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.
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
