Sourcebook of Alternative Technologies for Freshwater Augumentation in Africa


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<Sourcebook of Alternative Technologies for Freshwater Augumentation in Africa>

1.1.10 External Catchments Using Contour Ridging

Technical Description

A further variation of the contour ridging technique described in Chapter 1.1.6, this technology uses an external catchment and incorporates a stone spillway into the contour bund, providing for excess runoff to flow around the structure (Figure 16). Bunds are made of earth or, occasionally, stone, and, in Niger, they are usually covered with a layer of stone on the top and back slope Figure 17).

The area impounded by the bund is planted. The usual catchment to cultivated area ratio is 2:1 but reaches 5:1 in Kenya where off contour bunds are used as collection systems to channel runoff to cultivated plots.

Figure 16. Macro-catchment water harvesting system (Imbira, 1989).

For full utilisation of the cropping area, the spillway height should be level with the base of the spillway on the next contour uphill. Levelling of the ground between contours assists in water spreading when runoff is collected. The spillway height determines the depth of water retained and is usually about 10 cm.

Figure 17. Macro-catchment water harvesting in Niger (Critchley et al., 1992).

Extent of Use

Several thousand hectares are treated with a variety of bund systems, although the largest number of projects are in Niger and Kenya.

Operation and Maintenance

Maintenance is required to control erosion around spillways and bund wing walls. Achieving adequate compaction of bunds with manual construction methods is difficult and may result in breaches during the first year of operation. Grass planted on the bunds and spillways helps to protect these surfaces from erosion and reduces maintenance requirements, particularly since some resistance to the repair of breached bunds was reported in Kenya (Figure 18).

Level of Involvement

The Kenyan programme of bund construction is all done using manual construction methods, whereas, in Niger, the bunds are constructed by machine and only the stone is laid by hand. Construction of these storages has largely been done through food-for-work programmes and there is some concern about the level of true involvement of people. In at least one application, it was observed that there was little voluntary participation in the use of this technology by the community.

Figure 18. Spillway construction for bunds with external catchments (Pacey and Cullis, 1991).

Costs

In Niger, the estimated construction cost is about $500/ha for bunds, land preparation and fertiliser. In Kenya, 100 person days/ha are commonly devoted to construction, resulting in an approximate cost of between $52 to $202/ha.

Effectiveness of the Technology

This technology is effective in controlling erosion. Yield information is not available for Niger, but, in Kenya, the comparison with control plots has shown an highly significant increase in yields of sorghum and cow peas.

Suitability

This technology is suitable for low and unreliable rainfall areas, with an annual precipitation of 350 to 650 mm. It is also well-suited for use in the reclamation of degraded land.

Environmental Benefits

The technology has beneficial effects in the reduction and control of soil erosion.

Advantages

The advantages are the concentration of runoff to allow the cultivation of a crop where otherwise none would be possible. This results in increased and more reliable level of production.

Disadvantages

A high demand for labour in the construction and maintenance of these systems may partly be a reason for the low level of acceptance by the community. Approaches have not yet been found to fully involve the community.

Cultural Acceptability

These have not been fully assessed; however, there are no known limitations.

Further Development of the Technology

This technique is clearly an appropriate technology for the conservation of soils and the recovery of degraded lands. Nevertheless, the technology needs to be demonstrated as useful to the community and presented in a way that the community is willing to manage.

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.

Imbira, J. 1989. Runoff Harvesting for Crop Production in Semi-arid Areas of Baringo. In: D.B. Thomas et al. (Editors), Soil and Water Conservation in Kenya, Proceedings of the Third National Workshop, Nairobi, 1986. University of Nairobi, SIDA., pp 407-431.

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