The use of water harvesting for crops and the design and construction of channels, ridges and bunds are discussed in this chapter. Other applications such as the collection of water for domestic and animal watering are widespread and may be more easily achieved, but are peripheral to the coverage of this book. As well as providing supplementary water for an established crop, efficient water harvesting systems may allow an early start to the season in areas where farmers cultivate land away from their main residence and where they are dependent upon a supply of domestic water obtained from rainfall. Similarly, water harvesting can be used to promote early cultivation where farmers plant in soils with high bulk densities and soil wetness promotes easy ploughing and a moisture store for germination.
A convenient classification of water harvesting systems has in the past been regarded as somewhat problematic, a situation that may have more to do with the relative newness and dispersed nature of research into these methods of supplementing water availability, than with any inherent difficulty. This section on water harvesting covers three types of system that are classified on the basis of the scale of water movement and catchment size. As these two factors are the most important influences in determining the design, construction and the operation of the systems, this seems logical. It is inevitable that systems of one classification will grade into another and too much emphasis should not be laid on a strict adherence to the categorisation presented here or elsewhere, since all are to a greater or lesser extent arbitrary. Similarly there is no intent to enter into confusing discussions of water harvesting terminology, most of which is also arbitrary, though it is recognised that a clear and agreed definition of terms would be beneficial.
The importance of water harvesting lies in whether it can be used to improve agriculture or not. The categories used in this chapter are:
Micro systems - very small scale methods of concentrating runoff over very small distances, perhaps less than one metre, but which involve the construction or use of a catchment and which are not methods of encouraging in situ infiltration.
Meso systems - systems that redistribute runoff usually over metres or tens of metres, but which always use runoff gathered within the area of normal field boundaries.
Macro systems - systems that use runoff from outside normal field boundaries.
Water control and harvesting systems that provide supplementary water for crops have been used in widely different areas of the world, perhaps since 2,500 BC or earlier. This guide does not present an analysis or description of these systems because they are unimportant, but because documentation is more complete elsewhere than could be achieved here. These systems were used in the past in the Middle East, Nonrth Africa, India and other areas which are now regarded as under- or undeveloped. In many instances it appears that water harvesting provided a vital economic basis for the existence of the societies that practiced it, though these societies no longer exist in their past form and water harvesting has been largely abandoned. Water harvesting, of several different kinds from large to small scale, is currently practiced in areas of the developed world, such as the USA and Australia, but in forms that are inappropriate for the economic and social conditions that prevail in under-developed countries which, arguably, are in greater need of increased agricultural production. The financial and technological inputs to water harvesting in developed countries are impossible for poor countries to match. This chapter concentrates on methods of harvesting water that do not exclude poorer areas of the world.
7.1.1 Considerations for the Implementation of Water Harvesting
In many respects, successful water harvesting is a very complex mix of climate, soils, technology, social organisation and economic factors. Some of these are listed below, with plus and minus aspects, so that an unquestioning credulity will not be automatically assumed toward this form of farm practice. Under many circumstances successful water harvesting is very difficult to achieve.
Encourages water conservation and useful concentration around plants
May only operate effectively during large storms when it is not needed
Replenishes soil moisture reserve after periods of drought
Rainfall is unpredictable and totally uncontrollable
Helps reduce erosion and soil loss
Induces erosion by runoff concentration
Increases soil moisture, plant growth
Increases leaching in
organic material and
poor soils. It can cause
Encourages crop and other vegetation
Water harvesting catchments work least
growth and thereby provides favourable microclimatic conditions, grazing, fuel .
efficiently when covered by the vegetation that they encourage
Can provide, food, water and money for poor farmers
Restricts other activities, demands labour, fertiliser and time
There is litlle doubt that in some circumstances water harvesting can improve crop performance, but whether this improvement is widely attainable or, in some cases a priority, is another matter. In the general sense, it is useful to recognise that the technology of particular water harvesting systems cannot be applied haphazardly, many depend upon the physical constraints of availability of suitable materials at a precise locality and it is important to recognise that every system, to function properly, must be designed for the field upon which it is to work. An attention to detail is essential.
On the whole, the tropical and sub-tropical semi-arid regions are those expected to benefit most from water harvesting. They have generally low rainfall totals and long periods of drought. The supplementation of water for crops appears to be an obvious advantage.
Long term values of annual average rainfall have been proposed as a pointer to areas that should benefit from water harvesting, but actually such values are of little use. Rainfall in semi-arid areas is highly erratic and notoriously unpredictable. Large coefficients of variation exist for annual rainfall and the intensity-duration relations of individual storms. In many regions, for example Southern Africa, the start of rains is associated with complex weather systems entering from oceanic areas, and in this instance bimodal distributions of rain within the season are determined by the interaction of such systems with the Inter Tropical Convergence Zone. False starts and early closes to the rainfall season are common. In contrast, the start of the rainy season in West Africa is more reliably predictable.
Although relatively long term records of daily rainfall are available in such regions, the variability of rainfall distribution inevitably results in difficulties of prediction when the analysis of data is undertaken. High levels of spatial variability are also a problem. The fact that over many seasons an averaging effect occurs and totals for locations within a region may be similar, is of no comfort to a farmer who experiences such a dramatic shortfall of rain that crops fail. It is important to remember that farmers draw readily from experience and a negative experience with a water harvesting system that fails to work because of an unfortunate period of weather, may kill any interest for good.
The spatial and temporal variability of rainfall make the accurate assessment of water harvesting systems difficult to achieve from the results of short term projects. They make the prediction of extreme rainfall events difficult and demand that structures should be capable of dealing with large variations in the level of runoff. A level of over-design may be preferable, despite the extra cost. These estimates of extreme rainfall are translated to extreme runoff events, but runoff has a highly localised character; between regions, areas, fields and even within fields. The usefulness of any additional water supplied by this runoff will also vary according to regional climatic conditions; for instance whether rain falls in summer or winter.
Topography and Soils
Land form and topography are strong influences on the success of water harvesting. They influence, by interaction with other factors, the proportion of runoff that will occur from a given rainfall. They determine the manner in which overland flow collects, how it is distributed, the size and density of stream channels and thereby runoff velocities and peak flows. Although steep slopes give more runoff, other things being equal, the velocity of runoff increases (and in this sense the opportunity for infiltration decreases) by only the square root of slope. Increases of slope in low slope areas have a proportionally greater effect than similar increases in areas that have high slopes. The effects of microtopographic slopes is very important and was discussed in chapter 6.
Steeper slopes necessitate greater earth working to provide storage, but more water is retained over a smaller area. With structural limitations imposed by the nature of bunds and ridges, the horizontal distance between them is shorter on land with high slopes. Problems of channel erosion and over-topping with the subsequent destruction of bunds and ridges is a greater problem in areas of high slope. A more technical discussion of these matters is given in section 7.2, with recommended slope/area/bund lengths provided in Appendix D1.
Soil Me also exerts influence over water harvesting. Soil textural properties will determine rainfall infiltration and runoff production. Soil textures also control the structural capabilities of bunds (see section 7.2). Soil depth and soil texture will determine the extent of the soil moisture reserve; the amount of water that soils can retain for crop use. There is sufficient evidence that infiltration in very sandy soils is extreme and that any water added simply passes beyond the rooting zone of crops by downward percolation. The soil nutrient status will control crop yield, should the limiting factor of water availability be overcome. Leaching and subsequent nutrient loss, in the commonly poor soils of arid and semi-arid regions, can pose a serious problem that is exacerbated by the addition of harvested water. Waterlogging, in reality a combination of poor gas exchange conditions and nutrient leaching, has been seen to adversely affect crop growth even in medium textured soils. A considerable amount of work has been undertaken to solve the problems of soil crusting in the semi-arid tropics. Crusting acts as an inhibitor of seed germination and as a promoter of runoff. However, soil crusting appears to be as much a function of tillage practice as of inherent soil characteristics. Soils that are tilled to a modest extent, such as those used for subsistence agriculture, are much less likely to suffer than those which are well worked on commercial farms or the research station. In some cases where enhanced runoff is desired, crusting has been regarded as beneficial.
Vegetation cover is an extremely important factor in determining the runoff efficiency of a catchment, though authors attribute very different efficiencies to different covers and vegetation cover thresholds. Vegetation cover slows runoff velocities, encourages infiltration and inhibits soil erosion. On the other hand vegetation intercepts rainfall leading to its subsequent re-evaporation. Vegetation cover, retained on shedding areas to reduce soil erosion can be a serious problem, providing a vigorous seed bank for weeds and promoting the incursion into the crop area of invasive species. Once established, weed species that are difficult to eradicate will incur increased expenditure and difficulty for the farmer.
Agricultural and Social Influences
Although in many respects these two factors cannot be regarded as separate, some agricultural practices may be looked at from the technical viewpoint only. The different water harvesting systems described below are aimed at specific crops. Some are most suitable for tree production, which depending on the locality, may be fodder trees, fuel or fruit trees. Some systems may provide grazing for stock while others are most suited to arable agriculture. Crop types will depend upon climate and tradition, though the traditional crops of semi-arid areas are millet, sorghum and (to a lesser extent) maize.
Improved husbandry is necessary to exploit any advantage of increased water supply. The careful timing of planting is essential to exploit optimum conditions of soil moisture. This care may take different forms in different localities; dry planting to await predictable rains, planting after a certain date or a combination of planting after a certain date when sufficient rain has fallen, in other areas.
Weeding is particularly important at the early stages after germination, to avoid excessive competition. However, the amount and quality of weeding will depend upon the manner in which crops are sown. Weeding is much easier and more effective in row-planted crops, but this practice is by no means universal and in some regions may traditionally be undertaken only by farmers with access to draught power. Thus certain farmers may be at a great advantage compared to others and a socio-economic factor may play an important part in what appears to be a straightforwardly agricultural problem.
Plant population densities, in general, are reduced by the constraints of water harvesting technologies. In dry years this is an advantage, but in wetter years the full potential yield may not be realised and may limit the popularity of water harvesting. The tillering capacities of crop varieties can be adapted to take advantage of improved soil moisture conditions, or setbacks of the main growth due to exceptionally dry periods. Sorghum is especially suited to water harvesting since it can withstand both temporary inundation and drought. Inter or relay-cropping, whereby short maturity crops are introduced to take advantage of exceptionally good conditions are an important bonus that water harvesting systems can provide. Integrated systems, which promote the cropping of annual and perennial species, trees etc. may be suitable, but as yet have not been extensively tested.
