2 Availability of water
VIEW THE DOCUMENT3.4.3 Soils
VIEW THE DOCUMENT3.4.4 Topography
VIEW THE DOCUMENT3.4.5 Design end construction
3.5 Pumps and water lifting devices
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENT3.5.1 Types of pumps
VIEW THE DOCUMENT3.5.2 Pumps powered by humans and animals
VIEW THE DOCUMENT3.5.3 Animal-powered pumps
VIEW THE DOCUMENT3.5.4 Mechanically driven pumps
VIEW THE DOCUMENT3.5.5 Centrifugal pumps
VIEW THE DOCUMENT3.5.6 Propeller or axial flow pumps
VIEW THE DOCUMENT3.5.7 Mixed flow pumps
VIEW THE DOCUMENT3.5.8 Turbine pumps
VIEW THE DOCUMENT3.5.9 Sources of power
VIEW THE DOCUMENT3.5.10 Selection of pumps and power units
VIEW THE DOCUMENT3.5.11 Amount of water to be pumped
VIEW THE DOCUMENT3.5.12 The pumping lift or head
VIEW THE DOCUMENT3.5.13 Horsepower and efficiency
VIEW THE DOCUMENT3.5.14 Pump characteristic curves
VIEW THE DOCUMENT3.5.15 Power source and power Costs
VIEW THE DOCUMENT3.5.16 Pump location
VIEW THE DOCUMENT3.5.17 Pump installation
VIEW THE DOCUMENT3.5.18 Turbine and propeller pumps
VIEW THE DOCUMENT3.5.19 Intake structures
VIEW THE DOCUMENT3.5.20 Minimum water level
VIEW THE DOCUMENT3.5.21 Typical pump design
VIEW THE DOCUMENT3.5.22 Size of pumps
VIEW THE DOCUMENT3.5.23 Costs
VIEW THE DOCUMENT3.5.24 Evaluation of pumping plants
VIEW THE DOCUMENT3.5.25 Rower puma
VIEW THE DOCUMENT3.5.26 Hydraulic ram
3.6 Wells
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENT3.6.1. Methods of drilling: Percussion drilling
VIEW THE DOCUMENT3.6.2 Methods of drilling: Hand auger rig
VIEW THE DOCUMENT3.6.3 Other drilling equipment
VIEW THE DOCUMENT3.6.4 Hand dug wells

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

Chapter 1 - Introduction

1.1 The role and purpose of irrigation

Irrigation is defined as the artificial application of water onto cropland for the purpose of satisfying the water requirements necessary for growing crops. Irrigation plays a key role in stabilizing food production in a number of countries by either supplementing or replacing the need for natural precipitation for the purpose of food production.

Irrigation is a key to the ability of many farmers, and even nations, to feed themselves and provide an adequate standard of living. Irrigation not only protects against drought but brings with it numerous other benefits as well as occasional problems.

Irrigation has been credited with being a primary factor in the rise and fall of civilizations. For example, in the region of Mesopotamia about 4,000 years ago, a thriving civilization depended on a highly developed irrigation system. Waterlogging and salinization, as well as the erosion and sedimentation resulting from irrigation, were instrumental in bringing about the collapse of that empire. To this day, much of the land remains saline and has not been recovered for crop production.

Currently, about one-fourth of the cultivated land in the world is irrigated. In the United States, the 10% of cultivated land that is irrigated provides some 25% of the value of agricultural production.

Irrigation can result in a number of benefits for the farmer and his or her community. Irrigation stabilizes farm production by protecting against drought and by increasing crop yields and quality when rainfall is insufficient. It permits farmers to grow moisture-sensitive, high-value crops and crops that will improve their diet. In some areas with proper climates, irrigation allows farmers to raise two or three good crops in a year. It allows them to plant on time, thus optimizing market conditions. In some areas, irrigation systems are used for frost protection. There are numerous problems, however that can be caused by poor design, construction, and management of irrigation systems. Salinization and waterlogging are other results. Poor design and management of systems often result in irrigation of only one-half or one-third of the potential area. Thus, costs per unit area may be very high, and the benefits of irrigation may extend to only a portion of the farmers who could use the water.

Irrigation is only one of many inputs to a farmer's sustainable agricultural system. Cultural practices, farmer resources, farmer preferences, and other factors will affect the selection, design, construction, and operation of an irrigation system. Therefore, it is very important that those who work in irrigated agriculture understand clearly not only the benefits and consequences of irrigation but also what it takes to maximize or optimize the benefits.

This manual was written in response to the need for providing technical information to those who work with small-scale farmers in areas where rainfall is deficient and where irrigation water is available or can be developed from existing water sources.

1.2 Introduction to the irrigation reference manual

The Irrigation Reference Manual is designed to complement and support the materials covered in the Irrigation Training Manual. These reference materials should provide sufficient background information to allow trainers, trainees or Volunteers complete basic tasks in organizing and mobilizing communities, assessing and developing water sources, and designing and managing irrigation projects. The material is not intended to be all-inclusive but rather to provide enough technical coverage to allow basic concepts to be understood and basic construction or application procedures to be applied. The Irrigation Reference Manual also includes an annotated bibliography to point trainers or other users towards other references if more detailed descriptions of any topics are needed.

The Manual has been designed to correlate information directly with the format of the Irrigation Training Manual. All of the training section headings are represented in this reference manual. Trainers or other users should be able to immediately access technical descriptive or illustrative materials that will support a specific training topic. In addition, the Reference Manual concludes with an index to assist in locating materials pertaining to any particular topic area.

The Reference Manual is organized to discuss the following:

Chapter 2 - Physical and Biological Resource Base

Trainers or other users are provided with sufficient background to interpret basic hydrologic processes, measure the supply of water available to support projects, survey an area of land, describe the relationship between soils, water, and plant development, and prepare a simple assessment of potential project environmental impacts. This information represents the foundation on which any irrigation project will be designed, constructed, or managed. A variety of techniques are included for each topic area and all of the methodologies are appropriate to the typical working conditions experienced by Peace Corps Volunteers.

Chapter 3 - Developing Water Sources

This material allows users of the manual to begin working with simple construction practices necessary to capture, convey, store, and lift water supplies. Basic principles in the use of concrete are described. Illustrations and descriptions of pumping devices typically available to Volunteers are also included.

Chapter 4 - Estimating Irrigation Requirements

This material provides representative formulas, charts, and case examples that enable users to estimate the amount of water necessary to sustain a crop. Information allowing the users to calculate water use for more than 25 different crops is included.

Chapter 5 - Farm Water Delivery Systems

Detailed discussions of the concepts and applications of hydraulic principals are included in this chapter. Students typically benefit from access to careful and thorough explanation of these concepts, and the text has been developed to make these principals as practical as possible. The chapter also includes sufficient conceptual and illustrative information to allow trainers to communicate the factors involved in designing and implementing irrigation systems using surface, sprinkler, or drip application methods.

Chapter 6 - Farm Water Management

The Manual provides a thorough description of the principals and procedures followed in preparing a schedule of water use on farms and methodologies that can be used to evaluate how effectively water and land is being used by the farmer. The chapter concludes with a comprehensive set of illustrations that communicate the techniques of irrigation scheduling in a non-verbal manner.

Chapter 7 - Waterlogging and Salinity

Technical information is presented to enable trainers to describe and demonstrate techniques for assessing the degree of waterlogging and salinity problems that may be occurring and to apply chemical amendments or cultural practices that can minimize or avoid such problems. The chapter also includes a concise description of water quality as it affects irrigation projects.

Appendix A - Math Skills and Tool Use

Common formulas, conversion charts, and algebraic and trigonometric values are included to enable trainers or other users to easily apply mathematical formulas. A list of tools commonly available to Peace Corps Irrigation Volunteers is also included.

Appendix B - Community Organization and Development

A concise review of basic concepts important in working with people-centered agricultural projects is presented. The discussion emphasizes basic techniques that can assist a Volunteer's efforts to enter, interact, and participate in community project efforts. The level of detail in this appendix is limited because it is anticipated that Volunteers will be supplied with more comprehensive community organization literature. In particular, it is anticipated that trainers will attempt to provide each Volunteer with a copy of Two Ears of Corn, an excellent reference book to guide any community-based agricultural involvement.

Appendix C - Summary of International Irrigation Center (IIC) Training Modules

The IIC has prepared a collection of 40 video modules that present basic irrigation principals and practices. These modules were specifically developed for conditions in Ecuador, but many have universal applications. A description of the length, content, and applicability of each module to Peace Corps training sessions is included for those trainers who may have access to video equipment and the actual video cassettes.

Appendix D - Case Studies

Trainers can review these examples of different problems and strategies used to develop small-scale irrigation systems worldwide to support the material content in the technical training sessions. The case studies are intended to be representative of typical conditions that may be experienced by a Peace Corps Volunteer working with irrigation issues.

Appendix E - Annotated Bibliography

The Irrigation Reference Manual is intended to serve as a primary source for basic information needed to design, implement, and manage small or medium-scale irrigation projects. It is not, however, the only reference that a practitioner would want to use. The extensive bibliography includes a concise description of the specific values of each text that can be consulted for further information by trainers or Volunteers.

Appendix F - Glossary of Terms

Concepts and principals can be most easily grasped when they are presented in simple, concise descriptions. The glossary should assist trainers in preparing descriptions that will enable trainees to comprehend terms quickly and accurately. It is anticipated that the Irrigation Reference Manual will be made available to Trainees, Volunteers, and other irrigation practitioners on an as-needed basis. The material included in the Manual would be appropriate to support a Peace Corps Volunteer's efforts in the field throughout his or her term of service, and trainers may wish to request additional copies to distribute to Trainees during the training. Alternatively, sections of the Manual can be photocopied and distributed to support specific training sessions.


<<TOC2>> Chapter 2 - Physical and biological resource base

Reference:

Primary:

(2), (9), (14), (18), (20-21), (27-28), (39), (44), (56-59)


Other:

(16), (24)

2.1 Watersheds

A watershed is the area representing all the land draining moisture from the highest elevations, usually referred to as the headwaters, to a specified outlet point. A single watershed can include all of the lands draining into a main channel, often a river, including the tributary channels of the entire river basin. For example, the Senegal River basin in West Africa with its tributaries covers several hundred thousand hectares of land. A watershed can also be subdivided to include only those lands draining a single tributary, which may include only a few hundred hectares.

Watersheds are typically delineated using topographic maps. By following the contour lines on the map, the dividing points between drainage basins can be determined and the directions for flows identified (Figure 2.1). Once the watershed is delineated, soil, water, and ecological resources within the basin can be identified, and measures to protect these resources can be developed.

Irrigation specialists are concerned with protecting and developing the soil and water resources within watersheds and ensuring that irrigation projects do not result in any disruptions to balanced ecological processes. Specifically, it is important for an irrigation specialist to protect the quantity and quality of water obtainable from a water source, avoid conditions that promote flooding or extreme fluctuations in water availability, and ensure that erosion does not result in a loss of agricultural land or clogged canals and ponds.

Assessing and developing water resources requires an understanding of the hydrologic processes influencing the movement and storage of water within watersheds. Protecting the soil and water resource base requires a knowledge of watershed conditions and implementation of conservation measures that promote reliable water supplies of acceptable quality and minimal soil loss.

2.1.1 Watershed hydrology

The hydrologic cycle (Figure 2.2) is easily understood: water evaporates from the earth's oceans and other water bodies, is carried by air currents, condenses due to temperature changes, falls to earth again as precipitation, and finally flows back to water bodies to begin the cycle over again.

Within this cycle there are several processes that affect the timing and quantity of water moving through each phase. For example, as precipitation falls some of it will be intercepted by vegetation before reaching the ground. Some of this intercepted moisture will evaporate from plant surfaces directly back to the atmosphere while the remainder will reach the ground surface. Some water reaching the ground surface will evaporate, some will penetrate the soil surface, and some may run off as surface flow. The permeability of the soil surface will determine the rate and amount of water that seeps into the ground. Water infiltrating into the ground provides nutrients to plant roots to support their growth, recharges springs and aquifers, and moves slowly downslope through the soil pore spaces to recharge surface lakes and rivers.


Figure 2.1 Watershed Boundaries on Contour Map


Figure 2.2 The Hydrologic Cycle (Ref. 21)

Human resource management practices often greatly influence the hydrologic cycle processes. For example, the type of vegetation present will influence the amount of precipitation intercepted and the rate at which water can infiltrate into the ground. The area covered by vegetation also influences the amount of soil moisture that is recycled to the atmosphere through evaporation from exposed surfaces and plant transpiration. Land management decisions often directly influence the type and amount of vegetative cover present. Farming practices and other land use characteristics will influence soil characteristics and thus the amount and quality of water infiltrating into soils.

Water quality issues affect irrigation specialists primarily from the perspective of salinity and sedimentation. The problems of high salt content in water is discussed in detail in Chapter 7. Sedimentation is the result of poor land management that causes excessive soil loss. Eroded soils can clog canals and diversions, disrupt pipelines, fill in farm ponds or reservoirs, contaminate wells, or result in a loss of arable lands. Being able to measure hydrologic processes, and using this information to assess watershed conditions, is necessary in order for Volunteers to effectively control water quality concerns.

2.1.2 Hydrologic processes

There are standard methods used for quantifying and describing hydrologic processes. Specifically, irrigation technicians should be familiar with the concepts of precipitation, infiltration, surface runoff, evapotranspiration, streamflow, and groundwater yields.

Precipitation is usually characterized in terms of intensity, storm duration, and area covered. Rainfall intensity refers to how much precipitation occurs within a given time period. It is typically expressed in millimeters per hour (or inches per hour) and usually measured by seeing how much rain fills a container of known volume in a specific period of time. The length of time, or duration, of rain fall is expressed in minutes or hours and is directly correlated with rainfall intensity. For example:

Depth in mm

13.8

22.4

45

64

108

Duration (Min)

5

10

30

60

360

Intensity, mm/hour

166

134

90

64

18

Infiltration indicates how much water is absorbed into the ground during a specified period and is typically expressed as a rate (e.g. mm per hr or cm per hour). Infiltration capacity describes the maximum amount of water that will infiltrate into a particular soil within a specific time period. If the rainfall exceeds the infiltration capacity during the specified period, then the excess water begins moving over the soil surface as runoff. The infiltration capacity is determined by soil texture and structure. Soil texture indicates the relative amounts of sand, silt, and clay particles found within the soil. Soil structure indicates the way these particles are bound together by organic materials and other adhesive substances. Typical infiltration rates for different soil texture classes are as follows:

Soil Texture Class

Infiltration capacity cm/hour

Loamy Sand

2.5 - 5.0

Loam

1.25 - 2.5

Silt Loam

0.75 - 1.45

Clay loam

0.25 - 0.5

Many soils that are initially dry will absorb large amounts of water rapidly at first, but infiltration rates decline as the soil becomes wetter. Infiltration rates can provide an important clue as to the capacity of the soil to store moisture and the rate of runoff from a watershed.

Evapotranspiration, or ET, is the combination of evaporation and transpiration. ET is generally estimated using simple formulas described in Chapter 4. ET is typically expressed as a depth over a period of time, such as mm or inches per month.

Surface Runoff describes the process of water movement over the land surface that occurs when the precipitation rate exceeds the ability of the soil to absorb the rainfall. Surface runoff is usually measured as a depth or volume over time, such as mm or cm per hour or liters per second. A hydrograph is a graphic depiction of the rate of runoff plotted over a period of time for a particular watershed. Surface runoff supplies water to lakes, ponds, wetlands, rivers, and streams. In extreme amounts, it can result in flooding and severe erosion.

Volunteers should consult local hydrologists if they suspect that flow rates might be large enough to damage irrigation structures. Stream measurements conducted by governmental agencies such as ministries of water resources, irrigation, or agriculture are important sources of streamflow information.

Agriculture are important sources of streamflow information. For small streams the information may not be available, and the Volunteer may have to measure streamflows at various times of the year (see Section 2.2 of this manual). Adjustments may need to be made to account for wet and dry years.

Aquifers or ground water reservoirs are soil, rock, or mixed materials that are totally saturated with water (Figure 2.3). The surface (top most area) of this saturated zone is called the water table. The level of the water table may vary seasonally as recharge fluctuates and people withdraw water through wells. Due to gravityground water flows from a location where the water table is higher to where it is lower.

The permeability of aquifers vary according to the aquifer material. Aquifer materials are typically a mix of consolidated and unconsolidated (or fractured) rocks. Consolidated rocks are porous materials held firmly together by compaction and cementation and are represented by sandstones, limestones, and conglomerates. Unconsolidated materials include a mix of boulders, gravel, sands, and clays.

Gravel aquifers are the most permeable and yield water easily from wells. Gravel aquifers are often sources for high capacity wells. Permeability usually relates to the coarseness of the aquifer material.

Unconfined or water table aquifers (Figure 2.3) have a free water surface. Confined or Artesian aquifers (Figure 2.3) are bounded by an impermeable or semi-impermeable layer that maintains the water in the aquifer under pressure. Wells in artesian aquifers may flow freely without the need for pumping.

One type of unconfined aquifer is known as a perched aquifer, where an impermeable layer of limited size stops the percolation of water to a deeper aquifer, thereby creating a small underground reservoir of limited volume (Figure 2.3).

Surface springs or seeps occur in places where an impermeable rock layer emerges at the ground surface. Ground water flows by gravity along this impermeable layer and exits the ground at the spring site.

Aquifers function much like a surface reservoir, except in the case of some artesian aquifers, pumps are required to extract the water from below ground. Ground water quality may also be a concern in some locations. If irrigation wells have been operating without causing problems to crop growth, the water is probably of acceptable quality. In newly pumped areas, the water should be sampled and evaluated before use in irrigation (See Chapter 7).


Figure 2.3 Types of Aquifers and Location of Water Source

2.1.3 Assessing watershed conditions

An assessment of watershed conditions should be an initial task in the development of any project. Volunteers should be prepared to spend time walking the watershed to observe and record information first-hand.

Technical aids useful in conducting an assessment of watershed conditions include maps, aerial photographs, and data forms that can be used to collect field information. Topographic maps at scales of 1:25,000 are valuable for studying specific sites within watersheds or small watersheds of less than 2 km. Larger watersheds may require map scales of 1:100,000. Maps that have been prepared to document characteristics of soils, vegetation, climate, geology, hydrology, or social conditions should also be obtained.

Aerial photographs at scales of 1:10,000 to 1:15,000 should be used wherever possible. If stereoscopic aerial photographs are available, it is important that the overlap over the area depicted is not less than 50 percent and not more than 55 percent along the flight line and 15 percent between flight lines. This makes it possible to use the photographs to create three-dimensional viewing of areas using a stereoscope.

Data forms should enable the collection of baseline technical information quickly and with sufficient depth to make professional judgments. Data forms should be prepared for information on soil characteristics, climate, water sources, vegetation, social conditions, and land use practices.

Useful tools that will facilitate field work include:

· a shovel and soil auger for investigating soil characteristics and bags to collect soil samples;
· an Abney level for measuring changes in elevations;
· maps, photographs, and colored grease pencils for marking photos;
· small, clean jars for collecting water samples; and
· clipboards and data forms.

The inventory of watershed conditions should focus on evaluating the following characteristics of the watershed:

A. Soils: list general types evident in the watershed; note areas with high potential for erosion; link soil types with natural vegetation; note soils with potential for crop productivity.

B. Topography: link steep slopes with erodible soils; note slope lengths, particularly in areas of high erosion potential; describe slope aspects.

C. Hydrology: identify all potential water supply sources; note drainage patterns, floodplains, and areas of flood potential; interpret the amount of change occurring in the shapes, depths, or directional patterns of channels; identify factors that could affect water quality, including presence of grazing animals, areas of heavy sediment loading, or heavy agrochemical use; obtain streamflow data on a daily, monthly, or annual basis depending on the needs of your inventory.

D. Vegetation: classify general vegetation types in watershed and link with soils and climatic characteristics; estimate the amount of ground that is protected or covered by the natural or planted vegetation in each vegetation type.

E. Land Use Practices: map the ways all land is currently being used within the watershed, including grazing areas, crop production lands, harvested forests; identify areas prone to inappropriate land uses; also identify environmentally sensitive or unique areas, such as wetlands, important wildlife habitat, and archeological sites.

In evaluating watershed conditions the objective is to qualify or quantify the sensitivity and resilience of the watershed. Watershed sensitivity describes a watershed's ability to withstand stress and manipulation. Watershed resilience describes the ability of a watershed to recover from damage evident on land surfaces, in stream channels, or in water bodies. All watersheds can withstand some level of impact before the quality of the soils and water degrade to a level that no longer supports biological diversity and human goals.

