Semi-circular bunds: field layout. A diversion ditch may be necessary to protect the system against runoff from outside. When the stored water is to be used for post rainy season (Rabi) crops the water should be applied at the most critical growth stages. For ground slopes above 0.5% bunds aligned with a 0.25% gradient are used and are termed "graded bunds". 5.8 Permeable rock dams In erodible soils, it is advisable to place a layer of gravel, or at least smaller stones, in the trench. The vertical interval can be determined most easily by the use of a line-level. These should be replaced. It is sometimes more appropriate to approach the problem the other way round, and determine the area and number of bunds which can be cultivated from an existing catchment. 1.5k Downloads; Abstract. The basic methodology of determining C:CA ratio is given in Chapter 4, for the case where it is necessary to determine the necessary catchment size for a required cultivated area. 5.4.4 Maintenance When no machines are available, this should be done by foot or with a barrel filled with sand. Thus there must be planning for repair work where necessary and careful inspection after all runoff events. The earth beneath the bunds should be loosened to ensure a good mating with the bund. The reduced rainfall in the Sahelian zones over the last two decades has left much of the land in this area barren and encrusted with a hard surface cap. Construction begins with large stones laid down at the base and the downslope side of the trench, and then smaller stones laid in front and on top of this "anchor" line. The second, for slopes greater than 0.5%, is a series of graded bunds, each with a single short upslope wing, which spread the flow gradually downslope. When this has been done without any significant beneficiary commitment the bunds have been quickly abandoned. Soils: all soils which are not too shallow or saline. In this new approach the attempt is made to utilize water in all its stages and then excess water is drained out in to a farm pond connected to the field by protected grass water ways. 2. Therefore, it may be difficult to motivate the population to invest voluntarily, in the time and effort required for implementing and maintaining such a water harvesting system. Figure 20. When the pond is with murrum strata or under neath the total losses can be to the extent of 72% out at which 80% losses are due to seepage alone. Dimensions for bunds on different slopes are given in Table 23. The optimum design for an individual set of circumstances can only be achieved by a process of trial and error. ** Calculation of earthworks per unit includes only two of the sides around the catchment: the other two sides are included in the microcatchment above. Swales are not the only way of passively hydrating a landscape. The area of each unit is either determined on the basis of a calculation of the plant (tree) water requirement (see Chapter 4) or, more usually, an estimate of this. A diversion ditch should be provided above the scheme if there is a risk of damage by runoff from outside the block. "Limanim" (plural of Liman) may be constructed in series along a wadi bed. The uniform cross section of the bunds is recommended to be 60 cm high, 4.1 metres base width, and a top width of 50 cm. The spacing between bunds ranges normally between 15 and 30 m depending largely on the amount of stone and labour available. Construction of trapezoidal bunds on slopes steeper than 1.5% is technically feasible, but involves prohibitively large quantities of earthwork. Water harvesting is one way that farmers can deal with climate-change induced water scarcity in Ethiopia. The bund should be set into a shallow trench, of 5 - 10 cm depth, which helps to prevent undermining by runoff. PROFILE: Contour Bunds for Trees in Baringo, Kenya. A common variation is to build microcatchments as single, open-ended structures in "V" or semi-circular shape (see Figure 19). 5.6 Trapezoidal bunds For the wingbunds, the demarcation of the earthworks limits is slightly more complicated. It is not a cheap technique, bearing in mind that one person-day is required to build (on average) two units, and costs per unit rise considerably as the microcatchment size increases. A common distance between the tips of adjacent bunds within one row is 20 m with 30 m spacing between the tips of the lower row and the base bunds of the upper row (see Figure 38). There are three method of harvesting and recycling of runoff water. Soils: all agricultural soils - poorer soils will be improved by treatment. The first contour, at the top of the scheme, is staked out using a simple surveying instrument as described in the appendix. As recommended under "bund design" the bunds should be set at a spacing of 10 m for slopes up to 0.5% and 5 m for steeper slopes. SOILS: Should have the main attributes of soils which are suitable for irrigation: they should be deep, not be saline or sodic and ideally possess inherent fertility. In each case, crops or fodder are planted between the bunds. No structure in any water harvesting system is entirely maintenance free and all damage, even small, should be repaired as soon as possible to prevent rapid deterioration. A bund cross section of 1.38 square metres is assumed. The bund height assumed is 25 cm with 75 cm base