Sourcebook of Alternative Technologies for Freshwater Augumentation in Africa


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<Sourcebook of Alternative Technologies for Freshwater Augumentation
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1.1.9 Earthen Bunds

Technical Description

Earthen bunds are essentially an external catchment, long slope technique of water harvesting. Typically a u-shaped structure of earthen bunds which farmers build on their cultivated lands to harvest runoff from adjacent upslope catchments, this technique usually collects rainwater and, sometimes, floodwaters (Figure 14).

The base bund approximately follows the contour line and impounds the runoff. Two outer arms fulfil the same function and also act as conveyance structures which direct water to the cultivated lands. Sometimes, shorter inner arms are added which divide the land into smaller basins and improve the spread of captured runoff (Figure 15). A shallow channel is left on the inside of the bund to support the conveyance and circulation of runoff.

Figure 14. Typical element of the teras water harvesting structure
(van Dijk, 1995).

Excess water is normally drained along the tips of the outer arms which are reinforced with materials such as stones, brushwood or old tyres. Bunds are usually 0.5 m high and 2 m deep at the base, but these dimensions can vary greatly depending on both the slope and the amount of runoff expected in the area. The base can be between 50 to 300 m long, while the arms are usually 20 to 100 m long. The size of the cultivated area serviced by such a structure is 0.2 to 3 ha.

Figure 15. Overview of a teras water harvesting system (van Dijk, 1995).

Extent of Use

One of the few examples of traditional water harvesting technologies where the technique is applied over a wide area. The system is called teras in Somalia, where there are reported to be "thousands" of teras plots in some regions of the Sudan. The bund system is also widely used in Ghana, Burkina Faso and Mali.

Operation and Maintenance

This is a labour-saving technique. Generally, between 3 and 18 days/ha of work is required to ensure that the system runs efficiently. However, breaches of the bunds will require additional work in order to effect repairs. The local dynamics of a drainage system may also require that the conservation structures be continuously adjusted for best performance.

Level of Involvement

Entirely traditional and farmer-managed, earthen bunds may be built by hand using simple tools, although the use of hired tractors is becoming more common.

Costs

There are no data on costs available, but they are not believed to be high when earthen bunds are constructed manually by a farmer. Mechanical construction methods increase costs.

Effectiveness of the Technology

The technique allows the production of a crop of millet or sorghum. Based on data from the Sudan, yields may reach 750 kg/ha in a good year. Quick maturing millet should be planted immediately after the water from a storm has subsided. This crop grows and matures in about 80 days.

Environmental Benefits

Use of this technology reduces land degradation.

Suitability

This technology is appropriate for areas of the Sudan where the foothills reinforce high intensity and short duration rainfall, with 150 to 400 mm rainfall, annually. Low infiltration increases the generation of runoff from teras catchments. Catchments are normally 2 to 3 times the cultivated area in regions of 150 to 400 mm annual rainfall. Teras irrigation suits the lifestyle of the Beja in the Sudan, as they are often absent from the land, and this system lends itself to small-scale private enterprise. In West Africa the technology is widely found in valley bottoms.

Advantages

The technology is entirely farmer managed and, therefore, not subject to the organisational problems of other soil and water conservation techniques. Socio economic surveys have indicated that application of soil and water conservation practises contributed about an additional 75% to the total household crop production income in the 1980s and 1990s.

Disadvantages

The lack of a spillway can result in breached bunds.

Cultural Acceptability

There are no cultural restrictions. The Muslim population of the Sudan initiated the use of this technology about 40 years ago.

Further Development of the Technology

The development of spillways may improve the efficiency and reduce maintenance costs.

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.

van Dijk, J.A. 1995. Indigenous and Introduced Soil and Water Conservation in Sudan. Waterlines, 13(4):19-21.

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