Newsletter and Technical Publications
<Sourcebook of Alternative Technologies for Freshwater Augumentation
1.1.8 Flood Harvesting Using Bunds
|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. 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
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 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.
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.
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.
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
The traditional system has a problem in that contours are estimated rather
than measured, resulting in occasional waterlogging and bund breakages.
This technology is culturally-acceptable in Somalia as an indigenous water
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.
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.