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Newsletter and Technical Publications

<Sourcebook of Alternative Technologies for Freshwater Augumentation in Africa>


1.1.8 Flood Harvesting Using Bunds

Technical Description

This system is normally used where slopes are above 0.5% and there is significant runoff to be harvested. It is a long slope catchment system where water travelling in small gulleys is diverted onto a farm plot (Figure 13). The plot has contour bunds within the fields at a typical height of 50 cm and a base width of 150 cm. These bunds commonly extend across the whole plot and excess water spills around a short arm at one end. Fields are typically 1 ha in area, and there may be a second collecting bund behind the first to collect any overflows.

An alternative overflow system is the incorporation of a 10 cm plastic pipe into the bund. In cases of excessive flooding or water logging, the bund may be breached deliberately.

 

Figure 13

Figure 13. The laag system of capturing floodwaters
(Critchley et al., 1992).

 

Extent of Use

This technology is widely used by agro-pastoralists in Central Rangelands of Somalia where it is known as laag. There are up to 2 500 families using this system in Somalia.

A common way of using flood flows in a wadi or river channel is to construct a dam or a barrage to divert the water onto cultivated lands using a water spreading technique such that water is not allowed to form ponds. Rather, water is induced to flow over the land surface at low velocities. This is particularly appropriate in grassed areas used for forage production, and has been used experimentally in West Africa and Kenya.

Operation and Maintenance

There is evidence that the laag systems have developed over several generations, and, although breaching of bunds and occasional water logging occur, the cultivators are able to design, manage and maintain the system locally.

Level of Involvement

This is a traditional technique used for generations to harvest water for crop production. Structures are normally made by hand, and the system is simple and easy to construct. Rice farmers often hire tractors for the mechanical construction of bunds and for ploughing.

Costs

Costs of implementation of this technology are not known as it is a traditional technology implemented by farmers at their own expense. However, costs are believed to be less than $100/ha.

Effectiveness of the Technology

The technique is used to augment income from livestock production by crop cultivation and the simultaneous provision of fodder for the livestock. Sorghum is the usual crop of choice, although cow peas are also grown. Two crops can be grown if the rains arrive as expected. Yields are not specifically available, although, for this area, they are generally given as 415 kg/ha for sorghum and 330 to 530 kg/ha for cow peas. Local agro-pastoralists say that harvested runoff improves crop performance and, without it, crop failure may occur or crop production may not even be possible.

Suitability

The technology is used in Somalia in a region with an annual rainfall of between 150 to 300 mm, split between two rainy seasons. Crops are harvested in each season. Water harvesting is most common in zones of clay soils. This technique also requires an associated drainage gulley network, river system or system of road drains. Riparian situations may also be suitable.

Environmental Benefits

Limited environmental impacts are expected, although poor management of the flows can lead to erosion. Generally, this technology reduces soil erosion and improves vegetative ground cover.

Advantages

These systems are completely farmer-managed, simple to construct and allow the harvesting of floodwaters that would otherwise flow off the land surface. Provision of water to livestock and crops also contributes to improved food security.

Disadvantages

The traditional system has a problem in that contours are estimated rather than measured, resulting in occasional waterlogging and bund breakages.

Cultural Acceptability

This technology is culturally-acceptable in Somalia as an indigenous water harvesting technology.

Further Development of the Technology

Introduction of contour surveying instruments in the determination of contour lines, and completion of further work to determine the spacing between bunds and positioning of spillways, would improve the technology.

Information Sources

Critchley, W., C. Reij, and A. Seznec 1992. Water Harvesting for Plant Production. Volume II: Case Studies and Conclusions for Sub-Saharan Africa. World Bank Technical Paper No. 157, 133 p.

Critchley, W.R.S., C.P. Reij, and S.D. Turner 1992. Soil and Water Conservation in Sub-Saharan Africa - Towards Sustainable Production by the Rural Poor. International Fund for Agricultural Development.

Pacey, A. and A. Cullis 1991. Rainwater Harvesting. The Collection of Rainfall and Runoff in Rural Areas. Intermediate Technology Publications, London, 216 p.

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