Sourcebook of Alternative Technologies for Freshwater Augumentation in Africa


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<Sourcebook of Alternative Technologies for Freshwater Augumentation
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1.1.5 Contour Stone Bunding

Technical Description

A single line of stones, or a stone bund, depending upon the availability of stones, is laid along a contour (Figure 6). The resulting structures are up to 25 cm high with a base width of 35 to 40 cm (Figure 7). They are set in a trench of 5 to 10 cm depth which increases stability. The spacing between bunds varies but is usually between 15 to 30 m.

Figure 6. Artists impression, contour stone bunding (Critchley et al., 1992).

For rehabilitation of barren and crusted soils the farmers often use a combination of stone bunds and planting pits (zai; see Chapter 1.1.1 above). The contour stone bunds do not concentrate runoff but keep it spread. They also reduce the rate of runoff allowing infiltration, which is further enhanced through the use of the planting pits. Farmers often start at the lower points of a field and work upslope rather than the conventional wisdom which would suggest starting at the higher points in the catchment and working downslope. Stone bunds, however, are not easily damaged or destroyed by runoff, and, by starting lower on the slope, farmers can be certain to harvest sufficient runoff for production of a crop in a year of below average or irregular rainfall.

Figure 7. Detail of stone bund (Critchley et al., 1992.)

Extent of Use

The method was pioneered in the 1980s in Burkina Faso as a simple and effective technique for conserving water and soil resources. Since that time, it has been spreading rapidly. From 150 ha bunded by farmers in 1982-1983, the number of hectares had risen to an estimated 8 000 ha by 1989. Over 400 villages have participated in the installation of this technology. Thousands of hectares of barren land have been reclaimed by the combined use of stone bunds and planting pits.

Contour stone bunds, or simpler stone lines, are used in Mali on the Dogon Plateau.

Operation and Maintenance

There is limited, ongoing repair required as the stones are not vulnerable to erosion. However, silting behind the stone bunds requires that the stones to be relaid from time to time. Care must be taken that overtopping of the bunds does not lead to erosion on the downstream face, with subsequent gully formation and undercutting of the bund.

Level of Involvement

Beyond the initial, demonstration project in Burkina Faso, the technology has expanded in use of its own accord. Thousands of hectares outside of the project area currently use this technology. It is entirely farmer managed. In some villages, land management committees have been set up to look at a variety of activities related to improving land utilisation in their respective areas.

Costs

Where stones are in short supply, there are increased costs associated with their acquisition and transport. This will be self-limiting for the technology.

Effectiveness of the Technology

Contour stone bunding is effective when judged by the acceptability of the technology by farmers. Farmers use stone bunds on fields currently under cultivation and to expand cultivation to new areas. Stone bunding is particularly attractive to farmers because of its ability to be implemented on fields already under cultivation. Yields in the first year have been increased by an estimated 40%. When barren fields are rehabilitated, yields of 1 200 kg/ha have been achieved in the first year. Application of fertilisers has only rarely been necessary, and the expected decline in fertility has not been observed although it is expected that, ultimately, there will be a need for a limited use of fertilisers.

Suitability

The technology is particularly suited to semi-arid lands, where stones are available. In the areas of application of this technology, long term average rainfalls are over 700 mm, but during the 1980s, when this technology came into widespread favour, rainfalls have been below 600 mm. The technique has also been used in wetter areas of Mali to prevent erosion due to overgrazing, and, in addition, has produced profitable cash crops.

Environmental Benefits

The technology has noticeable, positive environmental impacts, leading to the rehabilitation of degraded lands and reducing soil erosion.

Advantages

Benefits to farmers have been evident, and the technology is simple to implement at the local level. Stone bunds do not readily wash away and, therefore, the technique is not vulnerable to unusual and variable intensity rainfall events. In Burkina Faso, the project has also resulted in increased attention to land use planning and the environment by villages.

Disadvantages

The popularity of the technique has resulted in shortages of stones and, therefore, a higher cost for latecomers.

Cultural Acceptability

There is a long history of soil and water conservation on the Dogon Plateau. Likewise, farmers in the Yatenga Region of Burkina Faso have traditionally used stone lines on their fields. For this reason, the further development of the concept into installation of stone bunds has been readily accepted. Farmer-to-farmer extension has been shown to be an effective tool which is underrated in many projects.

Further Development of the Technology

A concern over the eventual disposition of silt-in of the bunded areas has not yet been addressed, but it has been suggested that the planting of perennial grass on the bunds will maintain their function of slowing and spreading water and help to retain deposited silt within the bund basins. Extension support is required amongst the Dogon in Mali who have not yet adopted the improved methods based on the simple stone lines they currently use.

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. 1991. Looking After Our Land, Soil and Water Conservation in Dryland Africa, Oxfam, London. 84 p.

Lee, M.D. and J.T. Visscher 1990. Water Harvesting in Five African Countries. IRC Occasional Paper No. 14. 108 p

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