Watersheds that are in a declining condition typically demonstrate the following cycle of symptoms:

· reduced ground cover or increased density of drainage channels,

· increased peak flows of streams and rivers,

· deepened or widened channels resulting from the erosive power of increased peak flows,

· lowered water tables under alluvial floodplains, resulting from deepened channels and more rapid runoff,

· changes in the amount and type of streamside and floodplain vegetation, resulting from the lowered alluvial water tables,

· further channel degradation, and the development of many new side channels and gullies, resulting from changes in streamside and floodplain vegetation, and

· increased runoff rates leading to a decrease in available soil moisture, which further reduces ground cover and results in another cycle of decline in watershed condition.

Any land use or management practice that tends to speed up the delivery of precipitation to the stream channel(s) will tend to have a negative influence on watershed conditions.

2.1.4 Soil and water conservation practices

A primary objective in watershed management is to ensure balanced soil and water systems. Protecting water quality and supply and preventing erosion are the major focus of this objective.

Erosion is the removal of soil through naturally occurring processes, including wind, falling raindrops, water flowing on the surface of the ground, and the force of gravity. The impact of falling rain or wind-borne soil particles can cause more soil particles to detach and move under the force of gravity, moving water, or wind. Any factor that lessend the impact of rain or strong winds reduce the amount of soil particles detached and eroded. Removal of ground cover and changes to the natural drainage patterns are two primary causes of accelerated erosion.

Vegetative cover intercepts much of the rainfall and reduces the velocity and intensity of rain drops. Plant roots also create openings in the soil and increase infiltration of rainwater into the soil. This reduces the amount of water flowing on the surface that might otherwise accelerate the downslope movement of detached soil particles. Undisturbed forests and pastures frequently have infiltration rates that exceed rainfall rates, thus eliminating or reducing the amount of erosion due to water.

Erosion is typically classified into four categories:

Sheet Erosion - A uniform depth of surface runoff moves detached soil particles to tiny channels (rills) that have formed.

Rill Erosion - The surface of the ground is cut and deepened enough to concentrate runoff and soil particle movement in a tiny channel or rill. Rills are generally less than 1 foot or 30 cm deep.

Gully Erosion - Rills over 30 cm (one foot) deep are usually referred to as gullies. As more and more flow concentrates in rills, they deepen, speed up runoff, and lower the water table of alluvial lands. As the number of channels increase, peak flows increase and productive lands are lost.

Channel Erosion - As peak flows increase, their erosive force cuts away the banks or beds of natural stream channels, changing drainage patterns and frequently further accelerating flow rates and flooding.

Eroded soil particles are carried by flowing water or wind currents until the flow no longer has sufficient energy to carry or move a given particle. The particles are then deposited. In the case of water erosion, sediment deposits can reduce water quality, destroy the spawning and rearing areas for fish, reduce the life of.i.ponds or channels by filling and clogging, and increase downstream flooding through loss of channel capacity.

There are two simple ways to determine if serious erosion is occurring in a watershed:

1. Collect water samples at an outlet point in a drainage basin and observe the amount of sediments in the water over time.

2. Build simple runoff plots at several points in the watershed. Runoff plots can be built using large stakes or pins driven into the ground (at least 25 cm long) with a large washer at ground level. Measure the distance between the head of the stake and the top of the washer after drilling the pin into the ground. Re-measure this distance on a monthly basis for a rough estimate of monthly soil loss rates.

For example: over a one year period, the distance between the top of the pin and the top of the washer has increased by 10 centimeters (0.1 meter). Therefore, for every hectare (100 meters 100 meters) on the slope you can estimate 100 100 0.1 = 1000 cubic meters of soil loss.

It is also possible simply to use visual observations of the increasing exposure of tree roots or raised soil pedestals to indicate soil loss.

Measures for erosion control are based on either reducing the energy that detaches soil particles or increasing particle resistance to movement. Where it is not possible to reduce erosive energy and/or increase resistance to particle movement, it may be necessary to use methods that trap eroded sediments before they leave the site or are delivered to channels.

Reducing erosive energy can be accomplished by:

· increasing infiltration, primarily through increasing plant cover,

· reducing the length of slopes, primarily through berms and dips,

· diverting runoff away from disturbed areas using berms and drains,

· reducing slope gradient with check dams and land shaping,

· increasing the surface roughness of the ground to slow runoff, primarily through revegetation, mulches, and planted buffer strips, and

· avoiding the creation of unmanaged channels.

Increasing particle resistance to movement can be accomplished by:

· increasing ground cover,
· improving soil aggregate structure, for example, by increasing soil organic matter,
· lining channels, and
· conveying runoff through pipes or other medium.

Eroded sediments can be trapped using check dams, brush cover on hillsides, and earth or brush berms.

Techniques that can be applied to conserve or restore soils in a watershed or within an irrigated field include the following:

a. Protecting native vegetation.

b. Re-establishing native vegetation.

c. Establishing perennial crops (pasture, fruit trees, agroforestry systems), especially on steep slopes.

d. Practicing minimum tillage or mulching in crop cultivation systems that emphasize annual crops.

e. Using a crop rotation sequence rather than continual successive plantings of the same crop.

f. Planting strips of vegetation along the contour that serve to anchor the soil in place with their roots and slow down the movement of water downslope.

g. Constructing ditches along the contour at a 1 percent slope to divert excess water into protected drainageways.

h. Constructing terraces to provide a level platform for planted crops in combination with contour ditches, thus reducing slope gradient, slope length, and runoff velocity. Terraces are flat earth ridges, perhaps 3 meters (10 feet) wide at the base, usually constructed along a contour line. They must have enough slope to drain water, but should be less than 2 percent slope to minimize erosion. Next to tree cover, terraces probably offer the best measures for conserving soil and water on steep slopes.

i. Diverting flows in gullies and constructing check dams to trap sediments and encourage revegetation within the gully bed. Gully erosion control is extremely difficult, and success is not common. Gullies are often referred to as a kind of "cancer" on the landscape. When trying to use check dams or brush to slow the velocity of flow and encourage infiltration in gullies, it is important to work from the bottom (mouth) of the gully uphill towards the head. Starting at the top of the gully and working downhill usually results in undercutting of the check dams, as the force of water at the top is frequently too strong.

j. Constructing brush "carpets" on steep slopes by using wooden stakes to pin down leafy brush, thus slowing the velocity of the runoff and encouraging it to infiltrate into the soil.

k. Building wire mesh boxes filled with stones (gabions) and placing them in a stream channel in a manner that protects the banks and bed from the erosive force of streamflows. Gabions are flexible, permeable, and generally very inexpensive to make. They can be stacked against the sides of gullies or streams to prevent bank erosion, or staggered up steep slopes to slow runoff.

l. Protecting or replanting streamside vegetation to slow and filter runoff reaching streams and strengthen the banks of channels.

m. Reducing or eliminating unmanaged fires that would otherwise rapidly eliminate vegetative cover and increase nitrate and other contaminants in runoff.

n. Reshaping natural drainageways or digging artificial drainageways of a low, broad shape that drain excess water away from fields and protecting these drainageways from erosion by lining with rocks, planting grass, or placing drop structures or check dams periodically.

o. Selecting and planting crops in a pattern that provides maximum ground cover, aerates the soil through deep rooting, and reduces the force of runoff.

Cuttings and seedlings should be planted along with any temporary erosion control structures to insure long-term erosion control when the brushwood or stakes have decayed. Examples of these conservation measures are illustrated in Figures 2.4 through 2.17 and Tables 2.1 and 2.2.


Figure 2.4 Plugging of Smaller Gullies (Ref. 56)


Figure 2.5 Construction of a Rock Check Dam


Figure 2.6 Brushwood Check Dam (Ref. 56)


Figure 2.7 Pole or Log Check Dam


Figure 2.8 A Woven Wire Check Due


Figure 2.9 Sod Strip Checks on a Small Gully


Figure 2.10 Water Drainageway Protected Against Erosion by Rock Lining (Ref. 9)


Figure 2.11 Retention Well as a Site for Diverting Runoff eater (Ref. 9)


Figure 2.12 Discontinuous Narrow Terrace (Orchard Terrace) (Ref. 9)


Figure 2.13 Discontinuous Narrow Terrace (cross section) (Ref. 9)

- If initially constructed with an inverse slope of 15-20%, some self-compaction occurs resulting in a slope of approximately 10X.


Figure 2.14 Contour Planting Beds (Ref. 9)


Figure 2.15 Contour Infiltration Ditch (Ref. 9)

Figure 2.16 Bench Terrace Construction Sequence "A" (Ref. 9)


1. The lower most terrace is formed first and compacted thoroughly


2. me topsoil from the area of the next higher terrace is removed and distributed evenly over the lower terrace


3. The second terrace is formed and compacted, then covered with topsoil from the area of the third terrace.


4. Work progresses up slope, each newly formed and compacted terrace is covered with topsoil taken from the slope immediately above. Grass is planted along the risersof all terraces.

Figure 2.17 Bench Terrace Construction Sequence "B" (Ref. 9)


1. Terrace construction begins with the uppermost terrace and with the 2 meter segment nearest the drainage side. The topsoil is pulled over to one side of the section.


2. A well compacted section of the terrace is formed.


3. The topsoil is then redistributed over the same 2 meter terrace section.


4. Work progresses sideways a long the uppermost terrace


5. Work progresses downslope. Work begins at the drainage side of each terrace and progresses sideways


6. Grass is planted on terrace risers.

Table 2.1 Bench Terrace Construction Guide (Ref. 9)

SLOPE (%)

SOIL DEPTH* (Meters)

TOTAL TERRACE WIDTH (Meters)

PLATFORM WIDTH (Meters)


.2

1.68

1.30


.3

2.5

1.94

20

.4

3.34

2.60


.5

4.26

3.30


.6

5.02

3.90


.2

1.16

.80


.3

1.72

1.20

30

.4

2.3

1.60


.5

2.98

2.10


.6

3.46

2.40


.2

.90

.56


.3

1.32

.82

40

.4

1.78

1.10


.5

2.25

1.40


.6

2.68

1.66


.2

.74

.40


.3

1.10

.60

50

.4

1.46

.80


.5

1.84

1.00


.6

2.20

1.20

* "Depth of A Horizon" in original changed to permit use in eroded areas where horizons are often indistinct.

Table 2.2 Spacing of Contour Hillside Ditches (Ref. 9)

SLOPE (%)

ANNUAL CROP

PERSONAL CROP OR PASTURE


Distance (m)

Maximum Length (m)

Distance (m)

Maximum Length (m)

2

42.0

90



4

25.0

120



6

19.3

160



8

16.6

200



10

14.9

260

40.2

140

12

13.8

280

33.5

140

14

13.0

300

28.9

140

16

11.4

340

25.3

160

18

10.2

380

25.0

180

20

9.2

420

24.0

200

22

8.4

470

23.2

200

24

7.7

500

21.4

210

26

7.2

500

19.8

220

28

6.6

500

19.5

220

30

6.3

500

18.9

220

32



18.7

220

34



18.6

230

36



17.7

230

38



16.9

230

40



16.2

230

2.2 Water flow measurement

2.2.1 Units of measurement

Generally, water measurement units may be divided into two classes: those that express a certain volume and those that indicate a discharge or volume per unit time.

The units depend on the system being used, either metric or English. In the metric system, the more common units for volume are liters, cubic meters, cm-ha, and m-ha. A cm-ha is the volume required to cover an area of one hectare with a one-centimeter depth of water and is equivalent to 100 m3. In the English system, the common units are ac-in or ac-ft, which correspond to depths of one inch and one foot, respectively, over an area of one acre (43,560 ft2). All of these units are useful in expressing water requirements, or water applied, in terms of depths, and their equivalents to units of volume.

For discharge rates, or volume per unit time, common units in the metric system are m3/see (cum/see) or liters/sec (lps) and, in the English system, ft3/sec (cusec) or gallons per minute (gpm).

2.2.2 Measuring devices in open channels

Nearly anything that partially restricts the flow in an open channel can serve as a measuring device if it is calibrated. The majority of these restrictions are not standard, however, and there are no formulas or rabies available to determine their discharge. Even "standard" structures, if not properly built, installed, and maintained may not operate as expected. Standard devices include orifices, weirs, Parshall flumes, Cutthroat flumes, and broad crested weirs. Submerged orifices may be used under limited head conditions when trash and debris are not a problem. Weirs are useful and economical where flows are not too large and sufficient head is available in the canal. They are one of the oldest and most accurate devices when used under the proper conditions.

The flow measurement device selected for installation will depend on several factors, among which are:

1. The accuracy required. Most devices have an accuracy of ±10% if they are properly installed and maintained. Many have better than ±10% accuracy with careful fabrication, installation, and maintenance.

2. Ease of construction. A simple device that can be manufactured locally with the required precision may give better measurements than a more complicated one that is beyond the ability of local craftsmen to construct.

3. Ease of use. Readings must be easily made and accurately interpreted by the user.

4. Cost of the flow measurement device. Flow measurement devices must be economical to encourage their purchase and use.

5. Topographic conditions and geometric shape of the channel where the flow will be measured and the range of the canal discharges to be measured. Some devices require large differences in head and are most suitable for canals with significant slope while others will give satisfactory measurements with small differences in head. Many devices have a limited range of discharge for which they can be practically used.

Details of several common measuring devices such as orifices, sharp crested weirs, broad crested weirs, Parshall flumes, and Cutthroat flumes, as well as horizontal and vertical pipes, are provided in other publications. The following is a presentation of various common and simple methods of flow measurement.

Methods of Measuring Channel Flow

2.2.3 Float method

(Ref. 57)

The rate of flow passing a point in a ditch or other open channel can be determined by multiplying the cross sectional area of water by the average velocity of the water. Normally, the cross sectional area can be determined by direct measurement of the channel dimensions. The velocity can be estimated by timing the passage of a small float through a measured length of channel. The procedure for estimating rate of flow by the float method is as follows:

1. Select a straight section of ditch with fairly uniform cross sections. The length of the section will depend on the current, but 30 meters usually will be adequate. A shorter length may be satisfactory for slow flowing ditches.

2. Make several measurements of depth and width within the trial section to arrive at the average cross sectional area. The area should be expressed in terms of square meters.

3. Place a small float in the ditch about a meter upstream from the upper end of the trial section. Determine the number of seconds it takes for the float to travel from the upper end of the trial section to the lower end. Make several trials to get the average time of travel. The best floats are small rounded objects that float submerged. They are less apt to be affected by wind or to be slowed by striking the side of the channel. Among small objects that make good floats are a long necked bottle partly filled with water and capped, a rounded block of wood, or an orange.

4. Determine the velocity (or speed) of the float in units of meters per second by dividing the length of the section (in meters) by the time (in seconds) required for the float to travel that distance.

5. Determine the average velocity of the stream. Since the velocity of the float on the surface of the water will be greater than the average velocity of the stream, the float velocity must by multiplied by a correction coefficient to obtain a good estimate of the true average stream velocity. The correction factor varies with the type of float used and with the shape and uniformity of the channel. With floats that sink about 2 to 5 cm below the water surface, a coefficient of about 0.80 should be used for most unlined farm ditches. A coefficient of 0.85 is appropriate for smooth uniform unlined ditches. With floats that extend two thirds or more of the water depth below the surface, the coefficient should be about 0.85 for unlined ditches and 0.90 for lined ditches.

6. Compute the rate of flow. The rate of flow is obtained by multiplying the average cross sectional area (item 2) by the average stream velocity (item 5). The accuracy of these estimates of flow rates is dependent upon the preciseness with which average cross sectional areas and float velocities have been determined and upon the selection of the proper correction coefficient. The method is not accurate enough for conveyance loss measurements. An example of this method of estimating flow rates is shown in Figure 2.18 using Figure 2.19.

2.2.4 Weirs

In a weir (Figures 2.20 and 2.21), water is open to atmospheric pressure on both upstream and downstream sides. Types of weirs are identified by their shape. The most common are the:

· rectangular weir,
· trapezoidal weir, and
· triangular weir.

Sharp-Crested Contracted Rectangular Weirs

The standard contracted rectangular weir is built so that the outlet sides and crest are away from the bottom and sides of the canal in which it is set. The weir contracts the flow of the channel and causes it to fall over a crest.

Extensive experiments on weirs have resulted in the following guideline for accurate measurement of flow:

1. The upstream face of the bulkhead should be smooth and in a vertical plane perpendicular to the axis of the channel.

2. The upstream face of the weir plate should be smooth, straight, and flush with the upstream face of the bulkhead.

3. The entire crest should be a level, plane surface that forms a sharp, 90 edge where it intersects the upstream face.

4. The upstream corners of the notch must be sharp.

5. The distance of the crest from the bottom of the approach channel (weir pool) should not be less than twice the depth of water above the crest and in no case less than 20 cm.

6. The distance from the sides of the weir to the sides of the approach channel should be no less than twice the depth of water above the crest and never less than 20 cm.

7. The overflow sheet (nappe) should touch only the upstream edges of the crest and sides.

8. Air should circulate freely both under and on the sides of the nappe.

9. The measurement of head on the weir should be taken at a point upstream from the weir a distance of four times the maximum head on the crest.

Figures 2.20 and 2.21 indicate typical weir installation. Figure 2.22 summarizes the formulas used for different weirs.

FIGURE 2.18. Estimating Flow Rates by Float Method (Ref. 57)

* Assume a straight section of unlined irrigation ditch 30 meters in length. Representative cross sections at stations 00+0, 12+0 and 28+0 (Figure 2.19).

Cross Section Data

Station 00+0

Distance from left water edge

(m)

0.00

0.45

1.00

1.50

Water depth

(m)

0.00

0.33

0.35

0.00

* (1.00-0.45 = 0.55 and 1.50-1.00 = 0.50)

Station 12+0

Distance from left water edge

(m)

0.00

0.40

1.16

1.58

Water depth

(m)

0.00

0.26

0.32

0.00

Station 28+0

Distance from left water edge

(m)

0.00

0.27

0.58

1.00

1.46

Water depth

(m)

0.00

0.24

0.35

0.35

0.00

Velocity Data

Time for float (wooden sphere) to travel 30 meters

Trial Number

1

2

3

4

Average

Time (seconds)

95

91

90

88

91

Average stream velocity = 0.33 0.80 = 0.26 m/see

Flow Rate

Q = AV = 0.35 m2 0.26 m/see = 0.091 cubic m/sec
or 0.091 cum/see 1000 L/sec/cum/sec = 91 L/sec (lps)

Figure 2.19 Ditch Cross Sections for Example (Ref. 57)


Station 00 + 0


Station 12 + 0


Station 28 + 0


Figure 2.20 Rectangular Weir Used as Measuring Device and Drop


Figure 2.21 Ninety Degree V-Notch Weir (Ref. 41)

2.2.5 Siphon tubes

Siphon tubes (Figure 2.23), used to remove water from a head ditch and distribute it over a field through furrows, corrugations, or borders, are also used to measure the rate of flow into these distribution system.

These tubes, made of aluminum, plastic, or rubber, are usually preformed to fit a half cross section of the head ditch. The normal diameter range is from 2.5 to 15 cm (1 to 6 inches), although both smaller and larger sizes are available. The smaller sizes are used with furrows and corrugations and the larger sizes with borders. Various lengths are available.

Siphon tubes are portable. For this reason, a low number of tubes is required to irrigate a given area resulting in low initial cost for equipment. Flow into individual furrows or borders can be controlled effectively by using the number of tubes that will divide the total head ditch flow into individual streams of the desired size.

Figure 2.22 Summary of weir formulas

Measuring Device (all sharp crested)

Front Views
H = cm, L = cm

Formula
Q = Liters/Second

Rectangular Weir (with contraction)

*

Q = 0.018(L - 0.2H)H3/2

90° Triangular Weir

**

Q = 0.014H5/2


Rectangular Weir (with contraction)*


90° Triangular Weir**


Weir Side View

Siphon tube use is limited to fields with little cross slope in order to maintain a near-constant operating head on each tube. A disadvantage to their use is that each tube needs to be primed individually. This priming is the principal labor requirement when siphon tubes are used for surface irrigation.

The discharge of a siphon tube depends on: (1) the diameter of the tube, (2) the length of the tube, (3) the roughness of the inside surface and the number and degrees of bends in the tube, and (4) the head under which the tube is operating. When the outlet end of the tube is submerged, the operating head is the difference in elevation between the water surfaces measured at the entrance and outlet ends of the tube. When the tube is flowing free, the operating head is the difference in elevation between the water surface at the entrance of the tube and the center of the outlet end (Figure 2.23).