width. Although the first reports of such microcatchments are from southern Tunisia (Pacey and Cullis, 1986) the technique has been developed in the Negev desert of Israel. Figure 48. Points 3 and 4 are then pegged. If bunds are spaced at more than 2 metres, the ridge height must be increased. Infiltration pit with planting step. The general layout, consisting of a base bund connected to wingwalls is a common traditional technique in parts of Africa. Each contour line is then set out and pegged individually. This is particularly important in the arid and semi-arid regions of Africa and may help to explain the failure of so many projects that did not take into account the people's priorities. This technique is used exclusively for annual crops. The diversion ditch should be constructed before the contour bunds are built to prevent damage if rainstorms occur during construction. This lip should be pitched with stones for extra resistance to erosion. The amount of earthmoving necessary for trapezoidal bunds means that their construction usually requires organized labour or machinery and is beyond the scope of the individual farmer. The structure is finished off with a cap of large stones. Soil is excavated and placed downslope to form a ridge, and the excavated furrow above the ridge collects runoff from the catchment strip between ridges. Where a diversion ditch is necessary, an additional 62.5 m3 for each 100 metre of length of ditch has to be added. A permeable rock dam is a long, low structure, made from loose stone (occasionally some gabion baskets may be used) across a valley floor. 5.2.6 Socio-economic Thus the quantity of water actually utilized cannot be easily predicted from the catchment size. The diversion ditch should be constructed first to prevent damage in case a rainstorm occurs during construction of the microcatchments. The word "Negarim" is derived from the Hebrew word for runoff - "Neger". This technique is appropriate for small-scale tree planting in any area which has a moisture deficit. For higher slopes, smaller radii are required. Table 21. The top of the bund should be at least 25 cm wide and side slopes should be at least in the range of 1:1 in order to reduce soil erosion during rainstorms. This layered approach reduces the risk of damage by floods during construction. Design "a" is a short slope catchment technique, and is not designed to use runoff from outside the treated area, nor to accommodate overflow. The pipes run down the slope from roof into an underground tank. 5.8.4 Maintenance The "demi-lunes" were constructed by hand, using food-for-work rations during the construction time. The land must be sited close to a wadi or another watercourse, usually on a floodplain with alluvial soils and low slopes. 5.5.1 Background Conversely, in situations where the catchment is not of adequate size, interception ditches can be excavated to lead runoff from adjacent catchments to the bunds. Rainwater is collected at the roof top of the houses which is then sent to the storage tank via slope pipes. Gabions are best for spillways, as loose stone is easily destabilized by heavy floods. The farm ponds may be circular squared or rectangular. Any breakages must be repaired immediately. Cetyl alcohol gum mixtures polyethylene oxides. The project supplies transport of stone, by lorries with removable skips, free of charge. Conservation techniques conserve water within the biomass and the soil by reducing run-off and keeping the water where it falls, as much as possible. If the pond is located in upper patches water can be. The apex is now marked with a peg and the catchment sides (a-c) and (b-c) marked on the ground alongside the strings with a hoe. Six Methods for Harvesting Rainwater . The result was extensive bunding which was not useful - and not used. Alternatively, earth contour bunds can be constructed, with stone spillways set into them (Figure 48). Negarim microcatchment: details for 0.25 m bund size (for dimensions x and y see Table 18). Topography: need not be even - if uneven a block of microcatchments should be subdivided. For some species, seeds can be planted directly. As contour bunds are implemented on even land, contours need only be staked out approximately every 50 metres. The technique of floodwater farming using water spreading bunds is very site-specific. Semi-circular bunds which are used for fodder production normally need repairs of initial breaches only. A diversion ditch should be provided above the block of contour ridges if there is a risk of damage caused by runoff from outside the system. 5.5.4 Maintenance The yield of runoff from the very short catchment lengths is extremely efficient and when designed and constructed correctly there should be no loss of runoff out of the system. One important advantage of contour bunds for tree establishment is that oxen or mechanized cultivation can take place between the bunds, allowing crops or fodder to be produced before the trees become productive. For greater depths, a spillway is recommended. Permeable rock dams: general layout (Source: Critchley and Reij 1989), Table 25 QUANTITIES FOR PERMEABLE ROCK DAMS. Water spreading bunds can be used under the following conditions: Rainfall: 100 mm - 350 mm; normally hyper-arid/arid areas only. 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