Social and economic factors may be the most important constraints to the successful adoption of water harvesting systems, but integration of these systems into population groups may provide the basis of long term acceptance and benefit. In Malawi, soil conservation has a long history of practice and this provides the foundation of water control and harvesting, for many reasons. It has been promoted by an active extension group, but benefits for the farmers have been accrued by their own exertions. Some of the main reasons for the development of control and harvesting systems in Malawi can be summarised as follows:
Land slopes are high in much of the country and many years of trial and error have developed systems of water harvesting that are often individually tailored to a farmer's land out of sheer necessity. Such technologies are essential to conserves soil and water in these circumstances. In many areas the production of food crops for domestic consumption and the local market is crucial in a country of low economic development; the production of cash crops is a possibility while other sources of income are very few. Farmers live on or close to their land, in many areas soils are relatively deep and fertile and have a high water holding capacity. The addition of organic matter does not involve high costs of transportation and mulching with residues is frequently observed. Markets are widespread and transportation distances and costs are low. Produce is of high intrinsic value (maize, fruit, vegetables).
Botswana, with its erratic and marginal rainfall, widespread cultivation at the subsistence level and low rural incomes appears to be a very suitable area for the implementation of water harvesting schemes. In these respects it is a relatively typical sub-Saharan, semi-arid agricultural setting. However, various social and economic circumstances mitigate against any increased input to agriculture. Distance are long and therefore transport costs are very high. There are few markets other than the government purchasing agency in the capital, Gaborone, and the main crop is low value sorghum, with some millet. Soils are generally nutritionally poor and thin. Research has shown that manure spread at less than 10 tonnes per hectare is of little value and its transport is costly. Rainfall is highly erratic.
In Botswana, the traditional organisation of family life necessitates the need for three homes. A winter season village home is occupied while the fields are bare fallowed. At the beginning of the wet season, the male component of the family take the cattle (the traditional source of wealth) to the drier west, once ploughing has been completed. The female members travel to the arable land once it is thought that enough rain has fallen to permit ploughing. They stay there throughout the growing season to farm. The distances between these localities may be hundreds of kilometres.
Such physical and social circumstances do not lend themselves to the development of a more intensive agriculture based on a ready supply of water made available by water harvesting. The risks are high, the inputs great, the rewards are low and the social status of arable agriculture is negligible. Within the fastest growing economy in Africa, the opportunities exist for a farmer to earn more in cash in one month on a building site than in a good year on the land. Money is more attractive than sorghum. Botswana is by no means typical of sub-Saharan Africa, despite its annual average rainfall of about 500 mm, but serves as a good example of the strong influence that socio-economic factors can have on the suitability of water harvesting systems in agriculture.
Examples in the Text
Many texts on water harvesting systems concentrate on their construction and land form aspects and there is a dearth of agronomic and soil moisture data to assess the value, the total or partial success or failure of these systems, beyond the fact that they can actually be built. The underlying expectation that prompts the use of water harvesting is that a lack of available water for crops is the crucial factor that limits yield. The situation is rarely so straightforward. This is particularly true in arid and semi arid regions where labour and financial inputs can only be small, because returns are low. The rainfall regimes of these regions are notoriously unpredictable. Moreover, the availability of water for crops may be severely limited during dry periods within a season that has a more than average rainfall. The rainfall of one season may be too low to grow crops, whereas the next season may be so wet that soils are badly leached and crops are physically destroyed by the surfeit.
For this reason, research into an example system from each category of water harvesting is described at the end of each section. These examples give comprehensive field data on rainfall, runoff, soils, slopes, crop yields and, when available, soil moisture. The number of examples is limited by the availability of comprehensive information. However, this information is provided so that a personal assessment of typical systems can be made, bearing in mind that the successful capture and delivery of water to a designated area is only the first stage in growing crops. Apart from the obvious facts that systems need to work and be economic in construction, they need also to provide an environment within which plants can be easily sown, germinate and develop. They must also be viable from the farmer's point of view, so the difficulties experienced in the operation of these examples are also discussed.
7.1.2 Micro Water Harvesting Techniques
Almost all farmers practice these techniques, each time they plough. They plant crops in furrows surrounded by plough ridges that direct any runoff down the slope of the ridge sides towards the crop. Even farmers who broadcast seed and then plough achieve, effectively, the same result. Farmers who plough on the contour do this most effectively of all.
a. Tied Ridge and Furrow (TRF) System
Tied ridge and furrow systems encourage runoff by increasing the size of the ridges and extending the catchment area adjacent to crops. An attempt is also made to give advantageous degrees of slope to the ridge side. The runoff that is collected within the furrow is then retained by a smaller secondary ridge that is placed at right angles to the primary ridges and "ties" them together. Thus the dual action of encouraging runoff while overcoming its redistribution by local microtopography is achieved.
The construction of the ridges may be by hand labour or tractor and ridger, depending on the availability and cost of either alternative. The ridges may be constructed with indifference to the contour in low slope areas, but where slopes are greater than 2%, they are constructed as close as possible to the contour. A frequently-used ridge to ridge spacing is 1.5 m, with ridges built to 0.75m or so. The ties are hand-dug with a mattock or similar implement to 30-50 cm height, spaced according to the gradient, though distances of between 5 and 10 m are commonly used. Such microcatchments are extremely effective in retaining all the runoff of storms of 75 mm or so, and dimensions can be changed to suit particular circumstances.
The practical considerations of dimensions are important. When built by tractor, the spacing should be such that the tractor wheels run along the tops of the ridges during subsequent passes, to compact them and thereby improve runoff shedding, while avoiding compaction of the furrow and lower portions of the ridges where the crops will be sown. In most cases a ridger designed for the purpose is used to easily achieve the desired ridge height, rather than, say, a mouldboard plough.
Planting may be done in the furrow, or just above the bottom of the side of the ridges. The latter is frequently recommended because during ridge construction, top soil is removed and planting in the bottom of the furrow may be into subsoil. In addition, planting in the side of the ridge avoids any danger of waterlogging, especially during early stages of development, while allowing the crop root zone to develop adjacent to the area of enhanced soil moisture. However, soil compaction may arise during the ridging process, with attendant difficulties in the early stages of germination.
Modified systems of the basic TRF system have been used. An example is the wide ridge bed which places ridges at 1.5 - 2.0 m apart, with a flat bed between. The tractor wheelings are located at the base, not the top of these ridges. The central portion of the bed is planted with two crop rows. This system exploits the water harvesting and retention aspects of the TRF system to a lesser extent, but exploits the advantage of having the crop planted into top soil, while still benefiting from concentrated runoff at the root zone. In both systems, runoff usually travels very short distances.
Figures 7.1 (a), (b) show the TRF and wide bed systems, respectively.
Figure 7.1: (a) Tied Ridge and Furrow (b) Wide Ridge Bed
It is important to recognise that TRF systems provide a lower planting density than traditional ploughing methods. This is advantageous where water is short and crops must share it, but the natural consequence is a low per unit area compared to many traditional systems, when conditions are good. As TRFs also represent a significant economic input for the farmer, it may be important to grow higher profit crops (maize instead of millet or sorghum, for example), at least in part, to recoup some of this increased expenditure.
Modifications to orthodox bunds have led to the development of the "W" shaped catchment (in section) formed by alternating wide and narrow ridges, the former acting as shedding areas, the latter being used as the planting area. Inter-ridge distances are dependent upon wheel spacing. Planting densities maybe reduced in this, as other systems, but plants are lifted above the area of greatest saturation.
Tied Ridge and Furrow Example:
The following example from Botswana, is cited in detail for several reasons. The most important is access to comprehensive data over two seasons at several sites. The second reason is that Botswana represents some of the most marginal agricultural land in any semi-arid region, with poor soils and a low and erratic rainfall. Agriculture is exclusively subsistence farming. TRF systems that were adopted came from reasonably successful trials in Zimbabwe (see below) and Malawi, where experimentation on vertisols and medium textured soils had shown not only that the system was excellent at retaining water, but also gave increased yields.
Example 1. Botswana:
SE Botswana, approximately 24° 30 S, 26° 00' E.
AAR approximately 500 mm, but variable between 200 and 900 mm
Loamy sand/ Sandy loam
0.5 - 2.0 %, marked microrelief up to 1.0 m above general field level
Season 1: Rainfall, 692 mm for total season.
2 × 75 m strips were ploughed on.. On one a TRF was installed, on the other flat bed planting was done for comparison. These strips were aligned to cross the marked microtopography. Crop was sorghum (var. Segaolane) in both cases.
Results: Crop and Yields
The TRF crop failed to germinate and was replanted with maize (Kalahari Early Pearl) and thinned to 0.2 m (3300 plants ha-l). Growth / yield monitoring was therefore limited to within the strip, comparing performance of higher areas with and without ties, low areas with and without ties and a small flat bed transect planted at the same time as the TRF maize.
Table 7.1: TRF Maize production (areas + and - ties) and TRF versus Flat Row Planting
Comparisons showed that for high areas with ties, dry matter was greater than areas with no ties. The same was true for cob production, though the crop did not reach maturity.
Plants in TRF were more vigorous overall.
Comparisons with the small flat bed transect maize show that one of the main problems of TRF (and other water conservation systems) is that inherently low crop densities limit yield. Data are provided in Table 7. 1.
Tensiometers were emplaced below the TRF system to monitor soil
Figures 7.2 (a ) to (d) show soil profile data at the end of the season (April) after rainfalls of 26 mm (24 th) and 16 mm (26 th). The effective rooting depth was 250 cm below ridge and furrow by 21st April, prior to rainfall. The infiltrated wetting front after the rain had reached 170 cm under the furrow 6 days later, whereas under the ridge it had not attained 70 cm by then. Lateral redistribution of wetting enable it to reach 130 cm by 11th May. The driest part of the soil surface was the side of the ridge, which may be regarded as expected, compared to the furrow and the flat ridge top.
This observation has important implications for planting in this position, which is preferred for reasons explained in the text above, and is consistent with difficulties encountered in early establishment of the crop.