Method of Measuring Pipe Flow

2.2.6 Bucket and stopwatch method

This method for measuring flow is generally well adapted to small flows. It is very simple to set-up and conduct, requires no special equipment, and gives good results. The necessary equipment to perform this test are a bucket of known volume (preferably 20 liters), a 1 m long PVC tube (the diameter will depend on the flow rate), and an ordinary wrist watch with a second hand.

To use this method, first dam the water source and insert the PVC tube into the dam so that all the flow goes through the tube. The PVC tube will have to be high enough above the base level of the dam so the bucket can be placed under the tube. Measurements should not be taken until the flow in the tube stabilizes. The procedure works best with two people: one person filling the bucket and the other timing the event.

To measure the flow, record the time it takes to fill the bucket. The procedure should be repeated at least twice. The flow is calculated by:

2.2.7 Orifices

An orifice is an opening in a plate that has well-defined and sharp edges. It may be round or rectangular. The water surface upstream must be above the top of the opening, if orifice flow is to occur. The orifice is mounted on a flat plate or is cut out from a flat metal plate. The flat plate or wall on which the orifice is mounted is placed perpendicular to the direction of flow across the channel. Knowing the size of the orifice and the head across the orifice, the flow can be estimated. Figure 2.24 provides a definition of the terms.

Figure 2.23 Siphon Tubes: Bead Measurement and Discharge


FREE FLOW - Outlet of Siphon is not Submerged


SUBMERGED PLOW - Outlet of Siphon is Submerged


Discharge

For a freely discharging orifice, the only head measurement required is the water level height above the center of the orifice. For an orifice that is submerged on both sides, the head across the orifice must be measured. The equation and units are given in Figure 2.24. The conditions that should be met for accurate flow measurements are:

1. The upstream edges of the orifice should be sharp and smooth.

2. The distance from the edges of the orifice to the sides of the canal or stream bed should be greater than twice the least dimension of the orifice.

3. The face of the orifice wall should be vertical.

4. In a rectangular orifice, the top and bottom edges of the orifice should be level.

5. The cross-sectional area of the water in the canal should be at least 8 times the cross-sectional area of the orifice.

An orifice plate for measuring flows in small streams can be constructed easily by cutting either a rectangular or circular hole in a piece of sheet metal. The orifice should be carefully cut to the proper dimensions, and the edges should be sharp. The sheet metal plate can then be installed across a stream as part of a check dam while the measurements are taken. A 2 1/2 cm circular orifice is useful for flow rates to 1/2 liters per second. A circular orifice with a 5 cm diameter opening is useful for flows to 3 liters per second with 30 cm of head. A 10 cm orifice is useful for flows to 9 liters per second.


Figure 2.24 Definition Sketch and Formulas for Orifice

2.3 Surveying

Topographic surveying provides some of the basic information required for the design, construction, and operation of an irrigation system. Some of the most important aspects of surveying are:

1. Profiling. Measurement of the elevations of the ground surface along a route (for example, where a pipeline will be installed) or where a structure will be installed.

2. Area measurements.

3. Topographic mapping. Determination of ground surface elevations in a field in order to construct a contour map is necessary for determining land leveling requirements, placement of ditches or structures, etc.

Every Volunteer working with irrigation should have, at a minimum, a hand level (at least 4 power) or an Abney level; a surveying rod; a measuring tape (minimum of 30 meters); a carpenter's level; and a scientific calculator (capable of computing roots and powers). This will allow the Volunteer to determine elevation differences, profiles, and area measurements. Some topographic mapping can be accomplished with this equipment. For significant leveling work, however, an engineer's level and/or transit is often required. This equipment is not often available to the Volunteer. The theory and practice of land leveling is beyond the scope of this manual. The Volunteer should consult appropriate references and obtain assistance from an engineer or surveyor before undertaking significant land leveling.

This section covers the basics of profiling and topographic mapping. It also includes some appropriate techniques for laying out contour lines and determining elevation differences.

2.3.1 Profiling

(Ref. 21)

Accurate knowledge of the ground profile along a pipeline route is often critical for proper pipeline design. Correct profiling depends on correct use of simple equipment. "Eyeball" methods of profiling are sufficient only in the simplest of situations. The following is a general description of profiling methods.

Theory of Leveling

1. The line of sight of a properly used level is always at the same elevation, regardless of the direction in which it is pointed.

2. If the elevation at any point on the ground is known, the elevation of the level line of sight may be found by measuring up from the known point. Because most work requires knowledge of relative elevations only, the known point is often assumed to be 100 or 1000.

3. If the elevation of the level line of sight is known, the elevation of any point on the ground may be found by measuring down from the line of sight.

4. By successive use of the above concepts, the elevation of any point may be found.

Equipment

1. Surveying level and tripod or hand level. Levels are surveying instruments that have a telescope and means for orienting the telescope's line of sight on a horizontal plane.

2. A stick marked with distance measurements (e.g. feet or meters). This stick is called a "rod."

3. Distance measuring equipment, such as a measuring tape, engineer's chain, or optical distance estimating equipment. Pacing is adequate only for flat terrain or short distances.

4. A notebook, properly set up.

Theory of Profiling

1. Profiling involves measurement of elevations (leveling) along a line, together with measurement of horizontal distances.

2. Distances must be measured on a straight line between points for which elevations are taken.

Notekeeping

1. Notekeeping is one of the most critical portions of surveying. Many surveying mistakes can often be traced back to poor notation. A notebook should always be properly set up and time taken to make notes clear and readable.

2. A site sketch should accompany the measurements. This will help the notetaker remember important surface features of the area. The sketch should show salient features such as houses, streams, hills, and trees along the pipeline route. A North arrow should also be included.

Terminology

1. Sta = Station. This is the point on the profile line at which an elevation was measured. These are normally numbered by hundreds of feet. For example, a station 10 may be 1000 feet from the beginning of the survey. Intermediate distances are indicated by pluses: Sta 10 + 50 would equal 1050 from the beginning.

2. Bm = Benchmark. This is a monument or point of known description that includes elevation.

3. Tbm = Temporary Benchmark. This is an object that is relatively permanent, such as large rocks or trees, where the elevation has been determined.

4. Bs = Backsight. This is a rod reading at a point of known elevation.

5. HI = Height of Instrument. This is the elevation of the line of sight of the instrument.

6. Fs = Foresight. This is a rod reading at a point of unknown elevation.

7. Elev = Elevation.

8. Dist = Distance between points.

9. Tp = Turning Point. This is a point used primarily to serve as a reference elevation to move the instrument. Both a foresight and backsight are taken on the point. The point may be on or off the profile line but should be a solid point that is easy to relocate.

Profiling Procedure

1. Setup and level instrument.

2. Sight Benchmark (point of known elevation) for Backsight reading.

3. Enter rod reading in Backsight (Bs column 2).

4. Add rod reading (column 2) to Benchmark (column 5) to get Height of Instrument (HI column 3).

5. Sight point to be determined (Foresight) and enter reading in Foresight (Fs column 4).

6. Subtract Foresight (column 4) from Height of Instrument (column 3) to get elevation of Foresight (column 5).

Turning Point

1. Rodman maintains position at Foresight.

2. Move setup, and level the instrument at new location (Tp 1).

3. Sight rod at Backsight (last foresight station) and enter reading in column 2.

4. Add rod reading (column 2) to elevation of backsight (column 5) to get Height of Instrument (column 3).

5. Proceed with Foresight (steps 5 and 6 above).

Example 1: An example survey is presented in Figure 2.25. Notation for this survey is presented in Table 2.3

TABLE 2.3 Survey Notation (meters) for Figure 2.25 (Ref. 27)

1

2

3

4

5

6


+


(-)



Sta

Bs

HI

Fs

Elev

Notes

Pt A

2.5

102.5


100

Assumed elev.

Pt B



11.5

91


Tp 1

4.2

95.2



Pt B.

Pt C



12.3

82.9


The steps used in the example problem are different from those used by professional surveyors. They have been simplified in an attempt to reduce confusion and are more than adequate for the type of surveying that is necessary in small-scale piped water systems. When using this method, always remember the following simple calculations:

1. Known elevation + Backsight reading = Height of Instrument

2. Height of Instrument - Foresight = Next Elevation

Practical Hints for Surveying

1. Before starting, walk the course to be surveyed and mark the line to be profiled. If the survey is conducted for a piped water system, remember to keep in mind that pipe will have to be laid in trenches along the course. Whenever possible, avoid obstacles that will make laying difficult.

2. Mark with a sturdy stake all turning points, foresights, and backsights as work progresses so they will be visible if a recheck is necessary.

3. After you have finished your calculations, redo the survey if unacceptable errors occur. It is much easier to correct a surveying mistake before pipe has been laid in the ground.

4. It is desirable to recheck horizontal distances as well. Approximate methods, such as pacing, will catch major errors with a minimum effort.


Figure 2.25 Profiling: Example (Ref. 21)

Plotting the Profile

Once the horizontal distances and elevations are surveyed in the field, the data is brought back to the office and plotted on graph paper. This completed profile can be used for sizing pipelines and locating storage tanks, air valves, and washout points among other requirements. Normally, the vertical scale is greater (numerically smaller) than the horizontal scale. For example, the vertical scale may be ten times the horizontal: in the vertical scale, one cm may equal 1 meter and in the horizontal, one cm may equal ten meters. Other similar ratios may be used.

Example 2: Figures 2.26 and 2.27 show a ground sketch of the area to be surveyed and the completed profile for the survey. The following are measurements taken during this survey:

At

0 + 00

100.0 + 0.2 = 100.2

(equals HI)

At

0 + 47

100.2 - 9.7 = 90.5

(equals next elevation)

This is repeated for the next instrument setup:

At

0 + 47

90.5 + 1.3 = 91.8

(equals HI)

At

1 + 03

91.8 - 9.9 = 81.9

(equals next elevation)

Many different ground elevations may be found from a single Height of Instrument sight, as shown by the following:

At

1 + 03

81.9 + 4.2 = 86.1

(equals HI)

At

1 + 30

86.1 - 5.5 = 80.6

(equals ground elevation)

At

1 + 37

86.1 - 8.9 = 77.2

(equals ground elevation)

At

1 + 45

86.1 - 5.2 = 80.9

(equals ground elevation)

At

2 + 00

86.1 - 4.2 = 81.9

(equals ground elevation)

2.3.2 Steps in making a topographic man

1. Clear field of all debris, ensure clear line of sight to all points on the field.

2. Measure boundaries of the field and determine its area.

3. Collect and make a sufficient number of marker stakes.

4. Stake out a square grid over entire field. Use one side of the field (straightest side, if possible) as a starting point and set stakes at the recommended spacing of:

a. 10 m if broken or irregular land relief.
b. 20 m if flat or uniform land relief.

The use of a 3-4-5 right triangle will assist in laying out the grid evenly and at 90° degree angles.


Figure 2.26 Typical Profile (Ref. 21)

LOCATION:
SURVEYOR:
DATE:

STA

+B. S.

H. I.

-F. S.

ELEV.

NOTES

0+00


Assumed

100.0

Spring


0.2

100.2




0+47



9.7

90.5

TP #1


1.3

91.8




1+03



9.9

81.9

TP #2


4.2

86.1




+30



5.5

80.6

Stream West Edge

+37



8.9

77.2

Stream

+45



5.2

80.9

Stream East Edge

2+00



4.2

81.9

TP #3


9.7

91.6




2+50



5.3

86.3


3+00



0.7

90.9

TP #4


6.1

97.0




+30



3.7

93.3


+72



2.5

94.5

Top of Hill

4+00



7.8

89.2

TP #5


1.6

90.8




+50



6.1

84.7


5+00



9.6

81.2

E.O.L.


Figure 2.27 Survey Sketch and Notations (Ref. 21)

5. Establish a benchmark (permanent or semipermanent object) on or near the field.

6. Take readings of elevational differences for each stake in the grid with reference to the benchmark with some type of leveling device and rod.

7. Take field notes for the entire field.

8. Sketch topographical map of field:

a. Sketch field to scale on graph paper.

b. Place grid on sketch.

c. From the field notes, fill in all of the elevational readings for each point on the grid.

d. With a continuous smooth line, connect the points of the same elevation reading and extrapolate points in between where need be; this is a contour line.

e. Incrementally increase (or decrease) value of elevation reading and repeat step (d) for each new elevation.

f. Complete topographic map for the range of elevation readings. Make sure there is an incremental progression of elevation readings so that the contour lines will have equal elevational differences between them. The contour line increment will depend on the range of elevation readings, the detail required, and the size of the area being mapped. For example, a 1 meter increment between contour lines will provide more detail than a 10 meter increment in one given area.

2.3.3 Abney level surveying

(Adapted from Reference 27 with appropriate modifications. For greater details on surveying, and adjustment of the level, the user should consult Reference 27 or books on surveying. The Abney level should also be periodically checked to insure that it is properly adjusted.)

Abney level surveying is especially useful for rapidly determining elevation differences, particularly in hillside situations, where great precision is not required.

The Abney level consists of a square tube (dimensions of about 16 1.5 1.5 cm) with an eyepiece at the observer's end, a horizontal cross-hair at the objective end, a bubble level, a 45° mirror, and a moveable indexed arc.

Conducting a survey with the Abney requires two persons. An Abney level, a 30 meter tape measure, and a field book are necessary; a compass may be used if bearings are desired.

The survey is begun at some fixed reference point (such as the water source or some prominent landmark along the route) and proceeds long the route of proposed construction.

The surveying technique is simple: the surveyor sights through the Abney at a target held by the other person, and the ground distance between them is measured. This distance, and the vertical angle (angle measured by the Abney) are recorded in the field book. It is important that the target that the surveyor sights upon is the same height above the ground as the Abney, which is the same as the surveyor's eye-level. If the assistant is not as tall as the surveyor, then the assistant should carry a target stick cut exactly to the same length as the surveyor's eye-level. A red cloth can be tied to the top of the stick, or the assistant's hand can be placed over the end of it, to provide a clear target. It is also useful for the surveyor to use a forked stick as a stand to rest the Abney on, in order to obtain a steadier reading (in this case, the target stick should be cut to the same length as this forked stick).

Figure 2.28 shows the basic arrangement and calculation used in trigonometric leveling with the Abney: the surveyor and assistant are 28 meters apart (ground distance), and the vertical angle is - 16° (the negative angle indicates that the surveyor was sighting downhill). Use a calculator or trigonometric table to determine the sine of the angle. In this case the sin of 16° is 0.2756 so 28 0.2756 is 7.7 meters elevation difference.

Field Methods

While conducting the survey, the surveyor and assistant must also observe the terrain being walked. The surveyor must constantly keep in mind that, at some later time, other people will actually have to dig a trenchline along that route. Notes must be made about the type of terrain being traversed, such as stretches of jungle, cultivated fields, footpaths, gullies, soil conditions (e.g. gravel, soft dirt, bare rock). It is easy to survey across terrain over which might be exceedingly difficult or impossible to lay a pipeline!

The surveyor should make use of as many reference points as possible so that if a section of the pipeline needs to be resurveyed at a later time, a convenient starting point can be found. Reference points should be permanent or semi-permanent. Suitable examples are prominent trees, rock outcroppings, etc. If the surveyor carries flagging, spray paint, lime, nailpolish or enamel paint, landmarks can be identified with a permanent label.

Figure 2.29 shows a good, precise format for recording accurate and complete notes.

Figure 2.28 Trigonometric Surveying with an Abney Level (Ref. 27)

VERTICAL DISTANCE = GROUND DISTANCE SIN q

EXAMPLE ILLUSTRATED BELOW:

(= -16° (NEGATIVE SIGN INDICATES SIGHTING DOWNHILL)
SIN (= 0·276 (FROM TRIGONOMETRIC TABLE)
GROUND DISTANCE = 28 METERS
VERTICAL DISTANCE = 28 0.276 = 7.73 METERS.


Figure


ABNEY LEVEL


SURVEYING WITH AN ABNEY LEVEL


Figure 2.29 Example: Field Book Notes (Ref. 27)

Closing the Survey

Closing the survey means tying the survey to two reference points of known elevations, thus providing a check on the surveyed elevations. Closing a survey can be done by repeating it entirely, beginning from the original endpoint and ending at the original starting point, but not necessarily along the same original route.

Leveling Rods

A leveling rod is used to measure the vertical distance from the surface of the ground to the line of sight of a surveying level (Figure 2.28). It can be constructed as a land measuring rod but the "0" m should just be the end of the rod. The overall length should be about 3 m and the rod should be marked in meters, decimeters, and centimeters.

Attach a moveable target to the rod. This might be a small board held against the rod with something such as a short piece of rubber (from an old inner tube) to hold it in place after the height is adjusted.

When the surveying instrument is sighted on the rod, the target is moved up or down until it is centered on the line-of-sight. The vertical distance is then read.

2.3.4 Simple levels for use in surveying contour lines

(Ref. 9, with appropriate modifications)

In many areas, sophisticated surveying levels are not available to farmers interested in designing soil conservation structures. Even where they are available, it is often more practical for the farmer to build a cheap, simple, effective level for use in surveying contour lines. Although less accurate than more sophisticated levels, the A-Frame level (Figure 2.30), the Line Level (Figure 2.31), and the Hose Level (Figure 2.32), when properly constructed and used, are sufficiently accurate for the work on small hillside farms.

A. How to build and use an a-frame level

1. Construction

The materials required are 3 straight boards or sticks, 3 nails or screws, a thin string, and a plumb-bob -- a screw-capped glass bottle or uniform-shaped rock. A small level is very convenient and is easier to use on windy days.


Figure 2.30 A-Frame Level for surveying Contour Lines (Ref. 9)


Figure 2.31 Simple Line Level for surveying Contour Lines (Ref. 9)

Figure 2.32 Use of a Hose Level (Ref. 42)


No slope


Meter level


10% Slope


20% Slope

Important points to consider in building the A-frame level:

a. The symmetry of the level is important (two legs should be the same length and crossbar should be positioned identically on the legs so that it is parallel to the ground.)

b. The dimensions of the level are not important, but if constructed in larger dimensions than the one in Figure 2.31, the level should be assembled with screws so that it can be disassembled for transportation. Measuring an exact distance (2 m) between the feet makes calibrating the 1% slope position easier.

c. The plumb-bob must be attached so that it does not deflect the string to either side. If a screwcap bottle is used, it should be hung by a hole made exactly in the center of the cap. If a rock is used, it is important that a very uniformly shaped rock be chosen.

2. Calibration

The level should be calibrated every day before use, as warping of the wood can greatly change the results.

a. Calibration of 0%

1. The level should be positioned with both feet on firm surfaces but with one end obviously higher than the other.

2. The level is gently rocked, allowing the string with the plumb-ob to gently strike the crossbar.

3. When the plumb-bob stops swaying side to side and the string strikes the crossbar at the same point repeatedly (5-10 times), mark this position in pencil on the crossbar.

4. Reverse the position of the level so that the other foot is now at the higher point. Care must be taken to position the feet of the level in exactly the same points as before.

5. Repeat steps 2 and 3, obtaining a second mark on the other side of the center of the crossbar.

6. The 0% position of the level is exactly in between the two marks obtained in this trial. This position can be marked by measuring with a ruler or paper (half the distance between the 2 marks). Now when the feet of the level are even the string will strike the crossbar at the 0% position. This position is used to survey contour lines for barriers, terraces, or ditches be used for retention, rather than diversion, of water.

7. Once calibrated, a small carpenter's or line level can be fastened to the crossbar to facilitate use on windy days.

b. Calibration of 1%

1. Position the level so that the feet are on the same level and the string strikes the crossbar at the 0% position. The feet should be on firm surfaces.

2. Raise one foot by the distance required to position the level at a 1% slope. For example, if the distance between the feet is 2 m (200 cm), then a 2 cm tall object (e.g. a 2 cm tall stack of coins) should be placed under one foot. [2 cm (raised foot)/200 cm (distance between feet) = 0.01; 0.01 100% = 1%.]

3. Rock the level gently, now the string strikes the 1% slope position. Mark this position on the crossbar.

4. Since this type of contour line will be used to construct structures to divert water, an arrow should be placed pointing toward the lower foot to indicate the direction of water flow.

5. As in previous calibration, if desired, a small level can be fastened to the crossbar.

3. Use of the A-frame level

The A-frame level is used to survey contour lines by placing stakes at the position of the feet when the level gives the desired reading. Stakes should all be placed on the same side of the level, all upslope or all downslope, in order to avoid errors. When not being used, the level should be stored in a dry, shady place.

