Bunds

Being one of the most often used precipitation harvesting methods in agriculture, the main purpose of bunds is to slow down and filter runoff water from rainfall and hence reduce soil degradation. As the water flow is decelerated, higher amounts can infiltrate in the soil, leading to increased soil moisture (see also conservation of soil moisture). Furthermore, water is spread more evenly, which can prevent gully formation. Bunds are basically the opposite of field trenches, where slots and trenches are excavated to stop, store and infiltrate floodwater and surface run-off.

There exist two main types of bunds: contour bunds (contour ridges) and semi-circular bunds. The choice of the type of bund depends on the local ground conditions.

Contour bunds

Contour bunds can either be made of stones or soil (sometimes in variation with crop remains). They are constructed along a contour in order to best slow the water flowing down the slope, which increases the green water pool of the soil and prevents erosion. There should always be several bunds next to each other, whereas the distance between them depends on the slope and the soil type of the field: the steeper the ground, the closer the bunds. Contour bunds can be used for both yearly field crops as well as the planting of trees. Their use is widespread throughout Africa. In northwestern Somalia, contour bunds have reportedly increased yields of sorghum by up to 80% (MALESU et al. 2007; OUESSAR et al. 2012).

A special and well-known type of contour bunds is terracing contour bunds, often used for the production of rice. In comparison to normal contour bunds, terraces even out the slope of the ground and therefore make the paddies plain.

Semi-circular bunds

Due to their half-moon design, semi-circular bunds are well suited for planting individual trees. In opposition to contour bunds, each bund has to be made singularly by hand, making them much more time consuming in their implementation. As a plus factor, semi-circular bunds can also be applied to steeper fields.

It is important to choose the right measures for semi-circular bunds, as the amount of collected water depends on both its height and the position of its tips. A further factor for the appropriate size of bunds is the selected crop (see also crop selection): bigger bunds are needed for fruit trees, which require a radius and height of at least 0.6m. Furthermore, it is also important to leave free spaces between the bunds in order to enable excess water to run off.

Similar to planting pits, semi-circular bunds are usually filled with organic matter to add nutrients for improved crop yield (MALESU et al. 2007).

Adapted from RUFFINO (2009)

The costs for the implementation of bunds depend strongly on the choice of design. For contour bunds, working time of approximately 32 person days is estimated per hectare. If machinery can be used, the time required reduces, but the costs may rise due to higher investments (only possible for contour bunds, as semi-circular bunds cannot be made mechanically). For stone bunds, increased costs may apply where stones are rare.

The effectiveness of contour bunds may be enhanced by placing planting pits between the bunds and/or constructing upward ties. The ties are meant to create micro-catchments along the bund and can be built manually or with the help of special machines. If there is excess rainfall, it may be helpful to build cut-off drains in order to let surplus water run off (OUESSAR et al. 2012).

The required maintenance of bunds depends on the type of construction: constructions made of stones are very resistant to erosion and therefore only need limited on-going repair (ADB 2008). Earthen ridges on the other hand need to be rebuilt to their original height after each season (ANSCHUETZ et al. 2003). To limit maintenance to a minimum, it is important that the primary construction is done carefully and solidly. Grass growing on bunds can further enhance stability as its roots fix the soil. It should therefore not be removed.