Rainfall, Site 1 505 mm for the whole season
Sites 2 and 3 657 mm for the whole season
Ridges were made or re-made at three sites as for the previous year and the comparison between TRF and traditional flat bed planting continued. Both sorghum and maize were planted, but replanting at site 3 was necessary 2 months later because of poor establishment and at site I 10 weeks after original planting because of destruction by a violent storm. A deep-ripping comparison was also carried out within the TRF sorghum treatment to assess the value of increased water percolation.
With the exception of the ripped plots, the sorghum results are not presented because of late planting/poor grain filling. Analyses are compared within sites only.
Tillage was seen to make no significant difference to grain yield at any site although yields were greater for flat bed than TRF at sites I and 2. The reverse was true at site 3.
Total dry matter was significantly less for TRF at sites I and 2, no significance at 3, the trend was TRF flat bed.
Population densities confuse the issue (flat bed has a × 2 difference in row spacing from TRF) at sites 1 and 2, though the better performance of TRF at site 3 seemed due to better growth in individual plants as densities were similar. Growth monitoring indicated that TRF plants suffered a setback to early development, but recovered later. No significant difference found between the ripped and non-ripped rows. Table 7.2 gives components of yield.
Figures 7.2 (a) to (d) Contours of Hydraulic Head Beneath Tied Ridges and Furrows After 42 mm Rain
Table 7.2: Components of Yield for Maize at All Sites, TRF and Flat Bed Planting
Root studies showed that there were no statistically significant differences in root development between treatments, but several trends could be seen. Generally there were more roots at depth in the flat beds than TRF.
TRF roots tended to be concentrated near the soil surface and especially below the furrow, rather than below the plant perhaps due to ridge compaction and the greater availability of water in the furrow area.
Conclusions for Both Seasons
In a total of 18 trials, TRF did not out-perform the traditional flat bed systems on the basis of yield, but it is important to note that during the period of trials, rainfall was greater than the average annual rainfall (AAR) and no "dry" season was experienced.
The system proved very effective at preventing runoff, but difficulties in crop establishment and development were found consistently, despite evidence of improved soil water availability.
Evidence showed clearly that the mechanical and construction aspects of this kind of water harvesting are not difficult to apply, but their successful application does not guarantee improved crop yield.
It is important to bear in mind that there are complex soil/crop/water relations inherent in water harvesting systems and it should not be assumed that they will work, simply because they prevent runoff and concentrate it in the crop root zone.
In general, the system was not popular with farmers who disliked its intensive, high input nature. The results of this work should be compared with that from Zimbabwe, below, which is in marked contrast.
Example 2. Zimbabwe
The trials of TRF in Botswana were stimulated by the relative success of its application by the Agricultural Research Station at Chiredzi in low veldt Zimbabwe. Work in the period 1983-85 had found good results from TRF systems at various sites with high clay content soils. Results were not so good on lighter soils, as for the Botswana example above, and soil infertility and poorer water holding capacities were suspected. The results of later research, for the period 1986-1991, are summarised below.
Three land forms, TRF at 1.0 and 1.5 m spacing, and 1.0 spacing on the flat and three levels of fertility were used. Sorghum, maize and cotton were the crops.
1 986-87 Fertility levels were zero; 8 t ha-1 manure + 50 kg ha-1 N; 8 t ha-1 manure + 200 kg ha-1 8:14:7 NPK.
1987/88 to 1989/90
Fertility levels were zero; 100 kg ha-1 NPK + 50 kg ha-1 N top dressing; 200 kg ha-1 + 50 kg ha-1 top dressing.
These levels are referred to as low, medium and high.
Fertility levels were increased to 4:
zero; 25, 50, and 75 kg ha-1 + 150 kg ha-1 NPK. These levels are referred to as 1, 2, 3 and 4.
Planting was done after at least 15 mm of rain, plants were thinned to 22,000 ha-1 (maize) and 44,000 ha-1 (sorghum).
In all seasons there were significant differences in yield between TRF and flat bed at most sites.
These differences were greater in years of low or poorly distributed rainfall.
The 1.0 m spacing TRF performed better than the 1.5 m, and this was attributed to a greater loss of top soil in 1.5 m ridge construction.
The trend was for increased yield with increased fertility. This trend was stronger for maize than sorghum.
Poor rainfall distributions sometimes reduced the advantage of fertiliser applications.
A summary of results is given in Tables 7.3 and 7.4.
Table 7.3: Effects of System and Fertility 1986-87 and 1987-88, Grain Yield (kg ha-1) of Sorghum
Table 7.4: Effects of System and Fertility, 1988-89, 1989-90 and 1990-91 Grain Yield (kg ha-1), Maize
7.1.3 Meso Water Harvesting Techniques
These systems are constructed and operate within the field and do not receive important amounts of water from outside.
"Zay" are shallow pits dug into the soil, usually about 25 cm in diameter and 10 cm or so deep. Soil fertility and structure are enhanced by placing organic matter, usually grass and/or manure, mixed with earth into the pits. Termite activity commonly reduces the organic material to a state whereby it can be readily exploited by crops and an improvement in infiltration may be achieved due to their burrowing activity. The remaining earth is used to construct a small bund around the pit on the down slope side. They are staggered about 1m apart. This is a revived technique practiced in Burkina Faso to rehabilitate degraded land and is used in conjunction with stone bunds which reduce runoff velocities. Sorghum and millet are the usual crops. The labour input is large.
Figure 7.3: "Zay"
b. Contour Bunds or Ridges
Contour bunds are used to prevent runoff and soil erosion and supplement soil moisture for crops, often, though not exclusively, in high slope areas. Where water retention is of primary importance, ties are used to prevent any loss of water by lateral flow. In cases where erosion control is more important and where increased soil moisture is a bonus, the ridges are constructed with a slight gradient (usually about 0.5%) to allow controlled drainage and render runoff velocities non-erosive. Ridges can be broken to provide drainage and thereby, rudimentary water control.
The size and spacing of the bunds is dependent upon land slope, the practical limitations on bund height and the desired area of control. Construction may be by manual or mechanical means and the soil is excavated up slope of the bund which is under construction. Excessive depth of extraction must be avoided or the loss of top soil sufferes. Water naturally accumulates adjacent to the bund, where the top soil is removed. In areas that suffer from inundation, the crop is planted on the side of the ridges to overcome temporary waterlogging
Bund construction is widespread in Malawi, especially in the Highlands region, where it is used to control runoff from high slopes and reduce soil erosion; individual farmers modify the system according to their own particular needs. It is rarely observed in the Lower Shire Valley area where slopes are generally 1% or less. In the Baringo District of Kenya, contour bunds are not completely tied, but have small bunds that extend up slope, to reduce water loss.
One of the main problems with the implementation of contour bunds is the presence of microtopography which can lead to complex arrangements being necessary. Although a compromise can be reached by increasing the bund height at low lying locations, the natural tendency for runoff to collect in these areas increases the risk of over-topping, though it does allow for a simpler alignment of ridges. Once over-topping occurs, serious erosion can take place and the increased runoff volume imposes a threat to all down slope areas.
Figure 7.4 shows a simple plastic tube level that is easy and cheap to manufacture. It is now extensively used for the laying out of the contour system
Figure 7.4: Plastic Tube Levelling Instrument
c. Hoops (Demi-lunes) and Trapezoidal Bunds
Like Zay, the hoop system could be considered a macro or off-field measure, as external runoff may enter it, but probably most water is captured from local runoff. The harvesting structures are crescent shaped bunds that enclose an arable area, though in Kenya they are used for land rehabilitation and fodder production.
Construction may be undertaken with the dug furrow that provides the bund material excavated downslope, thereby retaining all topsoil within the hoops. In other instances the furrow is dug on the inside of the hoop, thus increasing water storage. Material moved is in the order of 35 - 50 m³ for each hoop, depending on size and slope.
Figure 7.5: Hoops and Trapezoidal Bunds
Usually, the semicircular bunds, about 30 cm high, are between 2 to 10 metres across and may be placed in lines or staggered to manipulate the catchment to crop area ratio. These ratios are usually estimated as between 4:1 to 12:1, depending on hoop density. Adequate distance is left for surplus runoff to pass between the hoops. The open arms of the crescents face up slope. They are reportedly liable to breakage with large runoff events, though this may be avoided by reducing the catchment area.
Trapezoidal bunds, as used in the Turkana District of Kenya are very similar in the manner of operation to hoops and demi-lunes, but are of a larger construction, though scale depends largely upon land slopes. The main bunds may be 60 cm high and 6 m in width, the tapered arm tips 120 m apart and 40 from the main basal bund. The main bund has a freeboard of about 30 - 40 cm, with the enclosed area filled with runoff when sited on low slopes. Construction is estimated to involve the movement of about 400 m³ of soil on a 1% slope.
d. Diamond Shaped Basins
This system is often regarded as a micro catchment system. It consists of diamond shaped microcatchments oriented with one corner up slope. The opposite corner is excavated to form an area of water concentration, where the crop is planted. The crops are usually trees grown for fruit/nuts (Israel) or animal fodder (Kenya) which are situated in the down slope, excavated corner.
In some cases, V shaped catchments are used, thus saving labour and allowing the inflow of water from external sources. Dimensions vary, but sides 5 - 10 m long are usually constructed, depending on local rainfall conditions. Some research has been undertaken in The Negev Desert on optimal shapes and densities of implementation, but the diamond/V system remains most common.
Figure 7.6: Diamond Shaped Catchments
e. Strip Tillage
Strip tillage is used for erosion control, in conjunction with vegetation cover manipulation and grassed bunds in some countries, though the advantages of strip tillage as a water harvesting system have been mooted.
Strip tillage is another contour system and in some respects is similar to the contour bund system described above. With strip tillage crops are planted in strips along the contour, downslope of a shedding area. Labour inputs are reduced, because ridges are not constructed. The natural land slope is used to shed runoff and strips may be made as wide as it thought suitable, no top soil is removed from the planting zone, so that the problems associated with this activity are avoided. Shedding strip to crop ratios of 1:1, 1:2 or 1:4 are typical, with a cropped strip of 5 m. As such, strip tillage represents a medium input system with no need for special equipment, but requires more management than most traditional systems. It is a system that is also open to manipulation on a seasonal basis, whereby fast growing crops can be planted on the shedding areas, should a season provide adequate rainfall.
