This Bulletin does not offer easy solutions to all the problems of soil and water conservation in semi-arid regions. There is no storehouse of tested methods and techniques ready to be taken off the shelf for immediate application. The conditions vary too much - the climate, the soil, and the social and economic factors. Instead, this Bulletin reviews methods and techniques which have been tested and found useful somewhere, and which might be suitable for use in other conditions.
We have not attempted to define semi-arid areas, and some of the examples come from arid or sub-humid regions. The objective is to make the bulletin relevant anywhere that rainfall is a problem because of amount, distribution, or unreliability. We have omitted the questions of salinity and alkalinity, wind erosion, and mechanization. Neither is there a discussion of the political, social, and economic issues, although in many cases these are as important as the technical problems.
The first three chapters are introductory, and outline the scale and importance of the problem, the difficulties and the possibilities for improvement. The bulletin argues strongly, and presents evidence, that drought is part of the natural order in semi-arid areas, and that the recent disasters of degradation and famine in Africa result from misuse and mis-management of the natural resources which reduced the region's ability to cope with the additional stress of drought.
Chapter 2 starts with a review of the extent of the erosion problem and the pressures on semi-arid ecosystems resulting from increasing human and livestock populations. There are problems which could be overcome, such as the shortage of information from agricultural research, which has in the past been mainly directed towards areas of higher agricultural potential. This has resulted in a lack of technology appropriate to the needs of sub- sistence farmers and nomadic pastoralists. The unchangeable problems are also discussed, particularly the variability of rainfall. The wide diver- sity of soils brings problems of soil chemistry and water storage.
Looking at the possibilities for improving agriculture in Chapter 3, a case is made for wider adoption in semi-arid regions of the existing techniques of resource inventory which would allow developing countries to make long-term plans for the optimum development of their resources, maximizing development of better land and minimizing stress on the marginal areas. To help the adoption of improved farming methods, Farming Systems Research is required to provide a better understanding of the motivation of subsistence farmers, semi-nomadic cultivators and pastoralists. The slow rate of adoption of promising new ideas is associated with failure to understand the farmers' thinking.
The first of the four technical chapters deals with soil conserva- tion under the headings Principles, Biological Soil Conservation, and Mechancial Conservation Works. It is argued that in semi-arid regions, with inevitable low production, much of the conventional approach is inappropriate because it has been developed in very different conditions. Expensive works using machinery are seldom justified, nor are the conser- vation tillage methods which are the backbone of biological soil conser- vation in developed countries with a more favourable climate. The requirements in semi-arid areas are that biological conservation through improved farming should show short-term benefit to the farmer, and that this need not necessarily be in terms of increased yield, for he or she may be more concerned with increasing the reliability of yield, or decreasing the labour requirement. Similarly, the emphasis should be on mechanical works which are simple, low-cost, and can be laid out and constructed and maintained with only small inputs of technical advice.
Chapter 5, called Water Harvesting, deals with methods to increase the amount of moisture stored in the soil profile or where there is some small movement as surplus run-off. The choice of methods of water conservation is difficult because the desired objective may change from one season to another. In a dry area it may be sensible to increase surface storage to improve crop yield in most years, but in a wet year this could cause waterlogging and reduce the yield. On the other hand, a drainage system may have the objective of increasing run-off from areas usually too wet, but also have the undesired effect of exaggerating the effect of a drought. The principle of many methods is to make use of marginally inadequate rainfall by concentrating run-off from an uncultivated part of the land onto a cultivated part which then receives enough moisture to grow a crop. The general term for methods of this type is "rainfall multiplier". Examples are conservation bench terraces, developed and widely used in the United States of America, and a number of simpler variations known as contour furrows, microcatchments, or contour bunds. The practice of ridging and tied ridging is discussed in some detail because there is a large amount of experience of this method in many countries, with varying degrees of success. The broad bed and furrow method is being increasingly adopted on the heavy black clays of India, closely associated with improved animal-drawn equipment which brings the method within the reach of peasant farmers. Other methods of localized surface storage are the semi-circular catchments and trapezoidal catchpits becoming popular in Burkina Faso and Kenya, which are variations on traditional methods of surface pitting.