B. How to Build and Use a Line Level

1. Construction

The materials required are 2 straight boards or sticks, a string of desired length, and a line level (Figure 2.31).

2. Calibration

The level should be calibrated every day before using as bending of the hooks on the line level or warping or chipping of the sticks can greatly change the results.

a. Calibration of 0%

1. Slots are cut in each stick at the same distance from one end.

2. The string is tied firmly to each stick so that it cannot slip out of the slots.

3. Hook the line level on the string and find a place on firm ground which gives a level reading.

4. Reverse the direction of the line level on the string while maintaining the position of the sticks. If the reading changes, the hooks of the line level must be adjusted slightly by bending them.

5. Repeat steps 3 and 4 until the line level gives identical readings upon reversal.

b. Calibration of 1%

1. Repeat the steps as in the calibration of 0%. However, this time the slots on the sticks should be placed so that a 1% drop occurs over the distance of the string. (For example, if the string measures 2 m then the slot on one stick should be 2 cm higher than on the other.)

2. Remember that the stick that has the slot located higher actually represents the lower ground surface when the reading of the string is level. Remember to mark the sticks so that no confusion as to the direction of water flow will arise when surveying contour lines.


3. Use of the Line Level

This type of level is easiest to use with at least three people, two holding the sticks and the third reading the line level and placing stakes. When not in use, the line level should be protected so that the glass vial and hooks are not damaged.

C. How to Build and Use a Hose Level

(Ref. 42)

1. Construction:

A simple hose level (Figure 2.32), useful for laying out a grade line, can be constructed using the following materials:

a. A transparent plastic hose, 16 m long and 1.5 to 2 cm in diameter.
b. Two thin rods or boards, or other thin, rigid material about 2.6 m long.
c. Strips of wire, rubber, or string with which to tie the hose.
d. Small cans of white and black paint.
e. Measuring tape.

The level can be constructed as follows:

a. Mark the rods every 5 cm, starting 10 cm from one end.

b. Tie the hose to the graduated rods so that it lies against the rods from top to bottom.

c. Place the two rods side by side on the same level. Stretch the hose downhill on a slope.

d. Fill the hose with water so that is rises to the 1 m mark on each end of the hose. Make sure that no air bubbles remain in the hose. The simple level is complete!

2. Use:

This simple tool can be used to determine slope as follows:

1. Separate the rods by 10 m.

2. Note how much the water has changed in the hose. If the water level at both rods are equal, then the two points are at the same elevation. The slope between the two points is 0%.

If the water level lowers 5 cm from the 1 m mark on the upper rod, it will rise 5 cm above the 1 m mark on the lower rod. (10 cm total difference.)

With each 5 cm change against either of the rods, there is a 1 percent change in slope, so that with a 5 cm change the slope is 1 percent, with a 10 cm change the slope is 2 percent, with a 50 cm change the slope is 10 percent, and with a 1 m change the slope is 20%.

If the water in the downhill end goes over 2 m, the slope is greater than 20% and should be measured as described below.

3. Slopes between 20 and 40 percent can be measured as follows:

a. Separate the rods by 5 m.

b. Note the change in water level on either side. A 50 cm change indicates 20 percent (for each 5 cm change there is 2% more slope). If the water level changes 75 cm, the slope is 30%. If the water level changes 100 cm, the slope is 40%.


The simple level can be used to mark a level contour line in the field in the following way:

1. Select a starting point and mark it with a stake.

2. Separate the rods by 10 m or some convenient distance.

3. Move the leading rod up or down, until its water level is the same as that in the rod at the initial point. Mark this point.

4. Follow the same procedure from point 2 to point 3, point 3 to point 4, and so on.

We can also mark lines with one, two, or three percent slope.

1. Choose a starting point.

2. Separate the rods by 10 m. The water level at each rod must change:

5 cm for 1% slope
10 cm for 2% slope
15 cm for 3% slope

If the water level indicated in the leading rod rises, the slope is downhill. If the water level drops, the slope is uphill.

3. Repeat the procedure for succeeding points.

2.3.5 Compass use

In the field it may be necessary to measure the angle between two different lines or points. This can be done by direct measurements with a protractor if the lines can be drawn on a flat surface. Another more accurate and easier method is the use of a simple compass. To do this, it is important that the compass have peripheral degree graduations that are 360° to make a full circle.

To measure a horizontal angle, the compass holder stands at a fixed point, usually a corner of a field, and visually aligns the north reading of the compass along the line of sight to another point (another corner in field). Since the needle automatically points to negative north, the compass holder notes the difference in degrees between the needle reading and the north reading on the compass. This is known as the negative bearing of the object from the fixed point. A second bearing is taken on a second object (another corner of field) and the difference between the two bearing readings will give the angle between the two points in relation to the fixed point.

The compass method can be used with a tape measure or long string for surveying the area of an irregular plot of land. The area is first walked over, and the points in the corners of the field are located and staked. With a tape measure, the distances between the corners are measured and recorded. At a corner of the field, the compass holder takes magnetic and adjoining corners bearings and determines the angle. This is done at each corner of the field.

With the aid of a protractor, the area is plotted to scale on a sheet of graph paper. The square units of the graph paper are given an area that will depend on the scale. The number of unit squares in the area of the field are totalled and an area is determined for the field.

In using the compass, be sure that it is free from the effect of magnetism due to iron objects carried by yourself or in nearby surroundings.

2.4 Soil-Plant-Water relationships

Proper irrigation requires knowledge of soil water storage and movement of water in the soil, as well as availability of this water to the plant. Irrigation also requires knowledge of plant root development, crop water use rates, and of the irrigation system itself. This section covers basic definitions and relationships that must be understood by those who work with irrigation management at the field level.

2.4.1 Soil moisture storage and availability

Figure 2.33 indicates the availability of soil water. A soil is at saturation or near saturation following a heavy irrigation or rainfall in which most or all of the spaces between soil particles are filled by water. The force of gravity is greater than the force with which soil particles hold water, so between saturation and field capacity (see below), water is free to drain through the soil by the force of gravity.

Field capacity (FC) is the amount of water that a soil can hold against drainage by gravity.

Permanent wilting point (PWP) is the moisture content in a soil at which plants permanently wilt and will not recover.

Available water (AW) is the water content that the soil can hold between field capacity and wilting point.

Readily available water (RAW) is that portion of available water that the crop can use without affecting its evapotranspiration and growth. This portion is often indicated as a fraction of available water (p) and is dependent primarily on the type of crop and evaporative demand. A p value of 0.5 is commonly used. Shallow rooted crops such as most vegetables, however, require high moisture levels for acceptable yields, so p is about 0.3. Deeper rooted crops will generally tolerate higher depletions, so p = 0.6 to 0.7. During critical stages of growth (for example, flowering in corn), less depletion should be allowed than at other stages.

Soil moisture is typically measured as a percent of dry weight of soil, or as a volume percentage. Expression as a volume percentage or depth of water per unit depth of soil is most common and convenient in irrigation management.

The most useful measurement gives available water-holding capacity (AW) as a depth of water per unit depth of soil expressed as mm of water per meter of soil depth (mm/m) or inches of water per foot of soil depth (in/ft).


Figure 2.33 Soil Water and its Availability

- Soil Moisture

The total available water (TAW) for a crop with root zone depth (D) is the product of the available water-holding capacity (AW) per unit of soil depth and the root depth in the same units, or:

TAW = D AW

The readily available water in the root zone (RAW) is:

RAW = p TAW

p = percent of allowable depletion not resulting in crop stress

Field Capacity

Although there are several lab methods for determining field capacity, it may be faster and more practical to estimate as follows:

(1) select a recently irrigated plot with no plants on it or make a small basin and fill with water;

(2) cover the saturated soil with canvas or plastic to prevent evaporation; and

(3) take samples after the soil has drained to field capacity. The time required is usually one day in coarse-textured soils, two days in medium-textured soils, and three to four days in fine-textured soils. Samples of the soil taken after the indicated time period will be approximately at field capacity. Table 2.5 can then be used to understand the concept of field capacity for different soil textures by the "feel" method.


Typical relationships, such as that illustrated in Figure 2.34, which indicates field capacity as a function of texture, are often sufficiently accurate for planning scheduling programs. For more details on determination of soil moisture in quantifiable terms, see Ref. 44.

Permanent Wilting Point

Permanent wilting point can be established by determining the moisture at which plants permanently wilt.

A simple criterion satisfactory for water management is that PWP is 50% of FC for coarse to medium-textured soils and about 67% of FC for clays and clay loams. Typical relationships, such as those illustrated in Figures 2.33 and 2.34 often provide sufficient accuracy in estimating wilting point for water management purposes.


Figure 2.34 Typical Relationship Between Soil Moisture Characteristics and Texture

Available Water

Available water-holding capacities for soils are a function of soil texture, structure, organic matter, and salt content. For general agricultural soils without salt, compaction, or other types of problems, information such as that in Figure 2.34 or Table 2.4 on water-holding capacities as a function of texture and soil water tension can be used for planning. The factors that affect soil available water-holding capacity are:

1) Soil texture: as Table 2.4 shows, the smaller the soil particles, the greater the surface area, and hence the greater the area on which water can cling. This results in a higher available soil water holding capacity. In the case of heavy clay soils, the soil water's availability to plant roots is limited by the soil denseness, since the water is so tightly held.

2) Soil structure is the arrangement of soil particles into groups or aggregates. The spaces between these aggregates provides places for soil drainage, soil aeration, and root growth. This is especially important in heavy soils with small soil particles. Four common soil structures are:

· Crumb or Granular: Roundish aggregates that are porous and easily worked.
· Blocky: Blocklike soil aggregates that are relatively porous.
· Columnar: Column-like soil aggregates that are relatively porous.
· Platy: Platelike, flat soil aggregates that can overlap and impair soil permeability.

A soil with poor structure and drainage will tend to trap water in the profile, increasing soil water-holding capacity, but also creating problems with waterlogging and restricted root development.

3) Hardpans are hardened or cemented layers caused by physical or chemical processes and restrict soil drainage. This results in a similar situation as poor structure.

4) Organic matter can increase a soil's water-holding capacity, mainly by improving its physical condition.

5) A soil's salt content restricts a plant's ability to take up water from the soil solution.


TABLE 2.4 Available Water-holding Capacities for General Soil Types

Coarse sands - gravelly sands

40 - 70 mm/m

Fine sands - loamy sands

70 - 100 mm/m

Sandy loams

120 - 160 mm/m

Loams

180 - 220 mm/m

Silt loams

230 - 270 mm/m

Silty clay loams, silty clays, heavy clays

160 - 200 mm/m

2.4.2 Estimating soil water characteristics on site

Soil maps developed by local or national agencies often provide sufficient information on soil water characteristics for developing irrigation management programs. If such sources are not detailed enough (as is often the case with very non-uniform soils), quick on-site evaluations can be conducted. This usually involves estimating soil texture and correlating it with available water through use of tables and/or experience.

Interviews with owners or managers of the farm will help to identify areas of different soils. Often simple observations of differences in crop development, superficial soil texture, and color of soil may help to define such areas.

The soil texture and structure can be evaluated by the visual and feel method using a soil probe or shovel and taking samples to typical rooting depths in increments. Table 2.5 describes properties often associated with specific textures. Figure 2.35 demonstrates an easy, visual method of assessing soil texture. Figure 2.36 shows typical water extraction patterns.

In tropical areas with high rainfall and good drainage, centuries of weathering have washed out much of the soils' mineral nutrients. This changes the characteristics of the clay fraction of these soils. These "tropical clays" are considerably less sticky and plastic than temperate clays described in Table 2.5. Unfortunately, most of the soils' natural fertility is also lost in this process. These soils are usually characterized by their distinctive red or yellow color.

Figure 2.35 Demonstrating the Particulate Make-Up of Soils (Ref. 9)


SANDY SOIL


LOAM SOIL


CLAY SOIL

Place soil in a bottle, add water, shake, and set on a stable level surface, me heavier sand-sized particles will settle out first followed by silt-sized and then clay-sized particles. This demonstration illustrates the particulate nature of soils and can be used to help farmers understand what soil texture means and how it can be important in affecting the drainage or erodability of a soil. The bottles should be allowed to remain undisturbed for 2 full day in order for the finer, clay-sized particles to settle out.

TABLE 2.5 Textural Properties of Mineral Soils

Feel and Appearance of

Soil Class

Dry Soil

Moist Soil

Sand

Loose, single grains that feel gritty. When squeezed, the soil falls apart when pressure is released.

Forms cast when squeezed that crumbles when touched. Doesn't form a ribbon.

Sandy loam

Aggregates easily crushed; very faint velvety feeling initially; continued rubbing gives gritty feeling.

Forms cast requiring careful handling without breaking. Doesn't form a ribbon.

Loam

Aggregates crushed under moderate pressure; firm clods. When pulverized it feels velvety, becoming gritty with continued rubbing. Casts bear careful handling.

Cast can be handled quite freely without breaking. Very slight tendency to ribbon. Rubbed surface is rough.

Silt loam

Aggregates firm, but may be crushed under moderate pressure. Clods firm to hard. When pulverized, it feels smooth, flour-like.

Cast freely handled without breaking. Slight tendency to ribbon. Rubbed surface has a broken or rippled appearance.

Clay loam*

Very firm aggregates. Hard clods strongly resist crushing. When pulverized, it feels gritty from the small aggregates left behind.

Cast bears much handling without breaking. Forms ribbon feeling slightly gritty when wet and rubbed. Plastic, sticky, and puddles easily.

Clay*

Hard aggregates and clods that strongly resist crushing. When pulverized it feels gritlike due to harshness of numerous small aggregates that persist.

Casts bear considerable handling without breaking. Forms a flexible ribbon and retains its plasticity when elongated. Satin feeling when rubbed. Sticky when wet and easily puddled.

* The properties described for the clayey soils refer to those of the clayey soils found in the temperate regions.


Figure 2.36 Typical Crop Water Extraction Patterns

2.4.3 Development of the soil water reservoir

The soil water reservoir available to the plant changes as the root system develops. Root depth varies with crop and variety, stage of growth, soil chemistry, structure, drainage and management. For example, excessive irrigation or inadequate wetting of the root zone may limit root development. The root system of a plant develops from seed depth at germination to a maximum depth at full vegetative development, or until it encounters impermeable barriers or other obstacles to root development. Typical rooting depths for several crops divided into four groups are described in Table 2.6.

TABLE 2.6 Rooting Depths of Various Crops (Ref. 44)

Rooting Depth (m)

Crops

0.3 - 0.5

Cabbage, celery, lettuce, onion, pineapple, potatoes, sisal, spinach, leafy vegetables

0.5 - 1.0

Bananas, beans, beets, carrots, peas, clover, groundnuts, peppers, soybeans, sugar beets

1.0 - 1.5

Barley, citrus, cucumber, flax, small grains, maize, melons, sunflower, sweet potato, wheat

1.5 - 2.0

Alfalfa, cotton, deciduous orchard, grapes, sunflower, sorghum, sugar cane

For management purposes such as irrigation scheduling, the root zone is often assumed to develop linearly from planting depth at time of planting, or shortly after, to typical maximum root depth at full cover. In monitoring the moisture on many field crops, the primary rooting system may be assumed to be from one to two times the crop height, or to the depth where hardpans or other obstacles are encountered. Moisture monitoring to the depth of plant height is adequate for many crops other than alfalfa, tree crops, and some other deep-rooted crops.

2.4.4 Soil water availability and crop use patterns

Readily Available Water and Allowable Depletion

Table 2.7 groups several crops according to permissible soil moisture depletion for maximum yield conditions. Table 2.8 indicates the fraction allowable depletion of available moisture (p) as a function of crop group and evaporative demand. The readily available water that can be extracted from the root zone without limiting yield is obtained by multiplying p by the total available water (TAW) to root zone depth.

Fungal and bacterial pathogens proliferate faster at higher moisture levels. Crop quality, such as protein in wheat and color in cotton, may improve with lower available water. Irrigation system flexibility or limited water supplies may dictate allowable depletions. Osmotic potentials created by salts in the soil create the same effects as soil moisture tensions. Salts may inhibit water and nutrient uptake from the soil, therefore, and maintenance of higher moisture levels than those indicated may be desirable in saline soils. Tables 2.7 and 2.8 should serve only as guidelines when water supplies are abundant and flexible.

TABLE 2.7 Crop Groups According to Soil Water Depletion (Ref. 11)

Group

Crops

1

Onion, pepper, potato

2

Banana, cabbage, grape, pea, tomato

3

Alfalfa, bean, citrus, groundnut, pineapple, wheat, sunflower, watermelon

4

Cotton, maize, olive, safflower, sorghum, soybean, sugarbeet, sugarcane, tobacco

Note:

Group 1 -- Most sensitive to water stress
Group 4 -- Least sensitive to water stress

TABLE 2.8 Practical Depletion Values (Fraction) not Resulting in Significant Water Stress

Crop Group

Hot High Water Use

Cool Low Water Use

1

0.25

0.40

2

0.30

0.50

3

0.40

0.60

4

0.50

0.70

Moisture Extraction Patterns

Plants maintained at high moisture levels will take their water from the root zone approximately in proportion to the concentration of roots. A typical extraction pattern of water from the soil is indicated in Figure 2.36, with 70% of the moisture extraction taking place from the upper half of the root zone. Moisture-sensitive crops such as potatoes, maintained at high moisture levels, may take 80% to 90% of their water from the upper half of the root zone. As plants are subjected to stress, they will take more of their water from where it is available in the lower reaches of the root zone.

2.4.5 Soil intake characteristics

Soils that take up water rapidly will wet the root zone rapidly after the onset of irrigation, and thus irrigations will usually be of short duration. The rate at which soils take water is called the soil intake rate, and the rate at which water goes into the soil is the infiltration rate. The intake rate of a soil will affect such management and design factors as irrigation durations, flow rates to be used, and dimensions of the system.

Factors Affecting Intake Rates

The most important factors influencing the infiltration rate of water into the soil are:

1. Soil texture and structure. The coarser the texture and the more highly structured, the higher the infiltration rates.

2. Soil surface conditions. Orientation of soil particles and compaction: after water moves over a soil surface, soil particles are rearranged and the soil surface tends to seal.

3. Soil moisture content and moisture gradients. Generally, the drier the soil, the faster the infiltration rate.

4. Time since the start of irrigation. Infiltration rate decreases with time until the basic intake rate is reached.

5. Salt content in the water and soil. Soils high in soluble salts will typically exhibit higher intake rates than soils from which salts have been leached.

6. High levels of sodium on the soil's exchange sites will severely affect infiltration if structure collapses. See Chapter 7 for details.

Infiltration rate, as used in border irrigation and sometimes in furrow irrigation, has the units of velocity (l/t) and is the depth of water entering the soil profile per unit time. It can also be thought of as the volume of water absorbed by a unit area per unit time. The metric units commonly used to express infiltration rate are mm/hr or mm/mint In furrow irrigation, where infiltration rate is expressed as a depth per unit time, an equivalent depth is usually implied since movement is horizontal as well as vertical. The depth is obtained by dividing the volume rate of infiltration per unit of furrow length by the product of unit length and furrow spacing. In furrow irrigation, infiltration rate is commonly expressed as the volume absorbed by a unit length of furrow in a unit time.

Most soils exhibit an initially high infiltration rate that decreases with time and eventually reaches a constant or nearly constant rate called the basic intake rate. Figure 2.37 demonstrates the typical infiltration rate behavior with time, as well as cumulative infiltration with time.

The basic intake rates for loamy sands and sands may be 2 to 3 cm per hour or greater. For sandy loams, it is typically 1 to 2 cm per hour. For silt loams and clay loams, it is typically 0.5 to 1.0 cm/hr. For silty clays, it may be 0.2 to 0.5 cm/hr or less. With poorly structured soils, these values may decrease by 25% to 50%. With highly structured and loose soils, the values may be 50% to 100% higher than those indicated.

Because infiltration can change so much during the season, infiltration data should be used with caution, and sound judgment should be exercised in interpreting it. To use such data requires knowledge of crops and cropping history, irrigation methods and management, tillage, soil type and structure, and time of season.

Infiltration Equations

In management of irrigations systems, several infiltration equations and methods for establishing these have been used. One common method for estimating infiltration rate uses ring infiltrometers. These are installed by penetrating the soil surface by 15 to 20 cm. They are then covered with plastic film, filled with water, and then the film is removed rapidly. A water level reading is taken immediately on removal of the plastic because water begins to infiltrate the soil at this time. The decline of the water surface is measured as a function of time and these results are recorded and graphed (see Figure 2.37).