Strip Tillage Example: Botswana
Location: SE Botswana 24° 30' S, 26° 00' E
Soils: Loamy sands/Sandy Loams
Slope: 2 - 3%
Work was initially undertaken in farmers' fields on a randomised block basis, but it was quickly realised that strip orientation relative to the overall land slope and microtopography could not be achieved precisely enough to regulate surface flow. Two further approaches were made to study the system:
1. Strip catchments on the crop station using bounded plots.
2. A final experiment looked at runoff redistribution within crop areas from shedding strips.
1. Bounded Plots
1989-90 and 1990-91
Runoff was measured from 2 replicates of 5 m, 10 m, 20 m shedding areas. Two 20 m shedding areas with 5 m crop strips were also used to measure runoff through the crop. Multi-slot dividers were used.
For the 1990-9 season, two replicates of 5 m strip with 5 m crop and 10 m strip with crop, were added to the runoff experiment.
Two replicates of 5 m strip with 5 m crop and 10 m strip with 5 m crop were planted separately from the runoff experiment. and the 20 m strip with 5 m crop were also monitored for crop performance. Figure 7.3 shows the details of plot layout.
Figure 7.7 Layout of Strip Tillage Shedding Catchments
A dead furrow, the natural consequence of ploughing was placed at the top of the crop to act as a sink for runoff and the shedding strips were kept weed-free throughout the season. In both seasons fertiliser (2:3:2) was added at rate of 400 kg ha~, to overcome any spatial differences in soil fertility. Six rows, 0.75 m apart were hand-dug and planted with sorghum beginning 0.5 m from the dead furrow. The central 4 rows were harvested when the crop was mature. Runoff was measured on duplicate plots. Control plots with no runoff were added to the experiment during the 1990-91 season.
Table 7.5 gives the mean runoff values for the two seasons.
Table 7.5: Mean Runoff from Strip Tillage Shedding areas (% of rainfall)
The smallest runoff strips were the most efficient, but volumes were small. On average, the cultivated areas gained the following percentages of seasonal rainfall: 5 m + 3%; 10 m + 3 %; 20 m+ 7%. The effect of individual rainfall/runoff events may be more illustrative of the usefulness of the runoff supplements than seasonal averages. When these are studied, the following points can be noted:
Only the 20 m strips contributed large individual volumes to the crops equal to between 10 and 45 mm or about 2 to 10 days' water lost to evapotranspiration, during the summer growing season.
Almost half the runoff passed through the crop strip and a "cascade" effect could, potentially, result. The construction of bunds down slope of the crops would be advantageous.
Table 7.6: Components of Yield for Final harvest
The even redistribution of runoff to the crop was difficult to achieve.
A large labour input was necessary to keep the strips weed-free and maintain runoff efficiency. Despite differences in received water, analysis of variance showed no significant differences in any component of crop yield, on a planted unit area basis in either season.
Water did not appear to be limiting in either season, though yields seemed to be smaller in the wetter 1990-91 season.
Figures 7.8 (a) to (d) show rainfall against runoff on an individual event basis.
Figures 7.8 (a) to (d) Rainfall versus Runoff on an Event Basis, Strip Tillage 1989-90 and 1990-91
Edge effects were seen in 1989-90 (dry) where the marginal rows gave significantly more yield.
When yields on a field basis were considered, a great difference was seen between the performance of the three crop to strip ratios. Results showed that the 5m strips would need to double yields to equal those of the non-runon control, the 10 m × 3 and the 20 m × 5. This seems to be a serious disadvantage, especially in favourable rainfall years, of many water harvesting systems which inevitably control overall field population densities.
2. Redistribution of Runoff on Crop Areas
The distribution of harvested runoff is an important issue for all but the smallest systems. Runoff flows along channels and collects in low areas, its natural inclination is not to redistribute itself evenly for the benefit of crops. This can lead to problems of uneven crop growth, differential waterlogging and nutrient leaching.
Ten planted, 10 × 5 m experimental plots were installed with different-sized runoff strips placed up slope to provide runoff. The runoff areas were 0, 10, 25, 50 and 100 m², covered with plastic to ensure maximum shedding. Fertiliser (2:3:2) was applied at 400 kg ha-1 and 20 rows of maize were sown 0.5 m apart. Crops were harvested before maturity because of lodging due to infestations of ants in the first season and because of late sowing in the second season.
Rainfall for the two seasons was about 10% below and 10% above average respectively: 1989-90 - 449 mm and 19909 1 - 572 mm.
No significant differences were seen between plots for any season . But a trend of inverse production of dry matter with runoff area was seen. Physical damage and yellowing was seen on areas with the greatest runon. Stunted and yellow plants were seen closest to the runoff areas. Growth distributions supported the hypothesis that too much water damaged plants. Plants recovered noticeably during dry spells.
7.1.4 Macro Water Harvesting Techniques
Off field-water harvesting systems have one great potential advantage over smaller systems: they are capable of exploiting much larger amounts of runoff by utilising much greater catchment to crop area ratios. This does have a concomitant problem: larger flows require more secure control, because their destructive capabilities are considerable.
Bund, channel and dam structures form the major components of such systems which fall into two general categories: runoff collection from broad, flat catchments by the intervention of stone or earth bunds, and the utilisation of water from ephemeral stream channels. In some cases, hybrids may evolve.
a. Runoff Collection from Broad Flat Catchments
The interception of what is essentially sheet overland flow necessitates the construction of large bunds, which concentrate runoff to an area of cultivation. These structures are usually aligned for the most part, on the contour.
b. Stone Contour Bunds and Lines
As currently practiced in Burkina Faso and Niger, stone bunds are discontinuous lines of stones, piled to extend perhaps 10 - 20 m, with a height of about 30 cm and laid in a trench to aid stability. Their action is to reduce runoff velocity by means of their permeability rather than to block the flow of runoff. This reduction in velocity encourages infiltration, reduces erosion and increases the deposition of suspended material. Unlike earth bunds, they allow the passage of water and are not so easily washed away. Given suitable material they are easy to construct and need no special equipment but a simple levelling tube. Farmers may depend on mechanised transportation of the material to site. Their location is usually on low slope areas in cultivated fields or on highly degraded land under rehabilitation. They are often used in conjunction with zay.
Rock dams are a logical development of stone bunds and are used in stream channels, often where soil erosion is a problem. The extremities of the dams extend beyond the channel on to the surrounding land to prevent lateral erosion, giving an overall length of 50 - 300 metres. The gully part of the dam may be well over 1 m in height, but the extensions are usually lower, with a width of 2 - 4 m. The down slope side of the bund is usually built to a 1:2 gradient (vertical:horizontal) while the up slope side is 2:1, giving stability to the structure. The largest stones are used on the outside and the inner portion of the bund is infilled with smaller grade material.
The collection of soil debris up slope of the dam can be considerable and crop yields of 1.9 t ha-1 upslope of the dam have been reported, though the areal extent of such yields is not declared. The main technical considerations in addition to the gradients of construction and the use of larger stones for the outer casing, are laying the foundations in a trench .
c. External Runoff with Enclosed Crop Areas
The collection of runoff from external sources, in a way similar to trapezoidal and hoop systems can be used to exploit large external catchments and to water more extensive areas of crop. Diversionary bunds are built diagonally to the contour, to collect runoff from an up slope area. This runoff is directed by the bunds to the crop area. It is usual to enclose this area with bunds, to prevent the loss of the harvested water. Examples of this general type are the " Caag" and "Gwen" systems of Somalia and the "Teras" of Sudan.
The redistribution of water within the cultivated area may be achieved by ridges and furrows, but in cases where runoff volumes are large, internal bunds and spillways are used. The bunds retain the water up slope until the first section of the crop area is filled, when the spillway freeboard level is attained the runoff passes over to the next section. Eventually the whole of the cultivated area is filled and any surplus water is allowed to drain via spillways positioned in the most down-slope enclosing bund.
Bunds are made of earth, sometimes with stone cores and the spillways and adjacent areas are made with stone or cement blocks to prevent erosion. Catchment and crop areas may be defined to suit the locality and farming practice.
An example of this general system is given below.
Off-field Water Harvesting Example: Botswana
Location: SE Botswana, 24° 30 S, 26° 00 E
Soils: Loamy sands/sandy loams
Slope: 0.5 - 1.5%
Off-field water harvesting involves the collection of runoff from a source external to the field under cultivation, the control and direction of this runoff and its subsequent redistribution over the crop area.
During the season 1986/87, an off-field water harvesting system was installed at Kgapamadi, approximately 15 km north of Gaborone by the International Sorghum and Millet Collaborative Research Program (INTSORMIL). The main aims of the research were to establish whether yield improvements could be obtained by increasing soil moisture availability and to estimate the reliability of receiving agronomically useful runoff.
The water harvesting system consisted of a 0.5 m high earth bund that intercepted runoff from a shallow natural drainage channel and conducted it to a runon area of approximately 0.5 ha, enclosed by 0.3 m earth bunds. Spillways in the bunds determined flooding depth and allowed controlled drainage of excess runon. Bunds were built by a tractor-drawn ridger and manual labour. Soils in the runon area were classified as Chromic Luvisol with an argillic B horizon. Sand, silt and clay contents were 75, 11 and 14%, respectively in the top 40 cm; and 70, 9 and 21% between 40 and 150 cm. The average soil water holding capacity was 12%, by volume and soil depth exceeded 150 cm. Figure 7.9 shows a plan of the water harvesting field, channels and bund structures and immediate surroundings.
Agronomic data were collected for the seasons 1986-7, 1987-88 and 1988-89. Runoff was measured by a different project for the 1988/89, 1989/90 and 1990/91 seasons by 0.90 m H-flume and water level recorder. Daily rainfall and intensity data were also collected. Because the overlap of hydrological and agronomic/soil moisture data is limited to one season, they are treated, to a large extent, separately.
Table 7.7 gives runoff intercepted and measured by the flume, which are the maximum Bows available for water harvesting. However, it is unlikely that these flows could be transferred to the runon area without some small losses due to infiltration in the transmission channel.
The potential amounts of additional soil moisture provided by the runon were very large in most cases. Four, two and six events for each season respectively, were measured as large enough to more than fill the soil profile. The timing of these runoff events is also important; in each season runon volumes were sufficient at the beginning of the season (October and November) for flooding to be practiced by the farmer. Having a deeply wetted soil profile in early season has been shown to be highly advantageous.