Under the section Water Spreading there is a discussion of various forms of localized use of surface run-off, either occurring naturally from hill areas or after it has been diverted and collected in channels. Several ancient examples of this type of run-off farming are described from the American Indians, Tunisia, and Israel. There are also more recent examples from Kenya, Pakistan, and the Yemen Arab Republic. Inundation methods are where floodwaters are impounded and retained long enough to saturate the soil so that a crop can be grown on the stored moisture. These range from small-scale examples from the Sudan to Brazil to the much larger schemes in semi-arid areas of India, some of which have been used for hundreds of years. The diversion and spreading of floods and spate flows without storage are discussed with examples from Pakistan, Yemen Arab Republic, and several countries in Africa. Sub-surface drainage is felt to have limited application in semi-arid regions, but several examples of simple surface drainage systems are described.
Methods where surface run-off is collected and stored in dams, tanks, or cisterns for later use are described in Chapter 6 on Water Harvesting and Use. The amount of run-off is critical to such schemes, so there is a discussion of ways of treating the catchment to improve run-off. This includes simple methods such as removing stones and vegetation, shaping the catchments as with the roaded catchments and flat-batter dams of Western Australia, and chemical treatments such as deflocculants and waxes, and going on to sealing the surface with waterproof membranes. Practical guidance is given on the design and construction of small earth dams and weirs. Sand dams are discussed, where water is stored in the pore spaces of sand retained by a weir, a method which may be particularly useful in hot climates because surface evaporation losses are reduced. Off-stream storage is described, including the traditional 'hafirs' of Arabic-speaking northern Africa, the tanks of India, and the small reservoirs used in Western Australia for stock watering. There is a discussion of methods for reducing seepage losses through the floor of storage basins, and evaporation losses from the surface, although the available solutions to both these problems are frequently too expensive for general use. This chapter ends with a discussion on groundwater, covering methods of recharge and extraction from wells, boreholes, and horizontal wells, including the ancient qanats of north Africa and western Asia, and modern drilled horizontal wells in the western USA.
Chapter 7 discusses applications of water conservation to grazing land and to trees and shrubs. The problem of overstocking and overgrazing was briefly mentioned earlier among the difficulties and problems in semi-arid areas, and is developed here in more detail. The evidence of satellite imagery proves conclusively that the degradation of the Sahel and of Namibia was man-made and caused by overgrazing, and not the result of drought. Examples are shown from Niger in the Frontispiece and from Namibia in this chapter. There is no doubt that national herds of livestock are increasing and numerical examples are quoted from Swaziland, and from the Mambilla Plateau on the border of Nigeria with Cameroon. The decreas- ing mobility of nomadic or semi-nomadic pastoralists is discussed, quoting the example of Botswana where the two main causes are settlement, and the provision of permanent water supplies at the cattle posts by replacing the previous natural water supply with boreholes. Two examples of successful group livestock management schemes are mentioned, from Niger and from eastern Senegal. Examples of good management are taken from the western USA and compared to the traditional management of the grazing by linking it to water supplies in the Butana region of the Sudan. Techniques for improving the grazing by reducing surface run-off are discussed with examples of large-scale mechanized schemes of creating furrows or pits. Small scale methods more suitable for Africa are semi-circular hoops, low banks of stone set out on the contour, and small basins formed by digging with hand hoes.
In semi-arid areas the landscape is always a mixture of woodland, bush, grazing land and arable land, and trees or shrubs are always a part of the ecosystem. They supply many needs; fuel, browse, construction materials, food crops, and cash crops, and usually a combination of more than one of these. Depletion of the natural stock of trees and shrubs is one of the indicators of excessive pressure on the land leading to an increasing rate of degradation. Ethiopia is the model for a study of the progressive phases of degradation starting with depletion of the tree growth, and ending in a massive exodus. In many countries the increasing scarcity of fuelwood is becoming as critical as the scarcity of food, and in this context, the need to expand the production of trees and shrubs is clearly a major challenge in the semi-arid areas.
The usual techniques for afforesation are seldom appropriate in semi-arid areas because of the unreliability of the rainfall. The methods suggested are variations of the rainfall multiplier approach discussed earlier for cropland. They range from a small basin in which a tree seedling is planted, perhaps with small channels to lead extra water to the basin, up to the 250 m2 microcatchments developed in the Negev desert in Israel. Other variations of this principle are quoted from several countries in Africa. Agroforestry is considered outside the scope of this Bulletin, but a list of references is provided.