To obtain an approximation of infiltration rate in a field situation, small 1 m 1 m basins are constructed with well compacted banks. A sheet of plastic is placed in the basin and 15 to 20 cm of water introduced to the basin. The plastic sheet is removed from the bottom, and the decline of the water surface is measured as a function of time. Results can be graphed to provide an approximation of the time required to infiltrate a certain depth of water. Base infiltration rates may require additional water to be added to the basin.

2.4.6 Soil chemistry and fertility

Soil is made up of mineral particles derived from the weathering of rocks and organic matter resulting from decomposing plants. Soil mineral particles decrease in size from sand to silt to clay. Sand and silt usually contribute very little to a soil's fertility but are important to a soil's pore space and drainage.

Clay particles, on the other hand, are very small and have tremendous surface area. The surfaces and edges of clay particles are negatively charged. These negatively charged sites on the clay particles act like weak magnets for positively charged ions or cations. Certain fractions of soil organic matter also have negatively charged sites. This organic matter and clays are termed the soil colloids by virtue of their charge and size. The sum of all the negative charged sites on clay and organic matter in a soil is called the Cation Exchange Capacity (CEC) and is expressed as meq/100 g.


Figure 2.37 Infiltration Curve

Many nutrients required by a plant are positively charged ions. They are loosely held by these negatively charged sites and are not washed out (leached) from the root zone when water travels through. For this reason, the CEC is a good measure of soil fertility. Clay particles and organic matter also have plant nutrients in their structure that are slowly released by soil weathering and decomposition. This natural fertility and the CEC of the soil are components of a soil's potential fertility. Negatively charged ions are not held by soil colloids and can thus be readily leached out.

Organic matter has a very high CEC and can dramatically affect a soil's ability to hold nutrients. Organic matter is thus very important to a soil's productivity. This is especially important in severely weathered "tropical clays,'" which rapidly lose most of their natural fertility and whose clay particles have few negatively charged sites. Addition of organic matter to these soils can be very beneficial.

Nitrogen is a very important plant nutrient involved in plant growth, photosynthesis, and plant proteins. In the ammonium form, nitrogen is positively charged and readily held on the negative exchange sites on soil. Soil microbes, however, change ammonium into nitrate, and this occurs at a very rapid rate in warm climates. The nitrate form of nitrogen is negatively charged and is easily leached out of the root profile. Proper irrigation water management is required to reduce the potential for leaching and ensure good nitrogen fertility levels.

Other nutrients are more or less resistant to being leached out of the soil, but it is a likely event when nutrient ions are negatively charged. As more of these nutrients are leached out and minerals continue to weather, the soil becomes more and more dominated by the hydrogen ion, resulting in acid soil. High rainfall tropical areas will often have tropical clay soils, red or yellow in color, exhibiting very acidic reactions (low pH).

Leaching of nutrients can also result in imbalances that can severely affect plant growth. Chapter 7 discusses the effect of excess ions (salts) in the soil solution. Soil chemistry is complicated, so a more complete reference should be consulted.

2.5 Conducting initial environmental evaluations of irrigation projects

Irrigation projects can result in a wide variety of impacts to surrounding ecosystems, both positive and negative. On the positive side, irrigation projects in Thailand have been credited with reducing the destruction of tropical rainforests by enabling rural people to increase food productivity on smaller parcels of land, thus eliminating the need for continuous clearing of forests for agricultural development. Irrigated fields have improved habitat for some species, particularly birds and small mammals. Some Asian farmers have integrated fish production into their irrigated rice fields, thus creating habitat while maximizing food productivity. In addition, the use of irrigation has improved human welfare dramatically, providing foods and fibers to a large percentage of the world's population that might otherwise have none.

Some irrigation projects, however, have contributed to significant environmental degradation, primarily through poor project planning and administration. Irrigation projects reshape the land surface and change the way water moves, is stored, and is recycled in the hydrologic cycle. Poor management of water applications in irrigation projects has frequently resulted in high salt content of soil, in some cases rendering these lands useless for further crop production. Poor drainage and poor scheduling of water applications have resulted in waterlogged soils in some locations.

For more than 7,000 years, people have constructed diversion and conveyance systems that enable water supplies to nurture crops on previously unproductive or less productive lands. The consequence has been a proliferation of crops, enabling regions to expand population levels, accumulate wealth, and develop cultural and political power. Improper management of the soil and water resources that support irrigated lands, however, can result in salinization of the irrigated soils, silting of canals, and contamination of water supplies. Several formerly powerful ancient cultures virtually disappeared, in part due to poor administration and management of irrigation systems.

Even today, productivity on approximately 25 million hectares (7 percent) of the world's irrigated lands is seriously affected by these problems (Ref. 3). Salinization of fertile croplands affects approximately 1.5 million hectares per year. World Bank studies in Pakistan and Egypt indicate that waterlogging and salinization have decreased the yields of major crops by 30 percent. Approximately 20 percent of the 40 million hectares irrigated in India are also affected by these problems.

Construction of water storage reservoirs and irrigation channel networks affect the amount of moisture in the soil, and the height of the ground water table, and can influence water quality through increased sediments, organic materials, and agrochemicals. Diversion of water for irrigation from the huge Aral Sea in central Russia has largely eliminated local fisheries and wildlife habitat, resulting in the loss of employment for people who fish, creating health hazards, and threatening the very existence of this sea.

Poorly developed and managed water supply sources for irrigation projects have resulted in a considerable loss of the world's wetland habitats and have created new vector breeding environments, contributing to the spread of serious infectious diseases, notably malaria, bilharzia, and yellow fever. In the Sudan, incidents of bilharzia (schistosomiasis) increased more than 80 percent after the development of reservoirs and canals for irrigation systems.

2.5.1 The role of environmental assessment

Many of these adverse impacts are difficult to predict accurately or quantify. In some cases, the effects are the cumulative result of several land alteration practices, with irrigation systems being only one of the causative agents. Still, the fact that irrigation systems can result in or contribute to environmental problems requires all irrigation workers to take full account of potential environmental impacts before any project activities are implemented.

In most countries, an environmental review is now required for virtually all projects that will involve some construction or modification of the environment. In most cases, the general public has insisted that projects consider several alternatives for achieving the same goal, and implement only the alternative that has been demonstrated to represent the least environmental risk. All signs indicate that both professional and public concern over adverse environmental impacts will continue to grow. The irrigation specialist will increasingly be required to document not only the wider impacts of his or her decisions but also the greatest balance that can be achieved between development and conservation.

Volunteers who will be participating in projects that divert water or otherwise alter water sources, construct canals, cultivate fields, or result in any other potential changes to the social or natural environment should be prepared to conduct a careful analysis of potential environmental impacts. The most important output from this analysis should be a documentation of potential impacts and the development of mitigative measures that will be implemented as part of the project to guarantee that any potential environmental problems will be avoided or minimized. Examples of mitigative measures include such actions as ensuring that the vegetation along streams is protected or restored; stocking Gambusia or other larvae-feeding fish in storage ponds to minimize the incidents of mosquito-borne diseases; and building terraces and drains in irrigated fields to minimize waterlogging and soil loss.

The process should begin with an initial environmental review that uses field observations and professional judgment to determine if a proposed project is likely to result in any significant environmental impacts. The review is intended to rely on qualitative analyses and to be done fairly rapidly. An illustrative form that can be used by Volunteers to complete an initial environmental review is included in the next section of this chapter. For most small projects that will not involve the construction of permanent dams, extensive canal systems, or the irrigation of more than 10-15 hectares of land, this initial environmental review should be sufficient to ensure that careful environmental management will be built into all project components.

Larger projects that are likely to result in more extensive environmental impacts can still be initially reviewed using this simple environmental review form. If significant impacts are identified it may be necessary to conduct a more thorough environmental assessment of the project. A comprehensive environmental assessment (EA) will require much more time and professional expertise than may be available to many Peace Corps Volunteers. If a comprehensive EA is deemed necessary for a project, then the Volunteer will most likely need to contact qualified government staff, private sector firms, or international donors to assist with the effort. Volunteers should be aware of the content of any comprehensive EA, which includes the following:

· A concise but thorough description of the proposed actions that will occur, a statement of project purpose, and a review of the roles and responsibilities of all agencies, organizations, or individuals to be involved in the project.

· A detailed description of the physical, biological, and social environment that will, or could potentially, be affected by the proposed project actions. This description should be quantified, wherever possible.

· A discussion of the relationship of the proposed project actions to existing land use plans, policies, and controls for the affected area.

· A detailed description of alternative strategies that can be taken to achieve the stated project purpose. This description should include the project plan (unamended) and a "No Action" alternative, which essentially indicates how the stated project purpose will be affected if the project is not implemented at all.

· A detailed description of predicted environmental impacts that should result from the implementation of each alternative. Environmental impacts should include both beneficial and negative consequences and secondary or indirect effects that may occur. This description should include quantified predictions wherever possible.

· Identification of environmental impacts that cannot be avoided with any of the alternatives, including the "No Action" alternative.

· Identification of the preferred alternative and use of some kind of quantifiable process to show why this alternative is the most preferable selection.

· A detailed description of measures that can be incorporated into the preferred alternative to ensure that any adverse environmental impacts will be minimized or avoided and to guarantee that careful environmental management will be built into the overall project effort.

2.5.2 Illustrative environmental review form for irrigation or water resource development projects

The following illustrative form provides a scheme that can be used by Volunteers or technicians to identify and rank probable environmental impacts from proposed field work prior to the initiation of construction or development activities. The form can also be used to help identify mitigative measures that will eliminate these environmental impacts. Impacts are identified as to whether they are of MINOR, MODERATE, OR SIGNIFICANT environmental concern. Any MODERATE impact should identify measures that can improve the scheme. Any SIGNIFICANT impacts should identify alternative activities that can replace the initial proposed project.

PART I PROJECT CHARACTERIZATION

Project Description
_______________
_______________

1. Obtain topographic map(s) covering the area within or including the proposed project development scheme.

2. Delineate the watersheds, natural drainage directions, and approximate affected area within which the project will occur. Affected area includes lands directly upstream and downstream from the proposed project, which the project could alter by changing runoff or flood flows, vegetation cover, or soil conditions.

3. Obtain land use, vegetation, soils, or flood maps for the affected area if available.

4. Obtain information on the likelihood of occurrence of important wildlife, fisheries, or habitat types, and the presence of threatened or endangered species in the affected area.

5. Obtain information on the likelihood of occurrence of important cultural resources in the affected area.

Note: Bring the topographic map to the field when doing the field review.

PART II FIELD EVALUATION

Based on available information and field observations, evaluate the predicted effect of the proposed project on the following resources. Include proposed mitigation resources, if necessary.

N

NO environmental impact or IMPROVED environmental conditions.

L

MINOR negative environmental impact.

M

MODERATE negative environmental impact.

S

SIGNIFICANT negative environmental impact.

U

UNKNOWN environmental impact.

I Water Resources

Use the topographic map to make a brief sketch of the proposed development scheme. Identify significant local features, including rivers, canals, ponds, lakes, forest lands, wetlands, other unique habitats, and archeological sites. Show the apparent flow directions for floods and note any obvious signs of flood depths or flood locations for a 10-year flood. Include a map scale.

1. The project is located in a floodplain area. (Yes) (No)

2. The project will increase/decrease/have no effect on the duration of floods (circle one answer). If yes, the ranking of impact is ________.

3. The project will change the location of floods. (Yes) (No) If yes, the ranking of impact is _______.

4. The project will change stream channel shapes, or the amount or type of vegetation alongside streams. (Yes) (No)


Impact Ranking

MINOR

MODERATE

SIGNIFICANT

Some change, but overall channel/vegetation remains the same.

Causes measurable change in shape or vegetation.

Causes major change in channel shape or vegetation.

5. Recommended measures to mitigate (reduce) effects: _________.

6. Explain how recommended measures will mitigate moderate or significant impacts.

7. What moderate or significant impacts cannot be mitigated and why?


II Fish and Wildlife

1. Project will result in destruction or degradation of habitat for fish or wildlife. (Yes) (No) If yes, the ranking of this impact is ________.

2. Project will block fish or wildlife migratory pathways. (Yes) (No) If yes, the ranking of the impact is ________.


Impact Ranking

MINOR

MODERATE

SIGNIFICANT

Impacts localized and no impacts to migratory pathways or habitat.

May temporarily block pathways or eliminate habitat.

Will block migratory pathways or will eliminate habitat.

3. Endangered or threatened species are likely to occur in the project area. (Yes) (No) If yes, list.

4. Proposed project will result in loss of individual threatened or endangered species. (Yes) (No) If yes, which one(s) and how?

5. Will any loss of habitat affect the survival of species? Explain.

6. Recommended measures to mitigate effects: ________.

7. Explain how recommended measures will mitigate moderate or significant impacts.

8. What moderate or significant impacts cannot be mitigated and why?


III Vegetation

1. Vegetation types found in the area include _______.

2. Project will result in cutting of trees, or loss of vegetative cover. (Yes) (No) The ranking of the impact is _______.


Impact Ranking

MINOR

MODERATE

SIGNIFICANT

No loss of important plant communities or changes in runoff.

Some loss of important plants and change in runoff patterns.

More than 50 percent of plant cover lost and runoff patterns vary greatly.

3. Recommended measures to mitigate effects: ________.

4. Explain how recommended measures will mitigate moderate or significant impacts.

5. What moderate or significant impacts cannot be mitigated and why?


IV Soils

1. Changes in soil moisture:

(a) waterlogging ________.
(b) drought ________.

2. Project will cause soil loss from farm fields. (Yes) (No) If yes, ranking is ______.

3. Project will result in adverse changes to soil physical or chemical conditions. (Yes) (No) If yes, ranking is ________.


Impact Ranking

MINOR

MODERATE

SIGNIFICANT

Soil loss less than 5 tons/ha/year, and no salt or chemical concentration.

Soil loss greater than 5 tons/ha/year, and some salt or chemical concentration.

Soil loss greater 10 tons/ha/year, and high salt or chemical concentration.

4. Recommended measures to mitigate effects: ________.

5. Explain how recommended measures will mitigate moderate or significant impacts.

6. What moderate or significant impacts cannot be mitigated and why?


V Cultural Resources

1. Proposed project will result in the loss of archeological, historical or cultural resources. (Yes) (No) If yes, identify resources and rank the impact.


Impact Ranking

MINOR

MODERATE

SIGNIFICANT

Project will make cultural resource less accessible.

Project will result in loss or damage to less important resources.

Project will result in the loss or damage of important resources.

2. Recommended measures to mitigate effects: ________.

3. Explain how recommended measures will mitigate moderate or significant impacts.

4. What moderate or significant impacts cannot be mitigated and why?


VI Summary of Moderate and Significant Impacts and Recommended Mitigative Measures ________.


<<TOC2>> Chapter 3 - Developing water sources

Reference:

Primary:

(5), (15), (20), (28), (39), (54), (58)


Other:

(13), (28), (59)

3.1 Diversions

3.1.1 Types

Diversion structures are used to separate all or part of the flow in a stream to a location where it will be used or stored. Diversion structures may be either temporary or permanent. If high flows make a permanent structure very expensive, then temporary structures that can be easily and economically reconstructed might be installed.

In streams or springs that have constant flow year-round, or stream beds that are very wide, a permanent diversion can be used to channel water, generally at an angle, towards the bank. The diversion may be constructed across only a part of the stream's bed.

In streams with narrow beds and small flow rates, the diversion may span the stream and pond up the water.

3.1.2 Types of construction materials

Depending on the availability and cost of materials, and the size and purpose of the structure, the following materials are commonly used:

· soil,
· boulders and rocks,
· logs,
· sand bags,
· gabions,
· masonry, and
· concrete.

3.1.3 Construction and maintenance factors

1. Many diversions are only temporary structures, so they should be built with the understanding that they will be destroyed by flood water during hard rainfall.

2. Diversions can be constructed so that part of the structure is permanent and the other part temporary. In areas where there is a large variation in stream flow during a relatively short period of time, this system of diversion reduces the amount of labor required to reconstruct the structure after each rainfall.

3. Design of any permanent structure should take into consideration the amount of sediment that will settle upstream and reduce water storage volume. Sediment traps can be constructed upstream of the diversion to reduce some of the sediment. Maintenance will have to be performed after each rainfall to clean the sediment traps. In watersheds with steep slopes, special precautions should be taken for the probable movement of large rocks in the stream bed during high flow.

4. If the diversion structure will be used as a check dam to allow for water storage, the site of the diversion should have a natural basin upstream to maximize water storage volume and decrease the necessary height of the diversion or dam.

5. Gabions are baskets made of heavy duty galvanized wire mesh that are filled with rocks and wired shut. The gabion provides an easily constructed unit that is large enough and heavy enough to remain stable in moving water. Gabions can be made to any convenient size ranging from 2-4 m long by 1 m wide by 0.5-1 m deep. The wire mesh is usually 2-3 mm in diameter. Evenly graded stones are used so that the gabions are well packed with few empty spaces, and the largest stone should not be more than two-thirds of the minimum gabion dimension.

6. Diversions constructed of materials that allow excess amounts of seepage may require some type of impervious core or barrier to reduce losses. These barriers are generally installed during the construction of the structure. A common core, if available, is clay. Other barriers include heavy grade plastic sheets, used fertilizer/animal feed sacks, and sod.

7. Diversion structures require upstream and downstream side slopes to assure their stability. The ratio generally depends on the construction material, but, for small structures, values of 2:1 upstream and 1.5:1 downstream are acceptable.

8. A spillway, overflow structure, or other bypass structure is necessary to allow water from the stream to pass over the diversion structure and flow back into the natural watercourse without washing out or damaging the structure. Water needs to be channeled through a bypass so that it does not flow over the top of the entire length of the diversion, which could cause erosion problems. Additionally, the energy of the water falling from the outlet to the stream bed must be dissipated. Rocks or other durable materials can be placed at the base of the downstream side of the bypass structure for protection.

9. If the diversion provides water to a pipeline, a trash removal system may be required to prevent blockage of the pipe. Sometimes a simple screen over the inlet may suffice. If sediment is a problem, an offstream sedimentation pond may be required.


<<TOC3>> 3.2 Concrete

Concrete is widely used for irrigation systems. It has several desirable properties that make it a versatile and popular building material. Freshly mixed concrete can be formed into practically any shape.

Rules for making good concrete are simple:

· Use proper ingredients (cement, sand, and gravel).
· Proportion the ingredients correctly.
· Measure the ingredients accurately by volume.
· Mix the ingredients thoroughly.

Table 3.1 provides an initial estimate of the ratios of ingredients to be used to make common small-scale concrete structures:

TABLE 3.1 Concrete Ratios by Volume

Coarse Aggregate,

Cement

Sand

Coarse

Water

Maximum Size, mm (in)



Aggregate


10 (3/8)

1

2 1/2

1 1/2

1/2

13 (1/2)

1

2 1/2

2

1/2

19 (3/4)

1

2 1/2

2 1/2

1/2

25 (1)

1

2 1/2

2 3/4

1/2

38 (1 1/2)

1

2 1/2

3

1/2

3.2.1 Hand mixing

Mixing should be carried out on a clean, hard surface or in a mortar box. Concrete should not be mixed on the ground. Commonly, the correct proportion of ingredients is measured by shovelfuls. A small bucket is also a good measuring device. The ingredients are first thoroughly mixed together dry until the mix is of a uniform color and consistency. Clean water is added slowly and in small quantities; with each addition of water, the mix is turned over a few times with a shovel. Water is added until the concrete is at the desired consistency. Concrete should not be any runnier than is necessary to work with because it loses strength if it is too watery.

Wet concrete is poured; it should not be dumped from any height since separation may occur. Before it sets, the concrete must be settled and compacted (rodded) to eliminate air pockets and make sure that there is good contact with the foundation materials. Excessive trowelling causes segregation of the aggregates and should be avoided. Forms are removed after the concrete has hardened (a few days). It should be kept wet for at least 3 days, preferably 7, to assure adequate curing time.

<<TOC3>> 3.3 Designing structures for springs and seeps

Protective structures for springs and seeps assure a clean water supply to irrigation systems. The protective structure increases the volume of water that can be diverted from the spring and protects the site from contamination by runoff or animals.

Developing a spring or seep requires some understanding of ground water flow (see Chapter 2) and preparation of a thorough construction plan. The construction plan should include (a) a map of the area identifying the location of the spring, the locations of water use, distances from source to use outlet points, and surveyed changes in elevations; (b) a complete list of all labor, materials, and tools needed; and (c) a spring box design with diagrams of the top, side, and front views, and the dimensions of a cover. Spring box structures can be costly in terms of the amount of time and finances invested, so careful planning is essential.