On average, rainfall in the range 10-15 mm was sufficient to give some runoff, though it was clear that larger rainfalls (>20 mm) were needed to produce useful runoff amounts. The likelihood of receiving one 20 mm daily rain was calculated from historical rainfall data to be 82% during November-December, 90% during December-January and during grain-fill, 94%.
The precise area of the catchment was difficult to calculate as the smallest scale maps available only provided 15m contour intervals, which indicated an area of about 400 ha. From field observations this was felt to be a gross overestimate. The use of 1:7,000 air photos indicated a catchment between 40 and 100 ha, with a variable contributing area, depending on storm size.
Overall percent runoff was low only a few percent, sometimes less, of storm rainfall.
Figure 7.9: Off Field Water Harvesting Scheme
Table 7.7: Runoff Received by the Water Harvesting Scheme, 1988-89 to 1990-91
The agronomy experiments were undertaken in co-operation with the farmer and cultivation consisted of mouldboard ploughing after harvest and before planting. Sorghum variety "Segaolane", (Sorghum bicolour (L) Moech) was planted 5 Nov. 1986, 7 Dec. 1987 and 2 Nov. 1988 (replanted 3 Jan.), in the runon area. Control plots were planted on the same days in the field, except for 1986, when it was planted 23 days later. All plots were weeded and birdscaring was employed as necessary. At maturity, sorghum plants were harvested, counted and threshed with grain weights adjusted to 12.5% moisture. Fertiliser effects were evaluated each season. During 1986-87 two fertiliser experiments were established after emergence in a low area where crop growth was poor. In separate experiments sorghum and maize (Zea mays, var. "Kalahari Early Pearl") were grown at 0 and 83 kg ha-1, in a 2x2 factorial replicated three times. Single row plots 10 m long were used.
In 1987-88 in the runon area, a combination of 10 t manure ha-1 and 30 kg P ha-1 was applied to a single 700 m² area. Yield was measured on six 20 m² plots in the fertilised area and three 40 m² plots in the unfertilised area. Two 40 m² plots in the field were used as controls. In 1988-89 an experiment was conducted with two treatments, no fertiliser and a compound supplying 45 kg N, 30 kg P and 15 kg K ha-1. Three replicates were made in the runon area, two on the field. The fertiliser was broadcast onto 250 m² plots immediately prior to sowing. Table 7.8 gives details of the size and number of experimental plots.
Table 7.8: Number and Size of Plots, Runon Area and Traditionally Managed Area
Sorghum grain yield was greater in the runon area for the seasons 1986-87 and 1987-88 (Table 7.9). Rainfall was 36% below the long term average in the first season and 11% above in the second, but it was poorly distributed in both seasons. During 1986-87 (Fig. 7.10) only 48 mm fell between 46 and 103 days after planting (22 Dec. to 26 Feb.) and in 1987-88 (Fig 7.11) only 13 mm was received between 16 and 63 days after planting (23 Dec. to 8 Feb.). The 2 Nov. planting of the 1988-89 season was replanted on 3 Jan. and the lack of yield differences probably reflects favourable rainfall (Fig. 7.12) and weather conditions after replanting.
Table 7.9: Sorghum Grain Yield in Runon and Control Plots
In 1987-88 and 1988-89, soil moisture was monitored to 150 cm at 20 cm intervals by neutron probe and because of the effects of runoff redistribution by microtopography, the access tubes were placed 5 m apart at low, middle and high areas, in both control and runon areas. Soil moisture levels measured during 1987-88 illustrate the differences between the field and runon areas (Fig. 7.9). Sorghum used more water in the runon area than in the control during the 44 day intra-seasonal drought and this was associated with greater root depth and mass. In the runon area, for 1986-87, P fertiliser significantly increased yields (P < 0.05), but N did not. In 1987-88, P combined with manure and water harvesting gave the highest yields. No comparable results were obtained for 1988-89 because of stalk borer infestation.
Figure 7.10: Rainfall 1986 -87 Season (F indicates water harvesting flood dates)
Figure 7.11: Rainfall 1987-88 Season
Figure 7.12: Rainfall 1988-89 Season
The off-field water harvesting system proved in general to be successful. Problems of up-slope runon, not associated with the ephemeral stream exploited for water harvesting, occurred. Protective bunds 30 cm high proved inadequate because of microtopography, but despite the high level labour inputs, the farmer was convinced that the system was economically viable.
Feasibility and Practical Considerations for Off-Field Water Harvesting Using the Botswana Example
A wide range of factors must be taken into account in the location, design and operation of off-field water harvesting systems, which will ultimately determine their success or failure. In many respects off-field water harvesting is more complex, but potentially more rewarding than on-field harvesting, because much larger volumes of water are available. The main practical aspects of off-field water harvesting are discussed below, in the light of experience with the scheme described above.
Location and Opportunities
Several important observations were made using a simple non-stereoscopic survey of air photographs of the area around the water harvesting scheme and practical knowledge of how the system worked:
- An aerial survey of 250 km² around Gaborone showed that in SE Botswana, the shallow ephemeral water courses such as that used for the water harvesting system described above were common and that at least in theory, considerable opportunities existed for the adoption of such schemes.
- The number of farmers that could use water harvesting systems was limited to those with fields located in a suitable position, usually low-lying in the landscape. The majority of fields were not located in valley bottoms and for the farmers of these fields, water harvesting of this kind is not an option, though the exploitation of up slope runoff could be possible.
- However, many fields share the potential for the use of runoff and while this is not a problem at present, the unregulated use (and possibly disposal) of runoff may bring different farmers into conflict with one another. The water rights aspect of the interception and use of runoff in Botswana, are at present not clear. Legislation covering water harvesting rights on agricultural land is non-existent.
- In parts of the area with steeper slopes and larger catchments, water courses were seen to be incised. In such circumstances, considerable difficulty could be encountered in obtaining runoff, as the channels were 1-2 m deep and overall land slopes were shallow.
- In many cases diversion bunds or channels would need to be outside the farmer's field if the stream were to be exploited. They could be placed on common land in some instances.
Design Problems of successfully harvesting water can be illustrated by the scheme described above, even though this was a favoured location.
- The area flooded was limited to 0.5 - 1 ha, of a total field area of about 5 ha. Most of the field was at a higher elevation than the channel and runoff could not be directed onto it. To increase the floodable area to 2 or 3 ha, using a channel slope of 0.5%, the take-off point for collected runoff would be 200 m outside the farmer's field. This limitation is one that will be met at many sites.
- The design of the water harvesting system was very simple. The diversion ridge was not difficult to make, though access to mechanical draught power was necessary and the farmer had to acquire the rudimentary skills of bund and channel alignment.
- The exact design of each system would need to be individually prepared.
- Early preparation for the onset of the rains and the maintenance of structures throughout the wet season were necessary.
In the example above, runoff was introduced into the runon area simply by breaking a section of the bund and allowing water to flow in. When the farmer regarded the runon as adequate, the breach was repaired. No mechanical devices were used (for example sluice gates), but the presence of the farmer was essential.
A study of 15 hydrographs of the larger runoff events for 1988-89 to 1990-91 gives a good idea as to the time of day (or night) that large storms take place and the time-distribution of flow, which together dictate the farmer's opportunity to avail him/herself of the runoff. Figure 7.13 shows example hydrographs with low peak flows (less than 50 1 s-1). Low peak flows are more manageable and present a reduced risk of channel and bund erosion, but more of the runoff must be harvested to provide an adequate supply of water for crops.
Note that the total period of flow is similar in all three cases (about 24 hours) despite the differences in peak flows. Note that the duration of high flows are also similar (about 6 hours for the period when flow is greater than half the maximum peak). These durations are important from the viewpoint of opportunities for the farmer to operate the system.
Figure 7.13: Water Harvesting Scheme: Low Peak Runoff Hydrographs
Figure 7.14: Water harvesting Scheme: High Peak Flow Runoff Hydrographs
Figure 7.14 shows the hydrographs with high peak flows (greater than 2001 s-1). Note that the overall duration of flow is similar to the low peak hydrographs, as are the durations of maximum flow. The recession curves are somewhat steeper.
Of the 15 events, 5 started in the morning, from 03:00 to 10:00 (3 of which occurred between 3 and 4 am.) and 10 started between 15:00 to 23:00 (3 between 9 and 11 pm). This poses some difficulty for farmers who have to be aware of heavy rain, assess that runoff is sufficient, operate the harvesting system and estimate when adequate runon has been collected, in the dark. The development of a more automated system, perhaps with sluice gates is indicated. Spillways were tested, but did not work well.
The average duration of flow was 20 hours, the longest duration 40 hours and the shortest 7.5 hours. These periods appear to give adequate time for the farmer to undertake the necessary action to harvest water. However, because the catchment area is small, the flows peak rapidly (average time 3 hours) and the hydrograph recessions contain only about half of the total flow volume. A farmer who does not act promptly will only have access to a rapidly decreasing supply of runoff. Experience has shown that protection from other sources of up slope runoff is important. The largest runoff event which occurred for the 1990-91 season (from rainfall of 49.6 mm on 25.11.90) caused substantial damage to the field and crop. Runon from up slope sources washed away much of the crop and several contour bunds. Runoff in the channel was sufficient to cause erosion (removal of soil to a depth of about 30 - 40 cm), such that the channel bed was lower than the area usually flooded.
The balance between using harvested water and preventing the damage from runoff, is difficult to achieve. The system of flood prevention that was used, field perimeter bunds about 30 cm high, was not adequate to prevent runon when heavy rain occurred. Runoff tended to concentrate in low microtopographical areas. However, the farmer was convinced of the value of water harvesting. Field observations showed that an increased water supply to crops can cause yellowing and poor growth. The reason for this appeared to be the leaching of nutrients from relatively infertile soils. The agronomy experiments showed greatly increased yield with manure and fertiliser applications. Traditional applications of fertiliser in Botswana are very low, indeed many farmers do not use them. When water availability is not a limiting factor, soil fertility can be.
It is advantageous that soils in low-lying areas where water harvesting can be practiced, tend to be heavier than usual. Soils must be deep to retain the water as soil moisture. Sandy textured soils not only have problems of poor nutrient status, but also allow the deep drainage of runon, with little benefit for the crop. Successful crop growth early in the season can pose difficulties. Crops can attract pests simply by being the only crops in the area. Successful farming will demand increased management, labour and money to protect crops from stalk borer, aphids and birds.
