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close this bookSustaining the Future. Economic, Social, and Environmental Change in Sub-Saharan Africa (UNU, 1996, 365 p.)
close this folderPart 2: Environmental issues and futures
close this folderDrought, desertification, and water management in Sub-Saharan Africa
View the document(introduction...)
View the documentIntroduction
View the documentDroughts in Sub-Saharan Africa and their implications for planning and development
View the documentDesertification
View the documentLand degradation and management of soil and water
View the documentConclusion
View the documentAcknowledgements
View the documentReferences

Land degradation and management of soil and water

The problem

From the discussion presented above on drought and desertification, it is apparent that the fragile soils of semi-arias lands of Sub-Saharan Africa are exposed to increasing threats of wind and runoff erosion. It is also evident that water is a scarce, high-value economic resource in the region. In addition to high rates of evaporation, because the rains usually come during the hot period of the year, other major routes of water loss include soil evaporation and runoff owing to poor infiltration in the soil. Rains in the semi-arid lands of Sub-Saharan Africa do not fall gently and evenly; they come predominantly in convective storms as described earlier. Rain that falls this way in torrents is more destructive than the gentler rain of temperate zones. More is lost in runoff and less filters into the ground.

Yet water management is less advanced in Sub-Saharan Africa than in any other developing region, including Africa north of the Sahara where water management is more developed. In Sub-Saharan Africa, almost half of the irrigated area is concentrated in two countries only: the Sudan and Madagascar. In the rest of the region, irrigated land is only about 2 per cent of the cultivated area. This compares with 8.5 per cent in Latin America and 29 per cent in Asia (Harrison 1987).

Most of the large-scale irrigation schemes and soil conservation work tried out in Sub-Saharan Africa in the past have met with little success. As Critchley (1991) points out, concern about soil conservation is nothing new in Africa. Several colonial administrations recognized from the early part of the twentieth century that there was an erosion problem. Programmes of one sort or another continued in most countries until independence. But the majority of these schemes were resented by the local people, who were forced to supply labour.

However, the cycles of drought that have affected Sub-Saharan Africa in recent decades have drawn renewed attention to the role of soil and water management in ensuring crop production in semi-arid lands. The need for conservation programmes is much greater now, because of population increase, than when the first unpopular programmes were started. However, new approaches need to be developed to avoid the many mistakes of the past.

The way ahead: How to make every drop count

At least two major lessons can be learnt from past failures (Critchley 1991): first, for subsistence farmers, the idea of preventing future loss of soil is irrelevant to present pressing needs; and, secondly, the farmers themselves have, in the past, simply not been consulted about their knowledge and understanding of the processes of erosion. Both traditional technologies and social organizations have usually been ignored.

The way ahead calls for alternative strategies to large-scale conventional irrigation schemes or soil and water conservation projects. One such alternative being implemented with success in the Sahel is a low-cost water-harvesting technique adapted by incorporating a traditional practice, in Burkina Faso, of placing lines of stones to slow down runoff. But slopes in the Yatenga region where the project was developed are too gentle (between 0.5 and 2 per cent) and levels are impossible to get right by eye, although runoff erosion is quite severe. In response to this constraint, the Oxfam project devised an inexpensive (and rather ingenious) tool: the water-tube level, which accurately identifies the contour lines. Originally developed in the arid Negev desert 3,000-4,000 years ago, the water-harvesting technique is now being tried out with various adaptations in many parts of the Sahel, and is now widely adopted by farmers in the Yatenga region in Burkina Faso (Wright and Bonkoungou 1985; Younger and Bonkoungou 1989; Critchley 1991). The success of the technique has exceeded expectations.

In the course of four to five years, the Oxfam project perfected a traditional technique into a now highly valued water-harvesting technology that successfully collects water and holds it on the field, preventing soil erosion and increasing water infiltration and crop yields. Thousands of farmers now use the technique, and the number is growing rapidly as others observe its success. Scientists are now studying the possibility of using agro-forestry technologies to improve the technique even further by planting suitable trees and shrubs along the contour lines.

Supplementary irrigation and rain harvesting are a useful way to increase the water available to crops. However, Cooper and Gregory (1987) indicate that this is only one of many ways to make every drop count. They point out that in rain-fed farming systems, where lack of moisture limits crop production, agronomists frequently assess innovative management practices in terms of water use efficiency, which is the ratio of dry matter produced to water used for the production of that dry matter. This is expressed in units of kg/ha/mm and can be increased either by increasing total water supply, as in the case of the water-harvesting technique described above, but also by increasing transpiration efficiency or by reducing evaporation from the soil surface, e.g. through mulching. In terms of water conservation, the principal effect of mulching is to reduce soil evaporation. This is often complemented by many other beneficial effects. In the acid sandy soils (Psammentic Paleustalfs) of the Sahelian zone in West Africa, Kretzschmar et al. (1991) report that crop residues play a key role in increasing pearl millet (Pennisetum glaucum L.) yield, a beneficial effect likely due to improvement of P nutrition through both an increase in P mobility in the soil and enhancement of root growth. Although alternative uses of crop residues for fuel and animal feed limit their availability as mulch, the development of agroforestry techniques could make leaves and small branches of multipurpose trees available for mulching, a traditional practice known to small farmers in the rain-fed agriculture region of the West African Sahel.

The success of the water-harvesting technique demonstrates that a project that proceeds with a low budget and pursues a low-technology method can yield significant returns. Of course, this does not imply that all agricultural research should follow such a strategy - many high-technology projects have also yielded high rates of return - but it does suggest that a place exists for simpler technological changes, and that planners should not forget them. Small-scale measures on a large scale could be a viable alternative to the large-scale projects that so conspicuously failed in the past.