3.3.1 Spring box designs

There are several possible designs for springs boxes, but, in general, their basic features are similar. Two basic design choices are a box with one pervious side for collection of water from a hillside and a box with a pervious bottom for collection of spring water flowing from a single opening on level ground. To determine which design to use, dig out around the area until an impervious layer is reached, locate the source of the spring flow, and design to fit the situation. Figures 3.1 through 3.10 and Table 3.2 provide sample drawings and work sheets.

General Construction Steps

The following steps are appropriate for either design choice:

1. Locate the spring site and mark out the area with measuring tape, cord, and wooden stakes or pointed sticks.

2. Clean out the area around the spring to ensure a good flow. If the spring flows from a hillside, dig into the hill far enough to determine the origin of the flow. Where water is flowing from more than one opening, dig back far enough to ensure that all the water flows into the collecting area. If the flow cannot be channeled to the collection area because openings are too diffuse, drains will have to be installed. Flow from several sources may be diverted to one opening by digging farther back into the hill. Always try to dig down deep enough to reach an impervious layer. An impervious layer makes a good foundation for the spring box, and provides a better surface for a seal against underflow.

3. Pile loose stones and gravel against the spring before putting in the spring box. The stones serve as a foundation for the spring box and help support the ground near the spring opening to prevent dirt from washing in.

4. Approximately 8 meters above the spring site dig a trench for diverting surface runoff. Use large stones, if available, to line the diversion trench and prevent erosion.

5. Mark off an area about 9 meters by 9 meters for a fence. Place the fence posts 2 meters apart and string the fence.


Concrete Construction Steps

In order to have a strong structure, concrete must cure at least 7 days. Strength increases with curing time. Be sure that all tools and materials needed to build the forms and mix the concrete are at the site.

1. Build wooden forms. Once the dimensions of the box have been drawn, cut wood to the appropriate sizes and set up the forms on a level surface. The outside dimensions of the forms should be 0.1 meter larger than the inside dimensions. An open bottom or back should be planned, depending on the spring source location. The size of the opening in the form depends on the area that must be covered to collect the maximum water. When building forms for a box with a bottom, be sure to set the inside forms 0.1 meter above the bottom for the floor. This is done by nailing the inside form to the outside form so that it hangs 0.1 meter above the floor. Make holes in the forms for the outflow and overflow pipes. Place small pieces of pipe in them so that correctly sized holes are left in the box as the concrete sets. Build a form for the box cover. Build all forms at the site.

2. Set the forms in place. Forms must be well secured and braced before pouring the concrete. The braces can be tied together with wire. Use a stick to tighten the wire and force the forms together. If the forms are set and concrete is poured at the permanent site, water must be diverted from the area to allow the concrete to cure. If diversion is not possible, pour the concrete near the permanent site, and plan to have 6-8 people available to help move the box later after the concrete has cured.

Clay/Cement Mixture

- Mix equal parts dry cement and dry powdered cement
- Add water till plastic (not too wet)
- Slap down mixture around spring onto nerd surface to start (It will not erode in flowing water)
- Build up walls capturing spring and add tubes as level is reached
- Add large rocks in water flow leaving a good water path
- Cover large rocks with progressively liar rocks and than gravel
- Finish off with a layer of sand or dirt


Figure 3.1 Spring Box Design


Figure 3.2 Spring Box with Open Sides (Ref. 39)


Figure 3.3 Spring Box with Open Bottom (Ref. 39)


Figure 3.4 Forms for Spring Box with Open Side (Ref. 39)


Figure 3.5 Forms for spring Box with Open Bottom (Ref. 39)


Figure 3.6 Excavating Spring Site (Ref. 39)


Figure 3.7 Placement of Rebar in Concrete Slab (Ref. 39)


Figure 3.8 Forms for spring Box Cover (Ref. 39)


Figure 3.9 Brick Spring Box (Ref. 39)

Figure 3.10 Calculating Quantities Needed for Concrete (Ref. 39)

(CaIculations for a box 1m 1m 1.0m with open bottom)

Total volume of box = length (1) width (w) height (h)

Thickness of walls = 0.10m (t)

1. Volume of top = 1 1.2 m w 1.2 m t 0.10 m = 0.144 m3
2. Volume of bottom = 1 0 m w 0 m t 0 m = 0 m2
3. Volume of two sides = 1 1 m w 1 m t 0.10 m 2 = 0.20 m3
4. Volume of two ends - 1 1 m w 1 m t 0.10 m 2 = 0.20 m3
5. Total volume = sum of steps 1, 2, 3, 4, 5 = 0.54 m3
6. Unmixed volume of materials = total volume 1.5; 0.54 m3 1.5 = 0.81 m3
7. Volume of each material (cement, sand, gravel, 1:2:3):

cement: 0.167 volume from Line 6 0.81 = 0.13 m3 cement.
sand: 0.33 volume from Line 6 0.81 = 0.26 m3 sand.
gravel: 0.50 volume from Line 6 0.81 = 0.4 m3 gravel.

8.

volume of cement 0.13 m3/.033 m3/bag = 4 bags.

9. Volume of water = 28 liters 4 bags of cement - 112 liters.

(NOTE:

1) Do not determine volume for an open side or bottom.
2) The top slab has a 0.1m overhang on each side.
3) The same calculations will be used to determine the quantity of materials for construction of a seepage wall.
4) To save cement a 1:2:4 mixture can be used.)

Table 3.2 Sample Materials List (Ref. 39)

Item

Description

Quantity

Estimated Cost

Labor

Foreman Laborers

______

______

Supplies

Portland cement

______

______


Clean sand and gravel, if available or locally available sand and gravel.

______

______


Water (enough to make a stiff mixture)

______

______


Wire mesh or rein forcing rods

______

______


Galvanized steel or plastic pipe (for outlets, overflow, and collectors)

______

______


Screening (for pipes)

______

______


Boards and plywood (for building forms)

______

______


Old motor oil or other lubricant (for oiling forms)

______

______


Baling wire

______

______


Nails

______

______

Tools

Shovels and picks (or other digging tools)

______

______


Measuring tape or rods

______

______


Hammer

______

______


Saw

______

______


Buckets

______

______


Carpenter a square or equivalent (to make square edge)

______

______


Mixing bin (for mixing concrete)

______

______


Crowbar

______

______


Pliers

______

______


Pipe wrench

______

______


Wheelbarrow

______

______


Adjustable wrench

______

______


Screwdriver

______

______


Trowel

______

______

Total Estimated Cost

________

3. Oil the forms. This prevents the concrete from sticking.

4. Prepare the reinforcing rods (rebar) in a grid pattern for placement in the forms for the spring box cover. Make sure there is 0.15 meters between the parallel bars and that the rods are securely tied together with wire. Then position the reinforcing rods in the form. Four bars tied together to form a square should be placed in the forms.

5. Mix the concrete in a proportion of 1 part cement, 2 parts sand, and 3 parts gravel. Add just enough water to form a thick paste. More gravel can be used to conserve cement.

6. Pour the concrete into the forms. Tamp the concrete to avoid pockets or voids. Smooth all surfaces. Make the middle of the cover a little higher than the sides to encourage drainage away from the spring box.

7. Cover the concrete with canvas, burlap, empty cement bags, plastic, straw or some other protective material to prevent it from losing moisture. The covering should be kept wet so water from the concrete is not absorbed. If concrete days, it no longer hardens, its strength is lost, and it begins to crack. Keep the cover on for as long as the concrete is curing.

8. Let the concrete structures set for at least 7 days, wetting the concrete daily. After 7 days, the forms can be removed and the box installed.

Installing a Spring Box

The spring box must be properly installed to ensure that it fits on a solid, impervious base, and that a seal with the ground is created to prevent water seeping under the structure.

1. Place the spring box in position to capture the maximum flow. Place gravel around the box or in the basin so that water flows through it before entering the box.

2. Seal the area where the spring box makes contact with the ground. Use concrete or puddled clay to form a seal.

3. Be sure that the area where the spring flows from the ground is well lined with gravel, and then backfill the dug out area with gravel. The gravel fill should reach as high as the inlet opening in the spring box so that the water flowing into the structure passes through gravel. For spring boxes on level ground, gravel backfill is unnecessary.

4. Place the pipes in the box. Use concrete to seal around the pipes and prevent leaks. Place screening over the pipe openings and secure with wire.

5. Disinfect the inside of the box with a chlorine solution. Before the spring box is closed, wash its walls with a light solution of chlorine.

6. Place the cover on the box.

<<TOC3>> 3.4 Ponds for irrigation water storage

Ponds or small reservoirs can be extremely important water storage structures for the irrigated farm. Some reasons for constructing water storage ponds are to:

· collect water from small spring flows so that it can be used efficiently when needed in large flow rates;

· provide overnight storage of canal water that is available at night;

· store water for times of critical need; and

· regulate flow.

Water for ponds can come from irrigation supply canals, natural streams, springs, wells, or even rainfall runoff. In selecting a site for a pond, one should consider the location of the water supply, the availability of water, the soils, and the topography of the site. One should not normally construct ponds and small reservoirs in streams, ponds, gullies, or places where severe storms could create high flows that would wash away the dam. Small, off-stream ponds and reservoirs generally will not have to handle intense storm runoff.

3.4.1 Location of the pond

The pond should be located on or above the highest part of the farm to avoid the need for pumping. Water can flow freely from the pond. The pond should be located downslope from the water supply, unless water must be pumped to a higher elevation. Preferably water will flow freely from the source to the pond. A topographic survey will indicate the correct elevations to optimize flow from the water source to the pond.

3.4.2 Availability of water

Generally, one liter per second will allow the farmer to irrigate one hectare of land. If water from a spring or other source will be stored and emptied at a given interval from the pond, the pond must be able to hold the maximum amount of water that the farmer needs to hold. For example, if a spring supplies 1/2 liter per second, and the farmer plans to store water for 12 hours, he must be able to store:

Volume = Q T =.0005 m3/sec 3600 sec/hr 12 hrs = 21.6 m3

3.4.3 Soils

Ponds should be as impermeable as possible to prevent leakage. The soils should contain a layer of material that is impervious and thick enough to prevent excessive seepage. Clays and silty clays are excellent for this purpose; sandy clays are usually satisfactory. Coarse-textured sands and sand-gravel mixtures are highly pervious and, therefore, usually unsuitable. The absence of a layer of impervious material over part of the ponded area does not necessarily mean that you must abandon the proposed site. However, the pond will have to be sealed in other ways.

You can generally determine if the soil has sufficient amounts of clay by wetting it, feeling it in your hand, and squeezing it between your fingers. If the soil can be molded, and if it can be ribboned out between the thumb and forefinger, the clay content is usually adequate.

3.4.4 Topography

Generally, reservoirs 2 to 3 meters in height can be constructed by any small-scale farmer if the design and construction are proper. Topography may determine the type of pond that can be built. Sloping topography will allow the farmer to fill and drain ponds using gravity. The topography may determine whether the farmer will be able to build an above-ground or below-ground reservoir or pond.

Some ponds are made by constructing a dam across a creek bed or gully to intercept water that would otherwise be lost after rainfall or to intercept water coming from a small creek or spring. This is possible only for very small watersheds that do not develop high flows during severe storms. One must be careful, however, to provide spillways and bypasses for excess water. Offstream ponds connected to a stream diversion structure provide protection against failures due to flooding.

3.4.5 Design end construction

A pond consists of 4 components - the walls (dikes or levees), the floor, the inlet works, and the outlet works. The walls should be impermeable (or nearly) and capable of withstanding the pressure exerted outward by the pond. The floor must be as nearly impermeable as possible. The inlet works must be able to receive the expected amount of water. The outlet works should have the ability to regulate flow from the pond and also provide for the overflow of excess water.

Figure 3.11 provides some ideas for outlet works. Inlets often consist of a simple pipe of sufficient diameter to carry the maximum expected inflow. The Rivaldi valve (a flexible irrigation hose that can be submerged to allow water to flow from the pond) provides a convenient means of regulating the outflow, as does the siphon tube(s).


Figure 3.11 Outlet Works for Small Irrigation Pond

The walls and floor are best made of a mixture of sand and clay. If the soil is of a sandy or sandy loam texture, clay or other lining materials must be used. If clay is hard to come by, a 15 cm layer of sand/clay mixture (at least 20% clay) may be laid on the bottom of the pond and used as a core in the side walls (see Figure 3.12.) In all cases, the soils should be well compacted to ensure that leakage is held to a minimum.

Waterproof linings made of plastic, vinyl, or butyl rubber may be used. These can be expensive, however, and, if not properly installed, can quickly develop leaks. All of these linings should be laid over a smooth bed and have an earth cover of not less than 15 cm (at least 8 cm of material not coarser than sand). Keep livestock out of the pond to prevent puncturing the lining. Linings should be overlapped 15 cm at the seams. All vegetation and roots of woody vegetation should be cleared to prevent the plants from breaking through the lining. Polyethylene plastic should be laid out with 10% slack.

The side slopes of the inside of the pond should be no steeper than 3 to 1 on the inside (water side) of the bank and 2 to 1 on the outside banks. Top width for ponds up to 3 meters in height should be a minimum of 2 meters to assure stability.

The freeboard, or distance between the maximum expected high water and the top of the berm (a berm is the wall of a pond), for small ponds of less than 3 meters in height and 1 acre (0.4 hectares) in size can be 60 cm.

The spillway should not allow water to come closer than 30 cm from the berm during periods of overflow.

If water has a significant amount of silt in it, a desilting pond may be necessary for removal of sediment before the water enters the storage pond.


Figure 3.12 Lining and Compaction of Irrigation Pond (Ref. 7)

3.5 Pumps and water lifting devices

Human and animal-powered water-lifting devices have been in use for thousands of years. These water-lifting devices have been used to provide water for irrigation as well as for drinking and other uses. More recently, machine-powered pumps have become the primary means of lifting water from wells and surface water sources.

The use of hand-dug wells as a source of water for irrigation also dates back several thousands of years. Hand-dug wells are still an important means of securing a reliable source of water in many developing countries. Hand-dug wells, however, are generally limited to areas where the water tables are within 20 meters of the ground surface. Modern well drilling technologies now permit wells to be drilled to several hundred, and even thousands, of feet in depth.

The design, construction, and use of deep wells requiring the application of modern technology is beyond the scope of this manual. Some basic concepts about shallow and deep wells, however are presented to allow the Volunteer to gain a basic understanding of types of wells and their characteristics. Hand-drilled wells and driven (sand point) wells are low-cost and appropriate technologies for many applications where water tables are shallow. These methods are presented in some detail. Other wells drilling methods that require the use of costly commercial drilling techniques and higher technologies are described very superficially. The Volunteer should consider these technologies where appropriate and can read a number of references for greater understanding (See Ref. 13). Design and construction of these wells should be left to the experienced engineer and/or commercial water well contractor.

The selection, design, construction, and maintenance of pumps to provide water from deep wells should be done with the assistance of experienced engineers and well installers. Basic concepts required for the selection, installation, and operation of some pumps and water lifters that can take water from streams, ponds, or shallow wells, are presented in this section. (Extensive information on the use of human and animal-powered water lifters is included in Refs. 28, 58, and 59.)

3.5.1 Types of pumps

The type or kind of pumping system selected depends upon many factors, including (1) the total dynamic head (TDH) or pumping lift; (2) the required discharge (Q); (3) the source of available energy; and (4) the operating conditions.

The use of mechanical power and modern pumps makes possible the lifting of large quantities of water to considerable heights.

The supply of oil and electricity have made modern pumping economically feasible. Worldwide energy shortages have recently caused concerns in countries where fuel oil must be imported. Man and animal pumps are still used in some areas where irrigation is primarily associated with subsistence agriculture. Several types of hand and animal powered pumps are illustrated in Figures 3.13 through 3.25.

3.5.2 Pumps powered by humans and animals

The simplest form of a manual lifting device is the water bucket or scoop used to lift water from shallow wells, reservoirs, or canals. The use of a bucket, rope, and roller or pulley increases the possible lift and allows the lifter to use his weight to assist in pulling down. Animals have also been used in some countries.

Labor costs of human powered irrigation are high unless there is no alternative demand for the labor supply. Where crop failure may result in serious malnutrition, the use of human and animal pumps for irrigation may be necessary.

3.5.3 Animal-powered pumps

Animal power has long been used for water lifting. Animals in good condition can develop about 0.10 HP per 100 kg of body weight. A 1,000 kg horse produces approximately one horsepower. Many different systems are used.

When animals are used for lifting water, the cost of owning and feeding them must be included in the cost of pumping water. A major problem in the use of manual and animal lifting devices for irrigation is the small amount of water that can be pumped. Irrigated areas are generally limited to garden-size plots of less than one-fourth hectare and very shallow lifts.

3.5.4 Mechanically driven pumps

Pumps powered by mechanical means other than man or animal can be classified as positive displacement pumps or variable displacement pumps. Positive displacement pumps (piston pumps, diaphragm pumps, gear pumps, screw pumps) are seldom employed for irrigation and will not be discussed in this manual.

Variable displacement pumps include the centrifugal, mixed flow, turbine, propeller and jet pumps, and the hydraulic ram. The first four of these are commonly used for irrigation.


Figure 3.13 Archimedean Screw (Ref. 28)


Figure 3.14 Hand (Piston) Pumps (Ref. 28)


Figure 3.15 Swing Baskets (Ref. 28)

Wicker swing basket of average capacity 8 litres.

Swing basket made from metal sheets


Figure 3.16 eater Scoop (Ref. 28)


Figure 3.17 Suspended Scoops (Ref. 28)


Figure 3.18 Noria: (Ref. 58) (a) With Fixed Buckets, Driven by Currents


Figure 3.18 Noria: (Ref. 58) (b) With Larger Paddles


Figure 3.18 Noria: (Ref. 58) (c) With Moveable Buckets


Figure 3.19 Modified Persian Wheel or Zawafa (Ref. 58) (a)


Figure 3.19 Modified Persian Wheel or Zawafa (Ref. 58) (b)


Figure 3.20 Picottah, Using Man as Moveable Counterweight


Figure 3.21 Picottah style Doon with Flap Valve (Ref. 58)


Figure 3.22 Gutters: (Ref. 58) (a) Single with Handle


Figure 3.22 Gutters: (Ref. 58) (b) "See-Saw"


Figure 3.22 Gutters: (Ref. 58) (c) Modified to Increase Capacity


Figure 3.23 Modifications of the Doon (Ref. 58) (a)


Figure 3.23 Modifications of the Doon (Ref. 58) (b)


Figure 3.24 Self- Emptying Not with Inclined Tow Path (Ref. 58)


Figure 3.25 Rower Pump (Ref. 28)

Each type of pump has advantages for a given set of conditions, such as suction lift, total lift, discharge, efficiency of operation, and cost. Pumping systems (combinations of pump motor and accessories) need to be designed or selected for the range of conditions expected under normal operation.

3.5.5 Centrifugal pumps

In centrifugal pumps, water enters the center of the impeller (Figure 3.26), is picked up by the vanes, and accelerated to a high velocity by rotation of the impeller. The water is forced to discharge into the casing, where much of the velocity energy is converted to pressure. When water is forced away from the center, or the "eye" of the impeller, a vacuum is created and atmospheric pressure pushes more water in.

Centrifugal pumps are available in a wide range of sizes and flow rates and can be used for both low and high head or pressure applications. Centrifugal pumps are recommended for pumping from rivers, lakes, canals, and wells. Usually, but not always, the pump is located above the water source.


Figure 3.26 Components of a Centrifugal Pump (Ref. 20)

3.5.6 Propeller or axial flow pumps

Propeller pumps are usually selected for pumping large volumes of water against relatively low (lifts) heads. Capacities range from 40 to 6,000 L/s (liters per second). Total dynamic head (TDH), which is pump lift plus friction losses, is usually from 1 to 2 meters but not more than 10 meters under certain design conditions. As the term "axial flow'' implies, the impellers lift the water and push it forward perpendicular to the plane of rotation, or parallel to the axis.

3.5.7 Mixed flow pumps

Mixed flow pumps both lift the water and accelerate it. Mixed flow pumps are used for intermediate lifts over a wide range of flow rates. Most mixed flow pumps are installed where head requirements do not exceed 15 m, although they are available for heads ranging from 6 to 25 m. Capacities vary from 40 to 6,000 L/s.

