Micro Basins

Micro basins are used in order to store precipitation water by collecting surface run-off: low soil ridges – optionally supported with stone walls - are constructed in an enclosed shape to form a basin, which helps to prevent the water to further run off. The collected rainwater is then used for direct irrigation or infiltration into the soil to enhance soil moisture and recharge groundwater (see also optimisation of water use in agriculture, conservation of soil moisture, surface and surbsurface groundwater recharge).

To use the water most effectively, pits are dug within the basin, storing more collected water and protecting it from evaporation (ADB 2008). As rainwater accumulates in these lower parts of the micro basins, crops or trees are best directly planted in the pits (MALESU 2007).

Micro basins show a similar design to semi-circular bunds, except that they are entirely surrounded by ridges. The benefit of this design using closed micro basins is higher water storage efficiency. On the other hand, as a disadvantage, excess water cannot run off and surface run-off has to be stronger to enter the pits.

Micro basins are just one type of micro-catchment. For other micro-catchment techniques, see also bunds, field trenches and planting pits.

 Different design types are possible, including half-moon, v-shaped, diamond-shaped, trapezoidal, as well as contour bund basins. Micro basins are mostly used for small-scale tree and bush planting in areas with moisture deficit (ANSCHUETZ et al. 2003). Each basin consists of a catchment area and an infiltration pit, which serves as the cultivated area. The field can be additionally protected from excess water runoff with the construction of a cut-off drain or a retention ditch (IRHA 2011).

V-shaped micro basins are also called “negarims”. They are particularly common for fruit tree production in Israel and in several districts of Kenya (MALESU 2007).

The height of the ridges relies strongly on the slope and the size of the catchment. Calculations may be done according to the following table:

The size of a micro basin is usually between 10 to 100 m2, even though larger dimensions are possible, especially if more than one tree is planted in a catchment or if they are applied on flat ground.

Constructing micro basins does not need special material apart from a shovel and a tube-level or some other instrument for finding the contour line. Otherwise, the implementation of micro basins is a comparably cheap measure.

Adapted from ANSCHUETZ et al. (2003)

The construction of micro basins cannot be mechanised and is done by hand following six steps:

• Detecting and marking the contour line (d), for example by using a tube level.
• The diagonal of the micro-basin is measured and marked along the contour line ((a) and (b)). Further points are detected along the contour line, using the same distance.
• Point (c) is found by using two strings with the same length fixed at point (a) and (b). A mark is made where the two strings meet.
• The following lines of micro basins are found one by one using the same procedure.
• The infiltration pits are then marked and dug out in each micro basin.
• The basins need to be cleared of all vegetation. Then, the bunds are constructed: first, they are to be constructed to half their final height. For firmness, this first layer is compacted. In a second step, more material is added to reach the wanted height. Finally, the bunds are again compacted and evened out.

Damage to the basins needs to be repaired immediately. If there is unexpectedly high rainfall, the danger for breaking embankments rises (MALESU 2007). Also, the micro basins have to be cleaned from unwanted vegetation. However. grass growing on the bunds of the basins can protect the walls from erosion.