Weed growth is enhanced by favourable growing conditions and more time and labour will be needed to control it. Unfortunately, row planting is not commonly practiced in Botswana, broadcast crops are more usual. Row planting, when practiced competently, gives better controlled planting densities, more even crop stands and facilitates weeding. Access to planters, draught power and the development of row planting skills would greatly enhance the farmer's ability to exploit harvested water and increase crop yield.
The implementation of water harvesting techniques for increased and sustained crop production on a national scale will need a range of favourable preconditions for success. They are listed below, in an approximate order of priority:
1. Farmer enthusiasm and commitment.
2. A suitable socio-economic background, especially a profitable market (be that increased food security or an increased cash income).
3. A co-operative communal framework.
4. A favourable soils environment.
5. Access to basic draught power and operating skills.
6. Simple but efficient agrohydrological designs
7. Access to fertilisers and pest control
8. Competent and effective research and extension planning.
The success of water harvesting schemes and the productive marriage of hydrology and agriculture depend on the identification of suitable systems for the regional or national social-economic-farming environment and the correct application of known engineering principles in the field. In this section aspects of the latter are covered.
Badly-designed and implemented systems cause more problems than they solve, interfering with natural drainage, promoting soil erosion and causing the destruction of crops. On land that has been developed already, it is useful to assume that all structures; roads, drains and field boundaries have been laid out with a total disregard for topography and drainage, because this will usually be the case. They should also be regarded as moveable. Catchments are natural features that are constituted of smaller subcatchments; fields are some of their components. In planning water harvesting and water conservation over large areas it essential to recognize the varying scales of catchment areas and their hierarchy. Field layouts aim to create artificial subdivisions of natural catchments into smaller units. A number of points should be considered:
- All layouts should effectively reduce erosion.
- Any redirection and discharge of water should not cause erosion elsewhere.
- Concentrations of water should be kept as small and travel as slowly as possible.
- As far as possible, water should be allowed to follow its natural route.
- Access roads and tracks should be planned as an integral part of any large layout.
- Structures should interfere as little as possible with natural drainage and farm operation.
- Future development should be borne in mind.
In general, it is sensible to first describe agricultural constructions at the small scale, adjacent to the crops that are being grown. Large structures, which are planned to deal with large, exceptional flows, can be described thereafter. There are two main types of structure which operate in different ways.
Contour Structures: are aligned parallel to a master ridge on the contour. They are designed to divide the catchment into smaller subcatchments and maximize infiltration. They do not aid in the discharge of runoff and may be tied to ensure that this control is effective. The greatest danger to these structures is that they fill and over-top.
Graded Structures: may be ridges or drains that are aligned at a slight slope and help in the discharge of runoff at low, non-erosive velocities. They prevent runoff from taking the shortest and fastest route downslope. Generally they are used in association with natural waterways.
7.2.1 Channels and Waterways
a. General Design Considerations
The design of channels that conduct water at non-erosive velocities is a common feature of agrohydrological practice and involves the basic channel hydraulic formula Q Va, discussed in the section on stream gauging. Manning's formula is commonly used determine channel design and several factors need to be considered:
Channel Size: larger channels carry more water than small channels, on the same slope.
Channel Shape: channels of the same cross-sectional area, but of different shapes will carry different amounts of water. Friction reduces velocity, so designs usually aim at reducing frictional resistance. The unit used to measure the effect of shape is the Hydraulic Radius of the channel (R):
R = a/w where (7.1)
R is the hydraulic radius
a is the cross-sectional area of flow and
w is the wetted perimeter of the channel
Generally, the smaller the value of R, the lower the velocity of flow. Channel Gradient: as the bed gradient increases, so does velocity. Channel Roughness: channel roughness is a factor that determines frictional resistance.
Manning's open channel formula is:
v = R0.667s0.5/n where (7.2)
s is the slope of the charnel (m m-1)
n is the roughness coefficient (dimensionless)
R is the hydraulic radius (m) and v is velocity of flow (m s-1)
The design velocity will depend upon the erodibility of the channel lining. Channels may be lined with earth, vegetation or artificial materials. Generally, vegetation, especially grass cover, is recommended for waterways to restrict velocity and to prevent erosion. High retardance species are recommended for use wherever they are available. However, it is difficult to compare the effect of different vegetation types. Vegetation varies from region to region and although much work has been done in countries such as the USA on retardance classifications according to plant species, these plant species may not be present in other areas. Grass and vegetation types may be divided into the following groups or classes of retardance, as presented in Table 7. 10.
Table 7.10: Retardance Classes for Vegetation
Note: condition and vegetation height are important factors in influencing retardance, as can be seen from Table 7.10 and although "good" conditions are tabulated, in real life the extents and conditions of cover are often "poor". In this case a value of Manning's 'n' = 0.040 is commonly used for vegetated channels.
It is also important to note that flow depth has a strong influence on retardance. With medium-height grass vegetation, Low flow (depth of water 10 cm) values of 'n' will be in the approximate range 0.20 to 0.50 whereas Intermediate flow (depth sufficient to just submerge vegetation) values will be around 0.30. However, when the vegetation is completely submerged, values of 'n' drop rapidly to the range 0.030 to 0.040. Bare channels have permissible velocities according to soil texture, though the encroachment of vegetation is common. Table 7.11 gives examples of permissible velocities for unvegetated channels.
Table 7.11: Permissible Velocities for Earth Channels
Permissible velocities for soil types with medium and very good grass cover are given in Table 7.12
Table 7.12: Permissible Velocities ( in m s-1) for Channels with Grass Cover
Table 7.13: Approximate Values of Manning's 'n' for Various Channel Conditions and Materials
Calculations of flow velocity from Manning's formula necessitate the estimation of the roughness coefficient 'n'. Table 7. 13 gives values of 'n' for various channel conditions and the various artificial materials that may be used to line water ways, as well as values of 'n' for dug earth channels: The calculations of velocity can be made from formula 7.1, or nomograph, Figure 7.15, may be used to read off velocity values.
Figure 7.15: Nomograph for use with Manning's Equation
Designs should be made to account for the lowest level of retardance that is likely to be encountered in the rainfall season, according to changes in vegetation growth.
Most commonly, channel cross-sections are parabolic or trapezoidal. Triangular sections should be avoided unless they are on very low slopes (< 2 %) and are well grassed, as they concentrate flow and may cause erosion.
Over time, natural channels tend to a parabolic cross-section and trapezoidal channels, by the natural processes of sedimentation' tend to this form also. As a result, their capacity will decrease. It is useful to remember that if tractors and similar equipment are to cross the channel, then the slope of the sides should not be greater than 1:4, to allow access. Figures 7.16 and 7.17 give the details of hydraulic radii (R) and cross sectional area of Trapezoidal and Parabolic channels, according to their dimensions.
Figure 7.16: Trapezoidal Channels (Metric dimensions)
Figure 7.17: Parabolic Channels (Imperial dimensions)
Flow velocities increase as slopes become steeper, and the danger of scouring and destructive erosion becomes more likely. It is important to recognize that to reduce erosion on steeper slopes, channels can be made wider and shallower to spread the flow and thereby increase the effective retardance of the lining. On shallow slopes channels can be made deeper and narrower with less risk, and narrow channels have the advantage of occupying less land.
b. Methodology of Channel Design
The procedure for determining the design of channels at any location point is as follows:
- Determine the maximum discharge from the procedures in Chapter 2. Remember that the important factor is peak design flow .
- Estimate the channel roughness ( 'n' ) from the details given in Tables 7.10 and 7.13 above. Remember that values of 0.0025 and 0.04 are usually used for earth channels and poor cover grassed channels, respectively.
- Calculate the actual gradient in m m-1 or set a design gradient that will be used in construction.
- Select from Tables 7.11 or 7.12 above, the maximum permissible velocity for the channel design.
- Find the hydraulic radius from Manning's formula (7.2) or from the nomograph, Figure 7. 15.
- Calculate the cross-sectional area for the maximum estimated or design peak flow / velocity
- Using the cross-sectional area and hydraulic radius, find the top width and depth.
- Add the freeboard.
- Assess whether the channel design dimensions are acceptable in the proposed location.
Gradient, channel roughness and velocity are open to variation if the dimensions are unsuitable. Various components of Manning's formula can be found according to which are known and which are not.
Controlled and Uncontrolled Gradients Two sets of circumstances will occur with regard to gradients:
1. Where gradients can be controlled, i.e. selected more or less at will, the velocity, lining and shape dimensions can be used to determine the appropriate gradient.
2. Where gradients are uncontrolled, for example natural streams following a course at right angles to the contours and which it is wise to utilise, the gradient is pre-determined and therefore termed "uncontrolled". In these circumstances it is necessary to determine the velocity according to the channel lining that is present. If time and resources allow, the type of lining can be imposed to select a preferred velocity. Velocity is used with peak flow to determine cross-sectional area, the shape and dimensions of the channel.
It is useful to remember that increases in the permissible velocity of a channel can be achieved by lining with vegetation, increasing the vegetation cover or using artificial lining materials. Grass cover for channels has the advantage that it often already exists in natural waterways. In this case it is best to calculate channel width and estimated peak flow with and without cover and if possible, to extend and improve this cover. Grass is cheaper to install and maintain than an artificial surface.
Design for Catchments
To aid correct design, catchments should be divided into subcatchments. Where the outflow of each subcatchment joins the main stream, the peak flows (known from previous data or calculated from the methods described in chapter 2) should be added together to determine the ever-increasing flow volume. Estimates of channel design should then be carried out at each point. The lowest value of permissible velocity for each section of channel should be taken as the greatest permitted value for that section.
Select and specify the points at which along the channel, the cross-section is to be designed, taking into account the subcatchment dispositions and:
- For all points determine the maximum runoff for these points.
- Select and specify the lining.
- Determine the most erodable soil type.
- Measure the slope of the channel bed segments that include these points.
- Determine the permissible velocity for the slope/roughness/vegetation.
- Determine the channel dimensions according to velocity, slope and runoff.
Any increase in dimensions, above and beyond the increase due to drainage from the increased area of catchment (for example a drop in velocity and therefore capacity), should be allowed for.