3.5.8 Turbine pumps

Turbine pumps are centrifugal pumps used in shallow to deep well applications. A turbine pump is designed so that it can be easily multistaged, developing several times the pressure obtained from a single stage pump. Deep well turbines are usually multistage types - that is, the turbines (impellers) or "bowls" are placed directly above each other. Each turbine picks up the flow and boosts, or increases, the pressure, thus making it possible to lift the water to higher elevations. Turbine pumps can be designed for discharges of less than one liter per second to more than 600 L/s. Deep well turbines are either installed with a motor at the surface, utilizing a long drive shaft, or with a submersible electric motor installed below the various stages of impellers. The submersible motor is often selected if the well has a crooked bore, if the drive shaft would be excessively long, if there is danger of flooding at the surface, and where economy and initial cost are favorable. A summary of pump types is given in Table 3.3.

TABLE 3.3 A Summary of the Type of Pump Needed to Meet various Pumping Conditions (Ref. 20)

Condition

Pump Type

A. Low to high lift, suction not exceeding 4 to 4.5 m, and low to moderate capacity.

Centrifugal (end suction)

B. Low lift and large capacity.

Propeller or axial flow

C. Low to moderate lifts and moderate to high capacities.

Mixed flow

D. Deep well with high lift over wide range of capacities (usually low to moderate).

Deep well turbine(semi-enclosed or enclosed multiple-stage impellers)

Centrifugal pumps in end section and deep well turbine configurations are illustrated in Figure 3.27

3.5.9 Sources of power

The kind of power used to pump or lift water depends to a large extent upon cost, availability, and the amount of water to be pumped. Countries that do not have, and cannot afford, an adequate supply of mechanical energy may use man and animal power. (Both of these require energy in the form of food or fodder.) Wind and solar power have been used to pump small amounts of water. These sources of power will probably increase significantly in use as technology improves.

Table 3.4 is a comparison of various energy sources. Note that a liter of fuel provides about the equivalent energy of three to five person days of work. A person's output is about 0.08 to 0.10 horsepower (HP) and, when pumping water, the overall efficiency is about 60 percent; therefore, the usable horsepower a person can develop is about 0.05. If water is pumped to a height of 4 meters, three people, working continuously during the growing season in 8-hour shifts each, could pump 1 liter per second, which is taken as the normal maximum irrigation requirement for general crops other than rice for one hectare in many parts of the world.

Where large amounts of water are to be pumped to higher elevations, mechanical pumps are necessary. Gasoline engines are suitable for small commercial developments. Diesel engines, however, are used for most commercial pumping operations where electricity is not available. Diesel engines are usually more economical than other internal combustion engines and have a longer life. Gasoline engines, including tractors, are more expensive to operate than diesel. They are lighter in weight, however, can be more easily moved, and are adaptable to many conditions where not more than 15 to 20 HP is required.


Figure 3.27 Pump Types and Configuration.

TABLE 3.4 Comparative Costs to Pump 1,000 Cubic Meters of Water to a Height of 2 Meters (Ref. 20)

Energy

Energy


Energy or Fuel

Time

Costs

Source

HP

KWH

Energy Units

Required

Required

(U.S. $)

Human

0.09a

0.32b

person-dayc

16.90 person-days

8 hrs

$84.50g

Animal

0.10e

1.79f

per day/100 kg weight

3.04 animal-days

8 hrs

$91.20g

Diesel

9.12h

6.82

liters

3.12 liters

1 hr

$ 0.82i

Gasoline

9.12h

6.82

liters

4.07 liters

1 hr

$ 1.36j

Propane

9.12h

6.82

liters

5.16 liters

1 hr

$ 1.08k

Electric

9.17h

6.82

KWH

6.82 KWH

1 hr

$ 0.61l

a In this table, a person is assumed to produce 0.09 HP.
b 0.09 HP 0.746 KW/HP 8 hr/day 60% eff. = 0.32 KWH
c 8 hours
d Labor costs of $5.00/person per day
e Assumes 0.10 HP per 100 kg of animal weight
f 500 kg animal 0.10 HP/100 kg 0.746 KW/HP 8 hrs/day 60% pump; efficiency = 1.79 KWH
g $30.00 per animal/day
h Pump efficiency 80%
i Diesel fuel at $0.26/liter
j Gasoline at $0.33/liter
k Propane at $0.21/liter
l Electricity at $0.09/KWH

The electric motor can be a fraction of horsepower or of many thousands of horsepower. Smaller sizes are usually single phase, while above 3 HP horsepower, motors are normally 3-phase. The initial cost is usually lower than for other motors. Costs of operation and maintenance are low, and they have a long and useful life. Electricity has often been considered the ideal source of energy, and electric motors are suitable for all sizes of installation, from very small to very large. There is much variability in cost and dependability of electrical energy, however, and availability, dependability, and uniformity of the electric power need to be considered. Voltage fluctuations can damage a motor. Power outages during critical periods can result in serious loss of crop production.

Wind has historically been an important source of power, particularly for domestic and livestock water. Wind velocities are not significant in many interior valleys. Conditions are more favorable on hill tops along coastal plains and on islands. Generally, the velocity increases with elevation above a base that breaks the wind. Costs of constructing windmills increase exponentially with the height of the tower. Windmills can pump water for garden-size plots if sufficient wind is available.

Water provides the lifting power for some irrigation projects and small enterprises. The noria (Fig. 3.18) or water wheel is a cheap and economical means of lifting water up to 3 or 4 meters (10 or 12 feet) above the water surface of a river or small stream. A large metal wheel welded onto a wheel from an old automobile, operating on its original bearings, is inexpensive and effective. In some developing areas, the use of pumping projects organized as small cooperatives has largely eliminated the use of both human and animal power.

3.5.10 Selection of pumps and power units

The selection of pump and power units depends upon several factors, including:

· amount of water to be pumped;

· the operating efficiencies (this includes the efficiencies of individual components such as impellers, gearheads);

· the pumping head (lift and/or pressure requirements);

· horsepower requirements;

· available energy (e.g. electricity, gasoline, diesel);

· cost and returns on investment; and

· the size of farm, type of irrigation, and the available labor supply.

Pumps are usually designed for a specific set of operating conditions. A significant departure from these conditions decreases efficiency; therefore, the pump must be operated at or near the design values.

In designing a pumping system, it is best to plan activities so that the pump operating conditions will be as constant as possible and that changes in water requirements will be compensated by increasing or decreasing the hours of pump operation.

3.5.11 Amount of water to be pumped

The amount of water to be pumped depends upon crop water requirements, the area to be irrigated, and the irrigation efficiency. The size of the pumping plant needed depends upon the amount of water required and the time that will be devoted to pumping.

The most economical use of investment capital is to select a pump for continuous operation. In irrigated agriculture, however, pumps are usually designed and operated continuously only during peak crop water requirements. During other growing stages, the pump is operated at intervals to meet crop water use. Some irrigators prefer to irrigate only during daytime hours. In the design of a system, the decision should be made so as to economize both labor and pumping plant costs.

A first approximation of the size of the pumping plant needed can be made by assuming that, for maximum crop growth, the plants will require an application equivalent to one liter per second per hectare (1 L/sec/ha), or 6.4 gpm/acre (gallons per minute/acre) of continuous pumping. This assumes that rainfall is negligible. Another rough estimate is that crop water requirements during the growing season are seldom less than 3 mm per day (0.12 inches per day) and usually will not exceed 8 mm per day (0.32 inches per day) during maximum use.

The amount of water a pump must deliver can be calculated from the continuity equation:

Qt = 28ad (metric)

where

Q = required pump discharge in liters/second
t = time in hours
a = area in hectares
d = desired irrigation depth in centimeters

and, in the English system, approximately by

Qt = ad (English)

where

Q = cubic feet/second (cfs)
t = time in hours
a = acres
d = inches

Note that in the metric system a pump discharge of 1 liter per second will cover one hectare to a depth of 8.64 mm in 24 hours, or 0.36 mm per hour. In the English system, 1 cfs will cover 1 acre to a depth of 1 inch each hour.

3.5.12 The pumping lift or head

The total pumping lift or amount of pressure that a pump must develop to force the water through pipes, sprinklers, etc., and to the desired elevation is referred to as "head" or total dynamic head (TDH). The relationship, if water were standing-in a vertical pipe, is


Head


Pressure

English system

100 feet

=

43.3 psi

Metric system

100 meters

=

10 kg/cm2

Figure 3.28 shows a typical pump installation for a centrifugal pump drawing water from a canal or pond and discharging through a sprinkler system.

The total dynamic head (TDH) is a measure of the energy per unit weight added to the pumped water by the pump, and is the sum of the changes in pressure, elevation, and velocity heads between the pumping water level in the well and the point of discharge, along with any friction losses between the two points.


Figure 3.28 Diagram of a Centrifugal Pump Installation Showing Head and Lift Concepts

Total dynamic head must be determined as accurately as possible in order to design a pump. The TDH in Figure 3.28 can be determined as follows:

TDH = hp + hz + hv + hf

where:

hp = pressure head. This is usually the pressure required at the discharge point, eg., to force water out through sprinklers.

hz = elevation head. This is the difference in elevation between the pumping water level and the point of discharge or the point at which hp is measured. The pumping levels should be determined while the pump is running and after the pumping levels have stabilized. In pumping from a reservoir or stream, the pumping water level may stabilize immediately. In pumping from a deep well, the pumping level may not stabilize for several hours. In measuring pumping water levels in deep wells, electric sounders, air lines, or other methods may need to be used.

hv = velocity head. This can be visualized as the vertical distance water would flow out of the end of the pipe as a result of its velocity. It is given by the equation:

g is the acceleration of gravity (32.2 ft/sec2 in English system, 9.8 m/sec2 in metric system).

Because of friction losses, possible water hammer and other structural damages may occur, and the velocity in most irrigation pipes should be kept below 2 m/see (7 ft/sec). Therefore, the velocity head is minimal. It is usually negligible when estimating TDH and thus is not even accounted for.

hf = friction head. This is the pressure or head that the pump must produce to overcome friction, i.e., the loss that occurs as water "rubs" against pipe walls and fittings.

Example: Determine the required TDH for a centrifugal pump as shown in Figure 3.28. Q = 411 m3/hr (1811 gpm); v = 2 m/see (6 ft/sec).


Metric

English

Elevation at discharge

933.00 m

3061.00 ft

Elevation at pump

922.00 m

3024.00 ft

Elevation at water surface

919.00 m

3015.00 ft

Pressure head (32 psi)

22.50 m

73.90 ft

Friction head



(18 in. Suction, elbows, etc., 20 ft.) 0.01 m

0.03 ft


(16 in. Diameter, 1,000 ft) 1.16 m

3.80 ft


Velocity head (v = 2 m/see, or 6 ft/sec)



0.20 m

0.56 ft

hz = discharge head + static suction



lift = (3061 - 3024) + (3024 - 3015) =

14.00 m

46.00 ft

TDH = hz + hp + hf + hv



Metric = 14 + 22.5 + 1.17 + 0.2

7.90 m


English = 46 + 73.9 + 3.83 + 0.56


24.30 ft

3.5.13 Horsepower and efficiency

The power output (energy per unit time) of a pump is the energy that the pump provides to the water in the form of discharge and head. This quantity is called water horsepower (WHP). With discharge, Q. in gpm, and TDH in feet, the equation is:

If Q is in L/sec, and TDH is in meters, m, the equation is:

The power needed at the output shaft of the power unit to run the pump is called brake horsepower (BHP) and is determined by the WHP and the pump efficiency (Ep). For Ep expressed as a decimal fraction, the equation is:

The efficiency given in the manufacturer's pump characteristic curves is the laboratory efficiency. It is determined for a new pump under closely controlled conditions. Minimum length of column and pump shaft is used, and impellers are adjusted for ideal clearance. Ep in a field installation is usually several percentage points lower than the efficiency in the manufacturer's pump curves. A value of 70%, or Ep = 0.7, should generally be the minimum acceptable for new pumps that are directly coupled to an electric motor, even though theoretical values could be near 80% (Ep = 0.8).

A right angle gearhead will usually have an efficiency (Kg) of about 95%. If the power from either an electric or internal combustion engine is transmitted to the pump through a gearhead, the efficiency of the transmission device must be accounted for in determining the horsepower requirements as follows:

Example: For the previous example, Q = 411 m /fur, or 114 L/sec (1811 gpm), and TDH = 37.9 m (124.3 ft). If a pump with Ep = 0.70 will be powered by either a diesel gasoline engine or electric motor through a gearhead with Eg = 0.95, what size of engine or motor will be needed?

WHP = (114 37.9)/76 (metric) = 57, or

WHP = (1811 gpm
124 ft)/3960 (English) = 57

3.5.14 Pump characteristic curves

The amount of energy (head) a pump will add to a given discharge of water, the pump required, and the efficiency are all measured by laboratory testing. The results are displayed on a diagram known as a "pump characteristic curve." An example of a typical characteristic curve is shown in Figure 3.29 for a centrifugal pump. A pump that operates near the flattest portion of the efficiency curve should be selected so that a small change in conditions will not significantly change the pump efficiency.

The total head (TDH) that a pump will develop is plotted on the graph versus the pump discharge. Often more than one curve is shown. Each curve is for a different sized impeller or rotation speed. The efficiencies at which the pumps will operate under various conditions, together with the required brake horsepower curves, are superimposed over the other curves. In Figure 3.28, the total dynamic head for which the pumps would be designed would be the head required to lift the water from its source through the pipe and sprinklers or into the ditch as shown. In the example, it was 37.9 meters (124.3 ft) at 114 L/sec (1811 gpm).

After the required discharge (Q) and TDH are determined, a search is made through a manufacturer's pump curves to find the pump with a high operating efficiency that meets these conditions. In Figure 3.29, a pump with 13.75 inch impellers, operating at 1750 RPM, with an efficiency of 80%, meets the requirements of the previous example.

The calculated BHP at a pump efficiency of 80% is:

The operating efficiency of a pump in the field is generally lower than that given in the catalogs. Thus, a 75 HP direct couple (no gearhead) motor would probably operate quite satisfactorily, unless bearing, shaft, and other mechanical losses were high.

3.5.15 Power source and power Costs

To estimate precisely the cost per hour of pumping, one would need to measure the energy consumption of an engine or electric motor under normal operation. One would then need to multiply this consumption by the cost per unit of energy consumption. For new pumping plants that are properly designed, one can estimate what the cost per hour of pumping will be if one knows the required BHP and the cost per gallon or liter of fuel, propane, or diesel. For electric units, one needs to know the cost per kilowatt hour (KWH). The basic equations are:

Diesel. Gasoline. Propane (see Table 3.5)

Example: With gasoline at $0.30/liter, and with 100 BHP required, what will the per hour costs be?


Figure 3.29 Pump Curve

Electric (Assume electric motors are about 85% efficient)

$/fur = BHP required 0.85 cost per KWH

Example: With electric power rates at $0.07/KWH, and 100 BHP required, what will the per hour costs be?

$/fur = 100x.85 $0.07 = $5.95

TABLE 3.5. Performance Standards for Engines (Adapted from University of Nebraska Tractor Tests) (Ref. 20)

Energy

Brake Horsepower Developed

Source

BHP-Hr/$ U.S. Gal. of Fuel

BHP-Hr/Liter of Fuel

Diesel

14.75

3.90

Gasoline

11.30

2.98

Propane

8.92

2.36

3.5.16 Pump location

The physical location of the pump in relation to the water level in the sump (well, reservoir, pond) from which water is being pumped is critical. If the pump is too high, cavitation may occur. The liquid is pulled apart as it goes through the impeller, causing vapor pockets that collapse after they have passed the impeller. This process, called cavitation, can destroy a pump or cause it to deteriorate rapidly. The pump may operate very inefficiently. If the pump is too high above the water surface, it may also lose its prime.

The height or distance that a pump can be located above a water surface varies with elevation above sea level, properties of the water, friction loss in the suction pipe, and the net positive suction head requirements of the pump.

Net positive suction head (NPSH) is the head that causes water to flow through the suction pipe into the pump. NPSH required is the suction head (pressure) required at the inlet of the impeller to ensure that the liquid will not boil or form vapor pockets that result in cavitation. NPSH required is a function of the pump design and is supplied by the manufacturer (e.g., Figure 3.29). It varies between different makes of pumps and the capacity and speed of any one pump.

NPSH available represents the pressure head available to force the liquid into the pump impeller and is a function of the system in which the pump operates. It determines how high a pump can be located above a water surface and can be calculated for any installation. To operate successfully, any pump installation must have an available NPSH equal to or greater than the required NPSH at the desired pump condition. Thus:

To calculate how high above or below the pumping water level the impellers may be located, consider the following:

PL = atmospheric pressure - NPSH required - friction losses in suction - vapor pressure.

If PL is positive, the pump may be set above the pumping water level a distance PL.

If PL is negative, the pump must be set a distance PL below the pumping water level.

Example - for the pump with impellers from Fig. 3.29:

Discharge

408.6 m3/hr (1800 gpm)

Friction loss in suction pipe

1.16 m (3.8 ft)

Elevation of pump

930 m above sea level (3061 ft)

NPSH required

4.86 m (16 ft.)

Water temperature

15.6°C (60°F)

Vapor pressure of water at 60°F:

0.17 m (0.57 ft) (Appendix A)

Atmospheric pressure at 3000 ft:

9.26 m (30.4 ft) (Appendix A)

NPSH available = 9.26 m - (suction lift + 1.16 m + 0.17 m) = 3.06

Solution: PL = 9.26 m - 4.86 m - 1.15 m - 0.17 m = 3.08 m

This is also shown graphically in Figure 3.30. Thus, the static suction lift (height of pump above the pumping water level) cannot be greater than 3.08 m. In practice, stormy conditions can reduce atmospheric pressure by more than 10% at times. Therefore, to assure good pump operation and prevent cavitation, the pump may need to be placed as much as 0.6 m to 0.9 m (2 to 3 feet) lower than the NPSH would indicate at normal atmospheric conditions. For this reason, the static suction lift should be 2.1 m to 2.4 m (7 to 8 feet).


Figure 3.30 Graphical Solution of NPSH (Ref. 20)

3.5.17 Pump installation

Figures 3.31 and 3.32 show a top and side view of a recommended pump installation when pumping from a sump. Figure 3.33 shows a recommended installation when pumping from a well.


Figure 3.31 Recommended Pump Installation - Side View (Ref. 54)


Figure 3.32 Recommended Pump Installation - Top View (Ref. 54)


Figure 3.33 Recommended Pump Installation (Turbine) (Ref. 54)

Suction Pine Design

When pumping water from a sump, the proper design of the suction pipe is essential to good pump performance. If a centrifugal pump is designed with a suction pipe that is too long, with too many bends, or too small a diameter, cavitation and poor pump performance may result, much the same as if the pump is located too high above the water surface.

Another critical factor in the suction pipe design is the potential for entrapped air. Improper design can result in "high spots" in the suction piping that trap air and either decrease capacity or periodically permit large air bubbles to enter the pump. The pump may lose its prime, not operate at expected efficiency, or give somewhat erratic discharge and pressure increases. Remember that the suction line is generally under negative pressure, and air release valves cannot be used to remove air.

Improper approach just upstream of the pump entrance may cause flow to enter the pump with spiral velocities or non-uniform flow profiles. The pump will not perform efficiently at the design flow if the water is spiralling.

The guidelines for suction pipe design are formulated with these potential problems in mind. The closer the designer can follow the guidelines, the more trouble-free his or her pumping system will be.

The following recommendations apply to suction pipe design:

1. Keep velocity as low as possible. This is accomplished with larger diameter suction pipe.

2. Avoid bends in the suction pipe but, if bends are necessary, use long radius bends.

3. Keep suction pipe horizontal or continually sloping upward towards the pump. Avoid high spots in the line.

4. If suction pipe is to be reduced in diameter as it enters the pump, use a reducer of a length at least twice the diameter of the small pipe.

5. If the reducer is in a horizontal portion of the pipe, an eccentric reducer, placed with the flat side up, should be used. If a conventional reducer is used, the pipe must be inclined sufficiently to prevent air entrapment.

6. Make sure the joints in the suction pipe are well sealed, otherwise difficulty in pumping will be experienced and air will continually be sucked into the system.

7. Ideally, a straight pipe eight diameters in length just upstream from the pump suction inlet should be used to provide uniform, non-spiralling flow.

8. Suction pipe should be of equal or greater diameter than the pump suction connection.

9. Do not use screens on suction pipe inlet, as they may clog and reduce flow and pressure at impeller inlet. Screens should be placed at some distance from the suction pipe.

10. Use a streamlined suction bell at the inlet of the suction pipe if possible. If not possible, use an inlet that minimizes friction loss.

