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Conventional erosion control generally recommends the application of erosion control structures and cropping techniques that have shown an ability to retain water and slow down erosion in some other place and under some other circumstances, so it is hardly surprising that most projects involving erosion control have met with failure over the past 50 years (Hudson 1990).
Here the effectiveness of erosion control practices and structures for surface water management, as well as the problems raised by certain production systems are briefly examined and analysed, and then an attempt is made to show how Wischmeier's empirical model can be put to practical use (Roose 1977a, 1987b and 1992b).
At the beginning of this work it was shown how governments have been led to approach erosion control from the off-site viewpoint, or that of the general interest of their citizens, focusing on protection of the quality of water (rural development strategies). Here, however, these problems are considered from the on-site viewpoint, in an effort to solve the farmer's problems of land degradation. Agricultural development strategies are discussed.
CAPITAL DEVELOPMENT STRATEGIES
In this perspective, the watershed is the natural physiographical unit for management, particularly erosion control, and the following procedure is adopted.
• First of all, a map of land potential is drawn up.
The US Department of Agriculture (USDA) has defined eight categories of land on the basis of the constraints to large-scale farming.
Classes 1 and 2 are slightly sloping (from 0 to 2%) and are fairly well drained. Such land is suitable for almost all crops without any special intervention, apart from drainage.
Classes 3 to 6 cover arable slopes: the limitations on cropping increase in accordance with how shallow the soil is, to what extent pebbles and stones prevent mechanization, and how steep the slope is.
Classes 7 and 8 must be kept under permanent plant cover, protective forests or extensive grassland. Cropping is not permitted.
However, land potential must be classified in each individual case on the basis of local climate, landforms and soil. For example, in the semi-arid Sudano-Sahelian conditions of the Mossi Plateau, distinctions are traditionally made between (i) the steeply sloping ironstone or gravel top of the toposequence, which is reserved for animal husbandry and extensive rangelands, (ii) the broad, gently sloping pediment, which consists, first, of a shallow sandy area, limited in use, and, secondly, a clay-loam area on the lower part of the slope, which is where cropping will be concentrated, and (iii) the soil at the bottom of the slope, which is always to some extent hydromorphic.
Once the map of soil potential has been drawn up, a map of present erosion risks and another of land use are needed. When these 1:10000 scale maps have been compared, a map of intervention and rural infrastructure for watershed management can be drawn up.
• The permanent structures for this scheme are then decided.
First comes the road network, the drainage network and the bridges to enable people to move around the catchment, particularly when transporting harvests. At this point an erosion control system can be decided, for example:
• gradual terraces defined by strips tilled downhill, based on grass buffer strips or banks;
• a system of beds developed by ICRISAT for vertisols on slopes of less than 2% (Pathak, Miranda and El-Swaïfy 1985);
• a system of level benches for areas where population pressure is strong;
• a system of stepped diversion terraces such as those found in North Africa, or a system of individual terraces for orchards.
• The last step is to plan a production system to take account of both environmental and human conditions (people's needs, economic viability of production, the local market and level of knowledge, and food self-sufficiency). Next come rotations and fertilizing and conditioning systems. Land to be set aside for forestry, used as pasture, and to be cropped must be identified, along with the areas to be irrigated or drained. A drainage system must then be planned, adequate outlets built, rivers and gullies stabilized, and the regional market and transport organized.
Within the framework of this strategy, the officially designated watershed manager will decide which areas are to be closed off and which are to be used as pasture for livestock, together with a whole set of conditions for production in the region.
AGRICULTURAL DEVELOPMENT STRATEGIES
In this approach to rural development, the expert will start by trying to meet the farmers' needs, a three-stage process.
• The first stage is to establish dialogue with the local people and gain their confidence. The expert will question the farmers to learn about the present land-use system and traditional production systems and to discover where, when and how environmental problems arise. He/she will then look for links between runoff and erosion problems and the local farming system, seeing how farmers perceive the problems and how they try to solve them. Lastly, he/she will work with them to find ways of increasing the infiltration of rainwater in order to increase biomass, yields, and the returns on the farmers' labour, as well as how to promote plant cover and thus reduce erosion risks. Once he/she has grasped the problems and observed the possible solutions already found by the farmers, he/she can propose more technical approaches for their consideration.
• The second stage entails field trials on the farmers' own land, where risks of runoff and erosion will be assessed for different types of rainfall. Various cropping techniques or erosion control structures will be compared, again on their land, so that the feasibility, effectiveness and economic viability of each element of a solution can be evaluated as precisely as possible. This stage will require between three and five years, ending with an evaluation of results by both farmers and technicians.
• The third stage entails a land-use plan no longer confined to individual farm units, but encompassing a slope, hillside or watershed, or the area occupied by a rural community. At this stage, the maps of potential land use, present damage and erosion risks, and current land use (a 1:10000 scale aerial map) will be compared, and schemes acceptable to the farmers will be selected for each functional land segment starting with the cropped area, then the upper slopes, and lastly the valley bottoms.
This kind of land-use planning demands a more sophisticated approach to social and economic aspects: the entire rural population must participate from the project design stage onwards and be involved in the various phases of survey, demonstration and trial, and extension in the field. Observance of traditional water and fertility management methods will allow the methods best suited to the local environmental conditions and the socio-economic context of the local population.
Land husbandry is based on rational management of water and nutrients. Nutrient management must link fertilizer (organic and mineral) with erosion control. Water management must enable the available water to be used in such a way as to maximize soil productivity.
If a diagonal line is drawn through West Africa from the Sahara down to the subequatorial zone, four methods of water management can be distinguished, depending on climate and soil permeability. And each of these methods entails specific cropping techniques and erosion control structures.