An example pro-forma sheet for the design of channels is given below:
Figure 7.18: Example Channel Construction Pro-forma Sheet
Establishment and Maintenance of Vegetation
The value of establishing good conditions of growth in vegetated waterways should be recognised and undertaken where it will mean the effective control of erosive velocities. Fertility should be improved with available nutrients and any seed mixtures should include quick-growing annuals as well as hardy perennials for permanent protection. Any material that can be used to stabilise the soil while plants grow ( such as mulches), will be of value.
Channels are depressions frequently filled with water and as such often retain good vegetation during the early dry season. The use of waterways for excessive grazing and stock routes, as well as tracks should be severely discouraged. Care should be exercised when equipment crosses waterways and runoff discharge from terraces and bunds can do serious damage if not properly sited and controlled.
Vegetation should be mown or carefully grazed to stimulate root growth. Repairs should be made when necessary and velocities controlled to allow the addition of nutrients by sedimentation without the smothering of growth. Revegetation is an important and useful process.
7.2.2 Storm Drains
Storm drains form an important part of the overall control of water in a farm system. They may be used to prevent damaging inundations of runon or may safely relocate runoff from large rain storms, by controlling gradients and velocities to prevent erosive flows.
Storm drains are usually lined with unprotected soil, thus reducing the permissible velocities of water. Their controlled (i.e. selected) gradients allow flexibility in design. In some cases, where low slope arable land meets a steep upslope area, very steep channel gradients may be necessary in the alignment.
The general method for the design of storm drains is as follows:
- Design either for the same dimensions throughout the channel length, or in sections and subcatchments and increasing flow as discussed earlier. The latter choice may take several iterations of design before the most appropriate is found and will depend to a large extent on locality and a balance of cost.
- Estimate flow for the design storm, for example a 10 year return; calculate the flow from the methods in chapter 2.
- Determine the maximum permissible velocity for the most erodable soil in the channel (or channel section).
- Choose the most suitable gradient for the location and purpose of the drain.
- Calculate the design depth; construction depth (design depth + a freeboard of minimum 50 cm), design width and construction width (design width + width of freeboard).
Note that each storm drain should be designed for individual circumstances, but if drains merge, then the section by section approach to design must be followed.
Example Design An example pro-forma sheet for the design of storm drains is given below (Figure 7.19).
Bunds are ridges built within a field to allow, and control, the flow of draining water. They are placed with shallow gradients just off the contour and their locations need to be surveyed. Usually the vertical distance between bunds will be kept constant though the horizontal distance between them will vary with the slope of the land. In some special and rare cases of uniform slope, parallel bunds ma,, be used. The vertical distance between bunds and the within-field spacing (which determine the amount of runoff to be controlled), will be calculated according to slope and soil texture, and will be determined before the bunds are designed.
Bunds can be:
a. Narrow-based or Ridge bunds - which are formed by hand and are narrower in both channel and ridge than
b. Broad-based bunds - which are made by whatever machinery (animal or engine powered) is available.
In form the two types have the basic size relations:
bank width (b) = 0.75 freeboard (f) = 45 cm
bank width (b) = 0.95 t(f) = 30 cm
where t = the channel width in metres.
Figure 7.19: Example Pro-forma Sheet for Storm Drain Construction
The relative merits of narrow and broad-based bunds are listed below in Table 7.14
Table 7.14: Advantages and Disadvantages of Bund Types
Appendix D 1 gives diagrams that can be used to calculate bund lengths for different catchment areas and slopes.
General Method of Design
In many respects, this procedure is similar to that for earth channels, though in general bund designs cannot vary so widely as those of channels.
- It is assumed that all bunds in a field catchment will be of the same design as the largest bund, which will have the largest catchment area.
- Use the longest bund and the average distance from its neighbour, to calculate the catchment area.
- Calculate the runoff peak from this area for (as a realistic example) the I in 10 year storm.
- Determine the lightest and most erodable soil type and select the maximum permissible velocity.
- Use the surveyed, design gradient of the field layout.
- Using the table in Appendix D 2 with the appropriate velocity, gradient and capacity select the suitable width and depth of bund channel.
- If machinery is expected to cross the bund a wide, shallow broad-based design will be needed.
- Add the freeboard.
An example pro-forma sheet for the design of bunds on a controlled gradient is given below:
Figure 7.20: Example Pro forma for Bund Construction
Standard Gradients and Standard Bund Design
Sometimes, standard gradients are used for field layouts. The maximum permissible velocity is found from the soil texture, the gradient is known and the design is calculated to carry maximum runoff. In other cases, it is more convenient to specify a standard bund design for all situations, this is especially so when only standard farm machinery is available. A standard bund design is shown below in Figure 7.21
Figure 7.21: Example of Standard Bund Design
In Figure 7.21 the bund is capable of carrying 0.09 m³ s-1 at a gradient of 1:1000 or 0.25 m³ s-1 at a gradient of 1:200. If runoff is expected to exceed this capacity, the gradient may be made steeper, to increase the velocity to that which is the maximum permissible; or the vertical intervals between the bund may be decreased to reduce the catchment area (and thereby the volume and peak flow of runoff). Alternatively, more waterways can be interpolated, thus reducing bund length.
In some cases parallel bunds can reduce management inputs, but there are difficulties and in general parallel bunds are not recommended. Slopes must be shallow and free from microtopography; inter-bund distances must be reduced and the suitability of each bund must be assessed individually in the field. Velocities must be restricted and on the whole the process is time-consuming and complex.
Roads fall into two main categories:
In this case, rainwater sheds to both sides of the camber of the roadbed and will travel away from either side in response, thus ensuring that the road does not lie wet.
- The minimum width of a cambered road should be 1.5 m either side of the centre line (total 3 m).
- Shallow V shaped side drains should be planned and the material used to add to the construction of the road.
- Mitre drains should lead off from the side drains at frequent intervals and especially at low points. The gradient used should be 1 in 50, with the head turned upslope to intercept any road side drainage.
- Wheel ruts may prevent lateral drainage from the road where it is on a gradient. Gentle depressions across the road at locations of the mitre drains.
Where possible these roads should be integrated with the bund layout of land, by selecting particular broad based bunds and developing them as road lines. Such roads are developed to the extent to allow one-way traffic.
In areas without bund layouts, roads should be designed in the same way, but with sufficient up slope channel capacity to ensure that over topping does not occur.
The detailed methodology of surveying and the use of survey equipment is a large area of study and it is assumed for the purposes of this section that a rudimentary knowledge of maps and levelling equipment has already been acquired. If not, it is recommended that project staff consult texts that deal with surveying to familiarize themselves with basic procedures.
Marking out in the field should be as accurate as possible, to ensure that the designs operate as planned. Survey equipment is used to mark out from the large to small scale, in a sequence such as the one below.
Water ways: the boundaries of fields and catchments
Bunds: the boundaries of bund catchments
Marker and master rows: the guides for row and micro catchment construction
Other required features
When the exact positioning of these features is known, the dimensions of bunds, water ways, etc. can then be pegged.
Dumpy/quickset level and recording book
Chains and arrows
Compass and record book
20 m + strong string
Centre of crest
First/ last peg of graded line
red + yellow
bunds of storm drains
7.3.1 Methods of Marking Out
The natural topography of an area is the first to receive consideration for the survey of field catchments, because natural features define the field catchments into which all other subsequent catchments are integrated.
- Main crests then subsidiaries are marked out.
- Natural waterways are marked, first main channels then subsidiaries.
- Where field and stream catchments coincide, the bund lengths should be acceptable, where this is not the case and bunds are too long, the catchments must be subdivided by roads and artificial water ways to form smaller field catchments.
- Where suitable condition exist, details should be transferred on to the ground from aerial photographs (use transparent film) upon which the planned layout is set, otherwise the area must be levelled. In many cases an intermediate situation will exist.
Crests are the lines of maximum elevation and are the next features to be marked. Access roads to undeveloped areas should follow these crests as much as possible.
- Crests are marked on the plan layout (use transparent film) in increments along the approximate alignment, noting any levelled points.
- Along the approximate crest line on the ground, level at right angles at 10 m intervals to determine the true crest and peg.
- Proceed forward 50 m and repeat with guide pegs between levelled positions as necessary.
- An office plot is drawn, smoothing the outline and the final line is transferred to the ground and is best cut, to overcome problems of disappearing pegs.
The same method is followed, to establish the lowest points between crests. Often drainage is obvious on photographs and the ground.
- Peg the centre line of obvious channels by eye.
- Level if the channel is indistinct and peg the centre line.
- If the starting point of the channel is clear, swing the levelling staff down stream on the string, the highest reading gives the lowest point.
- If the channel is not obvious from the photograph use this method so long as a start point is available.
- If not, a grid survey must be undertaken and the channel defined.
- Natural water ways may have to be extended up slope to ensure that they intercept drainage from all the proposed bunded areas.
Subdivision of catchments with roads and waterways
Decide from field, map and/or photographs, where the longest contour line lies, from crest to channel.
- Calculate the maximum desired bund length from bund design (length should not exceed 400 m on light soils or 500m on heavier soils) and if the length is excessive, divide it by 3, to allow an alternating arrangement of crest and water ways.
- Locate new crests and water ways at the third intervals (see Figure 7.22 below).
- Extend intermediate roads and artificial waterways to meet the natural features. There must always be a water way between two crest roads.
- Water way widths are then marked according to design specifications, with pegs.
- Where designs join, a gradual, smooth transition must be planned.
Figure 7.22: Subdivision of Excessive Bund Lengths with Roads and Waterways
Once catchment and field areas have been defined, they are subdivided into smaller artificial areas by graded bunds.
The design of bunds has been discussed and guidelines to appropriate bund lengths and bunded areas are given in Appendix D1. Bunds can be set out according to vertical height or horizontal distance between them. In the case of maximum permissible vertical height, this relates to; slope and the danger of increased velocity and soil type and the associated danger of erosion. The steeper the slope and/or more erodable the soil, the closer bunds are set together .
The general formula for the determination of the Vertical Interval, the maximum permissible height between bunds, is:
VI = S + f/ 6.5 where (7.3)
VI is in metres
S = Slope in %
f is a factor related to soil and bund type and for the following different conditions has a value of:
Sand, Loamy sand, sandy loam
f = 4
Sandy clay loam, Clay loam, Sandy clay
f = 5
Clay, Heavy clay
f = 6
Subtract (-) 1 from 'f' for fine "rained sands, limited permeability above 1m, soil crusting, row crops steeper than 3%. But the minimum value of f = 3.