In summary, keep suction pipe velocities low, minimize bends and friction losses, avoid high spots, and direct the flow into the suction side of the pump with a uniform, non-spiralling flow.

3.5.18 Turbine and propeller pumps

In general, turbine and propeller pumps are used in situations where no suction piping is required. They are usually placed in a pumping pit with the pumps submerged, so no suction pipe is necessary. As a result, they are generally supplied with a suction bell attached to the bottom of the pump. No suction pipe design is required.

If these types of pumps are to be installed in a situation that would require suction pipe, the same guidelines for design as were advised for centrifugal pumps would be used.

3.5.19 Intake structures

Geometry and Positioning

Intake structures, pumps, or pits should be designed to supply an evenly distributed flow of water to the suction bell to prevent vortices that introduce air into the pump, reducing capacity and efficiency. Water should not flow past one pump to reach another pump. The suction bell(s) should be as close as possible to the back wall and not less than the suction bell diameter above the bottom of the sump. False back walls should be installed when the location of driving equipment prevents normal positioning. Centering pumps in the sump leaves large vortex areas behind the pumps. Figure 3.34 shows a top view of a recommended sump design when pumping from a canal, stream, or lake.


Figure 3.34 Recommended Sump Design for Installation of one or More Pumps (Ref. 54)

Inlet

The sump inlet should be designed with gradually increasing tapered sections, as shown in Figure 3.34. Abrupt changes in size from inlet to sump can cause turbulent, cascading water and a vortex effect, which can cause uneven operation of the pump, air entering the pump, or a vacuum effect, all of which can cause excessive wear on the pump.

The sump inlet should be below the minimum water level and as far away from the pump suction as the sump geometry will permit. No sharp drops or waterfalls should be permitted. The flow should not hit directly on the pump suction, or enter the sump in such a way as to cause rotation of water in the sump.

Protective Screens

Protective screens should be provided whenever there is any possibility of debris entering the pump (or pipe and sprinklers). Screens must be cleaned frequently to allow water to pass through to the pump easily.

Sump Volume

The usable sump volume in cubic meters should equal or exceed twice the volume pumped in one minute. This ensures adequate size to dissipate inflow turbulence.

3.5.20 Minimum water level

Minimum water level should be adequate to satisfy the particular pump design requirements to prevent cavitation, but suction submergence ought never be less than four times the diameter of the suction pipe (not the bell).

Some examples of pumping plant installations for pumping from streams, ponds, or shallow wells are provided in Figures 3.35 and 3.36.

3.5.21 Typical pump design

As stated before, a pump should be designed for the maximum crop water requirements and possible operational changes, such as a declining water table. Decreasing efficiency should be anticipated and provided for.

Listed below are typical designs:

1. Centrifugal pump - conditions:

a. Pumping from a river or reservoir into a canal at a higher elevation.

b. Area to be irrigated 2 hectares (4.9 acres).

c. Elevation 912 meters (3000 feet).

d. Irrigation efficiency = 67%, including conveyance losses.

e. Maximum monthly crop ET in July of 206 mm with no rainfall, or an average of 6.65 mm/day average continuous flow:


Irg = Irn/Ea
Ec = 6.65/.67 = 9.9 mm = 1 cm

from QT = 28 a d: Q = 28
2 1/24= 2.31 L/s (37 gpm)

f. Vertical distance from pump to canal (discharge head) = 62 meters (204 feet).

g. Vertical distance from water level to pump inlet = 3 meters (10 feet).

h. Friction loss = 2 meters (6 feet).

Figure 3.35 Designs of Centrifugal Pump Installations

PUMPING FROM A STREAM OR POND

Irrigation can be accomplished by pumping from ponds and streams if the lift is not too great. A small horizontal centrifugal pump set not higher than 10 feet above the water and powered by a gasoline engine is the equipment ordinarily used. Pumps of this type are made in sizes which will deliver from 30 to 1,000 gallons or more per minute.

PUMPING FROM SHALLOW IRRIGATION WELLS

Along river bottoms where the ground-water level is permanently within 10 feet or /055 from the surface, small irrigation wells may be successfully used for the irrigation of farm land. Casings for such wells are ordinarily made from sheet metal slotted to permit water to enter. In some instances old range boilers with the ends cut out are welded together and slotted with a welding torch Small centrifugal pumps set at the ground surface are quite satisfactory


PUMPING SYSTEM FROM STREAM OR POND


DIAGRAMMATIC SECTION SHALLOW WELL PUMP


DIAGRAMMATIC SECTION OF PUMP SYSTEM FROM STREAM OR POND


Figure 3.36 Pump and Driver Mounted on Pontoon in Water Supply (Ref. 58)

3.5.22 Size of pumps

If the pump is powered by an electric motor, a 3 BHP direct coupled motor would probably be selected.

If an internal combustion engine (diesel or gasoline) were used, a gearhead would probably be used to transmit the power and the operating conditions could be obtained by throttle adjustment.

In this case, the motor will also have to overcome losses at the gearhead.

The Peace Corps Volunteers working with irrigation need not be experts on pumps. They must know, however, the fundamentals of design, installation, and maintenance. This will enable them to provide the basic specifications to the supplier and supervise the installation. A pump supplier can provide the correct pump and accessories if provided with information on the flow, head required, the power source to be used, and the configuration for installation. The technicians (irrigation specialist) should in all cases demand that the pump curves and maintenance instructions come with the pump. Often, the pumps in stock are not the correct ones, and the Volunteer should not settle for pumps that are extremely oversized or undersized or which do not operate with good efficiencies.

3.5.23 Costs

In considering the economics of any pumping system, all costs and benefits should be included. Initial or capital investment costs are important because they are usually high, and the purchaser is often required to finance this capital investment by obtaining a loan. The best measure of the economics of a system is to compare the annual costs with the annual returns.

Annual costs should include:

· interest, depreciation, insurance, and taxes,
· repairs and maintenance,
· operating costs (energy),
· irrigation labor costs, and
· production and fixed costs.

The expected life of various components used in pump irrigation is included in Appendix B. In irrigation by pumping, the annual energy and maintenance costs may be significantly higher than the amortized capital costs. Therefore, it is extremely important that good estimates of these costs be made before a pumping plant is purchased.

3.5.24 Evaluation of pumping plants

Proper design of pumping plants is only the first step to successful operation. Equipment that is expected to last 10 to 20 years requires good maintenance. Inadequate maintenance causes shorter life of equipment, operational delays, and increases in the overall operating costs.

Pumping plant evaluations may be conducted to determine the following:

· discharge of pump,
· discharge and pressure at normal operating conditions and under varying conditions,
· pump adjustment for optimum efficiencies,
· well characteristics,
· whether the pumping plant is operating as designed (quality control),
· whether the pump and power unit are properly matched, and
· problems within components of the pumping plant or its management.

When the pump is new, and periodically after that, it should be checked. As a minimum, a pressure gauge should be installed at the outlet side of the pump to determine whether it is generating the desired pressure or head. If the pump has an open discharge into a canal or pond, the pressure will be zero and the flow rate will be the best indicator of whether the pump provides an adequate or close to the design flow rate. When possible, measurements of both discharge and head will provide a good indication of how the pump is performing.

3.5.25 Rower puma

The Rower pump (Figure 3.25) is a reciprocating-action piston pump whose PVC cylinder is inclined at an angle of 30° to the horizontal. A unique feature of this pump is that it is fitted with a surge chamber at the pump suction. This absorbs the impact of the accelerating and decelerating column of water within the tube-well pipe and so provides a steadier upward flow of water. This in turn enables the operator to make easier and quicker strokes. A person can pump 50% more water in a given time using a pump with a surge chamber fitted. The addition of the surge chamber enables children (who would otherwise be too small to operate a conventional hand pump) to pump water quite easily. The pump is being used to irrigate small plots in Bangladesh and other countries.

3.5.26 Hydraulic ram

The hydraulic ram is a pumping device for lifting water to heights of over 100 m. It works solely on the power of falling water carried in a drive pipe. It is completely automatic and has an exceptional record of trouble-free operation. It can be constructed from commercial pipe fittings and adapts well to low input rural use. The disadvantage in the use of the hydraulic ram, in irrigation in particular, is the low water yields that it produces. Figure 3.37 shows the simple assembly of a hydraulic ram and the necessary building materials.


Figure 3. 37 Hydraulic Ram (Ref. 55)

3.6 Wells

A well is a structure that consists of an open hole that penetrates into the water bearing strata below the surface of the ground. The walls of the well are kept open by a liner or casing, which is typically made of plastic, metal, or rock. Holes in this casing (the so-called screened portion) permit water to enter the well from which it is extracted. Wells may be dug by hand, drilled mechanically with hand or machine tools, or the casing may be driven into the ground, creating the hole as it penetrates. The hydraulics, hydrogeology, and methods of well drilling are generally too complex and extensive to be treated in this manual. The irrigation specialist who must know more about wells should review References 5, 13, and 15. In this manual, some basic concepts of well drilling are discussed. Details are presented on the construction of only driven wells and hand drilled wells. These wells are low cost and, in areas where water tables are within 30 feet (10 meters) of the surface, will generally provide sufficient water for irrigating garden-sized farm plots.

3.6.1. Methods of drilling: Percussion drilling

In percussion drilling, a heavy bit is repeatedly lifted and dropped, progressively boring through the earth. In rotary drilling, the drilling results from the continuous scraping of the bit under constant pressure. The hole is cleaned out as the drilling progresses, either with a drilling fluid (mud), with high velocity air or, in auger drilling, by the mechanical lifting of the auger.

Cable tool drilling is one of the most common methods of percussion drilling. It is usually done by commercial well drillers with motorized equipment. In some countries, however, manual means of raising and lowering the bit have been developed.

In cable tool drilling, a chisel faced bit is repeatedly raised and dropped. The bit breaks and pulverizes the materials. A slurry of water and cuttings, which is formed by the drilling action, is periodically removed by a bailer. Water is continually added to the borehole as needed. With manual methods, the 40 to 80 kg drill is lifted and dropped through a tripod and pulley arrangement operated by four to six people.

Wells may often be constructed by communities or individuals, without the need for commercial drillers. The three types of wells commonly encountered, which can be constructed relatively inexpensively, are the driven wells, hand augered wells, and hand dug wells. The description of driven and hand auger techniques that follows is extracted from "Appropriate Well-Drilling Technologies" by the National Water Well Association (Ref. 15). Sketches of other types of drilling equipment useful in drilling small capacity irrigation wells are included in Figures 3.38 through 3.44.

EQUIPMENT AND METHOD - Driven Wells

Whenever the water table lies at shallow depths (23 feet or 7 meters), a well screen equipped with a drive point may be driven through the overlying soil and into the water-bearing formation. This method employs a drive hammer. Three basic types of drive hammers are in common use: (1) the hand driver, consisting of a sliding weight and an attached pipe that fits over the riser pipe (Figure 3.38), (2) an internal driving bar, which strikes directly upon the driving point (Figure 3.38), or (3) a sliding weight and drive stem or guide that attaches to the uppermost riser pipe coupling (Figure 3.39).

The basic equipment required for a driving rig ranges from a 4-foot (1.2 meter) section of oversized pipe (used as a sliding hand driver) to more elaborate systems requiring a tripod, pulley, rope, and driving bar or drive stem and sliding hammer. The driving rig will also require two or three pipe wrenches and a shallow well hand pump to develop and remove soil debris from the well screen.

Geological Applications

Driven wells are generally one of the most efficient methods of drilling whenever the water table is within 23 feet (7 meters) of the surface and the soil consists principally of sand with minor quantities of silt and clay. Under ideal soil conditions, a small diameter well point may be driven to a depth of 25 feet (7.6 meters) in 15 minutes. In heavy soils such as stiff clay, or soils that contain numerous boulders, drilling with an auger or percussion bit is faster than driving with a well point.

Hand-driven well points of 1 1/4 to 2 inches (3 to 5 centimeters) in diameter can be driven up to 25 feet (7.6 meters). If heavy, 100-300 lb. (45 to 135 kg) drive hammer assemblies are used, 4-inch (10 centimeters) well points and casings can be driven to depths of 33 to 49 feet (10 to 15 meters).


Figure 3.38 Methods for Driving Well Points (Ref. 15) (a) Hand Driver


Figure 3.38 Methods for Driving Well Points (Ref. 15) (b) Internal Rod Driver

These assemblies provide an effective means for driving both well screens and casings.


Figure 3.39 Heavy Duty Sliding Hammer and Drive Stem Assemblies (Ref. 15) (a) Internal guide driver hammer & Drive stem with sliding hammer


Figure 3.39 Heavy Duty Sliding Hammer and Drive Stem Assemblies (Ref. 15) (b) Cross-section of sliding hammer and drive stem


Figure 3.40 Hand Auger Drilling (Ref. 15)


Figure 3.41 Typical Hand Augers and Equipment (Ref. 15) (a) Sand Augers for Non-Cohesive Soils


Figure 3.41 Typical Hand Augers and Equipment (Ref. 15) Helical Auger


Figure 3.41 Typical Hand Augers and Equipment (Ref. 15) (b) Cohesive Soil Augers


Figure 3.41 Typical Hand Augers and Equipment (Ref. 15) Bucket Auger


Figure 3.41 Typical Hand Augers and Equipment (Ref. 15) Typical Drill Rod Connection


Figure 3.41 Typical Hand Augers and Equipment (Ref. 15) Hard-Wood Rod Handle


Figure 3.42 Rod or auger Fork Locally Fabricated From 1/4" (6 mm) Steel Plato (Ret. 15)

- When placed around a drill rod or auger, the fork acts as a support for the drilling tools. In this manner, rods and augers may be added or removed from the drill string with little danger of dropping sections down the bore hole.


Figure 3.43 Manual Jetting Equipment (Ref. 15)


Figure 3.44 Hydra-Drill (Ref. 15)

- One person may operate this equipment to depths of 50 feet (15 m). Greater depths may be attained when the engine and drill rod are suspended from a tripod.

Labor Requirements

Given the proper soil and water table conditions, small diameter driven wells may be completed by one to two unskilled persons. Large diameter driven wells require a heavy-duty drive hammer and a tripod assembly. The crew necessary for operation of this equipment consists of six people for manual methods, and two or three for a motorized cathead system.

Fabrication Skills

All well point driving equipment can be constructed easily from locally available scrap pipe or steel bars and standard pipe fittings. The fabrication of simple drive hammers requires basic metal working and blacksmith abilities. Construction of heavy-duty drive hammers, which weigh in excess of 50 lbs. (22.5 kg), will require the aid of an electric arc welder or basic metal casting techniques.

Cost of Equipment

Excluding the initial cost of a well screen, drive point, and riser pipe, a locally constructed hand drive system that requires no tripod will be relatively inexpensive. Heavy-duty systems may cost more for the fabrication of both drive hammer and drive stem, depending on the type of tripod used. If a driving rig incorporates a motorized cathead, the system price could increase again. A hub-driven cathead would cost considerably less but would require a support vehicle.

3.6.2 Methods of drilling: Hand auger rig

EQUIPMENT AND METHOD - Hand auger Rig

The hand auger method of drilling is one of the oldest and most basic forms of low-cost labor intensive well drilling. In hand augering, the drilling action is applied by manually rotating a cutting blade or auger (Figure 3.40). AS drilling progresses, the auger fills with soil and must be periodically lifted to the surface and emptied. Drilling by this method is fairly rapid for the first 20 feet (6 m). Thereafter, the number of drill rod sections that must be coupled and uncoupled each time the auger is brought to the surface adds considerably to the drilling time.

The basic components of a hand auger rig are:

· support tripod,
· drill rod, fork, and auger handle,
· auger,
· rope and pulley,
· sand bailer,
· temporary casing to case hole through caving soil, and
· drill bit to break up hard soil and boulders.

Most light duty hand auger drilling systems utilize an inexpensive wood or pipe tripod.

Drill rods are constructed from locally available 3/4 inch (2 centimeters) galvanized or black iron pipe. All connections between drill rods and augers are of box and pin-type construction (Figure 3.41). Joining pins for the connections are made from either toggle bolts or standard nut and bolt assemblies. Both pin systems have proven to be highly reliable.

To avoid dropping a disconnected section of drill rod down the borehole, a rod fork or auger fork is slid under a coupling to support and retain lower sections of the drill stem (Figure 3.42). This rod fork may be constructed from a 1/4 inch (6 millimeter) steel plate, or from a notched hardwood board. In either case, the notch must be wide enough to slide around the drill rod, but narrow enough to retain a coupling.

An auger handle is constructed by clamping two hardwood handle sections around the drill rod (Figure 3.41). As the borehole advances, the bolts are loosened, and the handle is relocated to a more convenient height.

Auger construction falls into two main categories, those for use in cohesive soils and those for non-cohesive soils. The cohesive soil augers (Figure 3.41) are designed for use in soils that adhere or stick together. These soils commonly contain a mixture of sand, silt, and clay. Augers designed for use in non-cohesive soils (Figure 3.41) are best suited to loose sand and gravel formations.

Each type of auger can be produced locally using discarded sections of casing, pipe, sheet metal, or perhaps the tubular section of an automobile drive shaft. Local soil types determine the type of construction used. In general, the best performance in soft cohesive soils can be obtained with an open blade or helical auger. Hard clay soils may be excavated with the bucket auger. In non-cohesive soil, the tapered tube auger is most effective (Figure 3.41).

In addition to a tripod, rope and pulley are also primary components of the drilling equipment. On a hand auger rig, a rope and pulley are often used to handle the drill rods when a long pull of 20 or more feet (6 meters) is necessary to raise the auger. This long pull saves time and eliminates the need for disconnecting numerous drill rods.

A rope and pulley are also necessary to handle other drilling equipment such as well casings, wet bailers, and percussion bits. These accessories provide a useful means for continuing

the well when water saturated or hard-pan formations are encountered.

The most frequently used wet bailer on a hand auger rig is the flap valve bailer, also used in bail-down drilling.

A percussion drill bit is also commonly used in conjunction with hand auger equipment. Its ability to break up and loosen hard soil and boulders that cannot be excavated with a hand auger permits drilling under a wide variety of conditions.

Geological Applications

Hand augered wells are particularly well adapted to alluvial deposits consisting primarily of silt, clay, sand, and limited quantities of gravel. Maximum depths for hand augered wells range up to 122 meters (400 feet); however, normal hand augering, is best suited to maximum depths ranging from 15 to 25 meters (49 to 80 feet). Hand auger wells of these depths were used for an extensive hydrological survey of specific community well sites in Tanzania. Other significant hand auger well projects include wells in the following locations: Western Pakistan, Vietnam, Sri Lanka, India, Togo and the Ivory Coast Region of Africa, and in Ecuador.

Labor Requirements

The operation of a hand auger rig requires a minimum crew of four to five people. One member of the crew must be trained in basic well drilling, as well as development techniques. The remainder of the crew may be unskilled local labor. Under ideal conditions, inexperienced crews have drilled to depths of 49 feet (15 meters) in a single morning. However, greater drilling depths will require a considerably longer period due to the time and effort spent in removing and emptying the auger.

Fabrication Skills

All hand auger tools and equipment are easily produced using locally available sheet metal and pipe.

3.6.3 Other drilling equipment

Other types of equipment used in drilling operations are shown in Figures 3.45. and 3.46.


Figure 3.45 Drilling Equipment (Ref. 15)


Figure 3.46 Hydraulic Rotary Rig Mounted on a 1-Ton Pickup Truck (Ref. 15)

3.6.4 Hand dug wells

Hand dug wells have been used through the centuries as a source of water for domestic and irrigation uses. Figure 3.47 shows a schematic of one such well. Most hand dug wells are less than 20 meters in depth, even though some wells have been constructed deeper than 60 meters.

The use of hand dug wells is limited to areas with fairly shallow water tables (generally 20 meters or less), in contrast to some wells that can be drilled to several meters. The construction of hand dug wells is a very slow process, especially in rock formations that require blasting for the penetration process. These wells are generally limited to small flow rates, as the wells generally do not penetrate deep below the water table. Fluctuations in water tables will usually cause significant variations in the yield of the well.

Constructing a hand dug well is a labor-intensive process requiring a minimum of five workers for wells deeper than 5 meters. In loose soil, the excavation speed may be 1/3 to 1 meter per day, while in rock formations it may be even slower. Hand dug wells should be dug during the dry season, when the water table is lowest.

Details on the construction of hand dug wells are presented in ICE Manual M-9, entitled "Wells Construction." This manual should be considered indispensable for Volunteers who are contemplating well construction for the development of water supplies.


Figure 3.47 Hand Dug Well

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