Technical
Micro-catchments

There are FOUR main types of microcatchments:
1.  Negarim Microcatchments - for trees
2.  Contour Bunds - for trees
3.  Contour Ridges - for crops
4.  Semi-circular Bunds - for range and fodder


The type of technique chosen must take into account the socio-ecomonic situation of the region and also the technical aspects of the particular site.  

Technical Flow chart
to determine the appropriate technique



Four Techniques:

The Food and Agriculture Organization of the United Nations has a manual for design and construction of microcatchment systems:

A Manual for the Design and Construction of Water Harvesting Schemes for Plant Production
The manual provides guidance in determining:
1.  Water and soil requirments
2.  Rainfall-runoff analysis
3.  Design and construction of the 4 techniques mentioned here
4.  Husbandry
5.  Socio-economic factors


A brief introduction to each technique:

Negarim Microcatchments
Negarim microcatchments are diamond-shaped basins surrounded by small earth bunds with an infiltration pit in the lowest corner of each. Runoff is collected from within the basin and stored in the infiltration pit. Microcatchments are mainly used for growing trees or bushes. This technique is appropriate for small-scale tree planting in any area which has a moisture deficit. Besides harvesting water for the trees, it simultaneously conserves soil. Negarim microcatchments (Figure 3) are neat and precise, and relatively easy to construct.



Figure 3:  Negarim Microcatchment


Contour Bunds
Contour bunds for trees are a simplified form of microcatchments. Construction can be mechanized and the technique is therefore suitable for implementation on a larger scale. As its name indicates, the bunds follow the contour, at close spacing, and by provision of small earth ties the system is divided into individual microcatchments. Whether mechanized or not, this system is more economical than Negarim microcatchment, particularly for large scale implementation on even land - since less earth has to be moved. A second advantage of contour bunds is their suitability to the cultivation of crops or fodder between the bunds.   An example of a contour bund system is shown in Figure 4.

                                                       
                                                    Figure 4:  Contour Bunds


Contour Ridges
Contour ridges, sometimes called contour furrows or microwatersheds, are used for crop production. Ridges follow the contour at a spacing of usually 1 to 2 meters. Runoff is collected from the uncultivated strip between ridges and stored in a furrow just above the ridges. Crops are planted on both sides of the furrow. The system is simple to construct - by hand or by machine - and can be even less labour intensive than the conventional tilling of a plot.  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. Another advantage is an even crop growth due to the fact that each plant has approximately the same contributing catchment area.   An example is shown in Figure 5.

                                                                                                     
                                                                                                      Figure 5:  Contour Ridges

Semi-circular Bunds
Semi-circular bunds are earth embankments in the shape of a semi-circle with the tips of the bunds on the contour, as seen in Figure 6. Semi-circular bunds, of varying dimensions, are used mainly for rangeland rehabilitation or fodder production. This technique is also useful for growing trees and shrubs and, in some cases, has been used for growing crops. Depending on the location, and the chosen catchment: cultivated area ratio, it may be a short slope or long slope catchment technique.  Semi-circular bunds, (the term "demi-lune" is used in Francophone Africa), are recommended as a quick and easy method of improving rangelands in semi-arid areas. Semi-circular bunds are more efficient in terms of impounded area to bund volume than other equivalent structures - such as trapezoidal bunds for example. Surprisingly, this technique has never been used traditionally.

                                                                                                                                                                                       
                                                                                                                                Figure 6:  Semi-circular bunds
Reference:
Critchley, Will and Klaus Siegert.  A Manual for the Design and Construction of Water Harvesting Schemes for Plant Production.  Rome:  Food and Agriculture Organization of the United Nations, 1991

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