Add (+) 1 to 'f' for well drained soils, tillage that encourages infiltration and reduces surface detention. But the maximum value of f= 7.
For Narrow-based bunds controlling peak flows of up to 0.23 m s-1, f is 3 or 4. For Broad-based bunds f can be from 3 to 7.
VI increases with increasing slope, but decreases with the erodibility of soils. Where high annual rainfall and/or poor agricultural practice are encountered, VI should be reduced to 0.8 of the calculated value, and to 0.6 of the calculated value if a parallel bund system is used.
The general formula for the Horizontal Distance is
HD = VI × 100/% Slope (7.4)
For all practical purposes the horizontal distance as measured on the field (i.e. up or down the slope) and the actual horizontal distance can be taken as equal. HD is not usually used for spacing the bunds, but is needed to calculate the catchment area up slope of the bunds, and subsequently for the calculation of runoff and for channel design purposes.
The measure of % slope to determine VI can be taken as the average slope of the bund catchment found by continuous measurement while pegging out. This accounts well for variations of slope if these are frequent on a field and leads to bunds varying continuously in HD. Alternatively, on a relatively uniform field the minimum slope can be used to find the VI. This means that on parts of the field with higher slopes, the density of bunds is actually greater than is needed for conservation, but time is saved in pegging and measuring.
Field Layouts of Bunds
The critical factor is to ensure that velocities of drainage do not cause erosion, and the following must often be considered:
1. The best line for a bund must be defined so that obstructions (e.g. termite hills) do not interfere.
2. Where a pre-determined gradient (e.g. 1:250) has been selected, calculations of design must ensure that velocities do not exceed permitted velocities for the soil type.
3. If a standard bund shape has been selected, the gradient alone can be used to determine safe volumes and velocities for the soil.
4. A particular depth and gradient can be selected at the maximum velocity of the soil type, and channel width is then adjusted to account for expected flow.
Generally, gradients for within-field bunds vary between 1:250 and 1:1000. Obviously the lower gradients are more suitable for lighter soils. Catchment areas do not usually exceed 1.5 ha on sandy soils and 3.0 ha on clay soils and channels are not usually more than 3 m wide and 0.30 m deep.
Pegging Bund Lines
The pegging of bund lines is outlined below, even though such field survey practices are generally beyond the scope of this book. It is included here for two main reasons. First, it is likely to be encountered very frequently and second, it provides a good example of the general practice without entering into the details of field surveying. The method is as follows:
- Peg from the highest point so that if pegging is interrupted, runoff from upslope will do no damage.
- Inter-peg distances are 15 m on uniform land, 7.5 m on uneven land, so use string of these lengths.
- Locate the start point by measuring the average slope from the high point (or previous bund line) then calculate the VI to the next start point.
- This is recorded and added to the high point (or previous bund line) reading to give the new desired reading.
- Move the staff down the crest of field edge until the desired reading is found.
- This position is then pegged as the new start point.
- If the HD is pre-determined then it is simple to measure or convert to VI for the particular slope.
- Check the slope at various points below each bund line and remember that the slope refers to the catchment that will drain to the next bund down.
- Peg at desired or standard gradients.
Remember to check the grade and direction of flow before construction.
- Increase the grade by 0.6% over the first 15 m near the crest and waterway to help intercept any roadside or channel runoff.
- Normally peg from crest to waterway except for storm drains (the reverse procedure), where bunds must end opposite at waterways for cross access and around obstructions.
- Draw pegging diagram with grades, lengths etc.
In some areas obstructions will be found. In such cases the bunds should be placed at least 3 m away from termite hills, for instance, to avoid tight kinks in the line. It may be necessary to try a different grade to avoid the obstruction, pegging back from the 3 m distant location until the original peg line is met, but care should be taken. In some cases, excavation of the obstruction will be possible. It must be stressed that termite hills are very common in some areas and must be incorporated carefully to avoid bund failure.
When the lines are complete, kinks can be smoothed by moving low pegs uphill (NOT high pegs downslope). At least two consecutive pegs must remain in place undisturbed between moved pegs.
As soon as possible, the pegging lines should be ground-marked by hand or shallow disc cut. Any soil should be thrown downhill, so not to interfere with construction.
Contour Ridges Ploughed ridges and furrows are commonly used to prevent runoff and improve infiltration. Marker rows are first set out:
- Using a level, mark contour lines every 20 -30 m down the slope.
- Peg at 15 m or less
- Do not peg across drainage lines.
Master Rows and Microcatchments
Master rows are used to ensure that ridges have gradients no less than that of the main bund that determines their location.
Alternatively, they can be used to restrict runoff when cross-tied. A master row is pegged out between bunds either parallel to the upper bund where bunds converge or parallel to the lower bund where they diverge. Figure 7.23 shows how master rows are positioned in relation to bunds. Note that during pegging the string used to determine pegging positions is kept at right angles to both bunds.
Where a master row runs into a lower bund, move at right angles to the master row and start a new row at the upper bund. Ground mark the row. Microcatchments are made by cross-tying at the construction stage, to no more than two thirds of the ridge height. When pegging is complete, it is essential to draw a field diagram. This allows the farmer to assess accurately the area of land under cultivation, it provides construction details of waterways etc. and shows how much land is lost to obstructions, roads, bunds and other non-cultivated areas.
Figure 7.23: Pegging a Master Row
It is essential that the construction of water conservation structures proceeds in the correct sequence. In the case of simple water harvesting structures, such as Zay and Demi-lunes, construction is straightforward, but it is becoming increasingly recognised that to be successful, water harvesting schemes must be viewed as features integral to the landscape.
The overall control of runoff and its safe redistribution and disposal are also critical in preventing destructive water movement. This will quickly become self-evident in areas where extensive water control by conservation structures is attempted.
Waterways, Drains and Bunds
These are constructed according to permissible velocities and unless the design grass cover has been achieved, erosion will take place when runoff occurs. For this reason waterways must be constructed one or two seasons before other works. Careful planning is needed to integrate this time scale into project activities.
Subsoil exposure can be a serious problem. Grass cover will not be encouraged and every effort must be made to improve condition for vegetation growth. This can be done by spreading back top soil, including mulches, manure or fertiliser. Where possible, the creeping species of the waterway are best replaced by grasses with a bunching habit at the edge to restrict spreading onto the field. Water when necessary, if possible.
Waterways should never be used as roads or serious erosion will take place. Grazing should be controlled to encourage spreading growth.
The danger of obstructing the drainage from bunds into a waterway must be recognised, especially where the sides of a waterway are used by vehicles. This will lead to dangerous over-topping of the bunds and render the waterway useless, and must be avoided at all times.
Storm drains are constructed after waterways have been stabilised with vegetation and before bunds have been made.
Bunds are only constructed after waterways and storm drains, and construction starts at the top of the field, working downwards. If they are not, the drainage they cause worsens a difficult situation. It may be very important to explain this to farmers, who often see bunds as the total solution to water control.
Costs and Equipment
The equipment with which construction is undertaken will depend upon the locality, the economic status of the farmers and the inputs from the project and other external sources. The most appropriate equipment must be determined carefully according to local circumstances and the ability of farmers to sustain the inputs once project support has been removed.
Frequently, the costs involved in the construction of water harvesting and conservation systems are difficult to quantify. The data on costs are very approximate. Often labour costs are not accurately accounted and the cost of long term maintenance is not considered. Moreover, both costs and benefits are highly variable locally, for instance some farmers weed thoroughly and do not regard such inputs as being anything "extra", whereas other farmers do. Byproducts of farm production such as stover and residue grazing may not be accounted for, nor even exploited.
The financial costs of water harvesting structures in particular can be highly variable, perhaps from US$ 50 - US$ 1000 per hectare. Where mechanised transport is used to import rocks for bunds, and where tractors and bulldozers are used to move earth, costs are always very high compared to small hand made structures. However, the mechanised systems may allow water control over a larger area and permit a more fully integrated approach to water harvesting. This can bring benefits in erosion control at the large scale, but these benefits may not be recognised or accounted for.
Hand tools and manual labour will limit the size of structures that can be built and the scale of activities. However, the advantages of cost and sustainability should not be overlooked. Hand tools can provide a flexibility of operation that is not attainable by machinery and both ridge-based and broad-based bunds are best finished by hand. In many cases this equipment will be the most suitable and the control of such labour does not impose any serious technical difficulties, beyond limitations of scale.
Figure 7.24 shows the construction of a waterway by disc plough. The plough should be correctly set. Once the soil is pulverised it should be allowed to settle, encouraged by water (or rain) if possible. An increase in tractor speed will increase bank height. More discs are best suited to large straight structures, fewer discs for bunds. Other ploughs can be used.
Blades: only powerful machinery can move soil with a blade and the use of such equipment is rarely warranted, except for large structures.
Scoops: animal drawn scoops can be very useful for bund construction, after soil has been loosened by ploughing. Alternatively scoops can be moved by tractor.
Various ridgers, trenchers and ridge tying units have been developed to suit local needs.
Figure 7.25 shows the construction of bunds using a disc plough.
Numbers indicate position of equipment for each round. Apply water after each two phases, continue until required section is achieved. Final section is smoothed with a blade terracer or by hand.
Figure 7.24: Construction of Waterway with Disc Plough
Numbers indicate position of equipment at each round
Figure 7.25: Using a Disc Plough to Construct Bunds
All costs of locally made equipment are approximate. The costs of raw materials and especially labour are highly variable from country to country, but a good idea of cost magnitude can be gained from the figures quoted below. The costs of manufactured equipment are based on 1993 prices. Shipping costs, agents' fees and fluctuations in exchange rate cannot be taken into account.
Typical Approximate Cost in $
Abney level complete
Automatic level complete
Bund Length - Catchment Area versus Slope SAND V.I = % S + 3 / 2 (VI in feet)
Note: 1 metre 3.281 feet
1 hectare 2.47 acres
Bund Spacing - Horizontal Distance versus Slope (feet)
1 foot 0.3048 metre
1 inch 2.540 centimetre
1 acre 0.4047 hectare