The lower rainfall in semi arid areas compared with that in humid climates does not mean a corresponding low level of soil erosion by water. Indeed rainfall erosion can be higher in semi arid areas than in any other climatic zone. This is partly because the rainfall of semi arid areas has a high proportion of convective thunderstorm rain of high intensity and high erosive power. It is also because there is poor protective vegetative cover, especially at the beginning of the rainy season. Some of the soils common in semi arid areas are particularly vulnerable, either because they have poor resistance to erosion high erodibility , or because of their chemical and physical properties. An example from mexico is illustrated in plate 4.1 for example, alfisols suffer a particularly high loss of productivity per unit loss of soil stocking and peake 1985.

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Gully erosion can be severe in semi arid climates and the benefit/cost of gully control needs to be considered. Successful but expensive gully conservation like the australian example shown in plate 4.2 might not be suitable for third world countries. There are always strong links between measures for soil conservation and measures for water conservation, and this applies equally in semi arid areas. Many measures are directed primarily to one or the other, but most contain an element of both. Reduction of surface run off by structures or by changes in land management will also help to reduce erosion. Similarly, reducing erosion will usually involve preventing splash erosion, or formation of crusts, or breakdown of structure, all of which will increase infiltration, and so help the water conservation.

The approach by soil conservationists in the 1980s is moving away from using mechanical works and structures in soil conservation programmes paid for by a government or a donor funded project. An example is the increasing awareness of the ineffectiveness of terracing programmes alone. Also, we are moving towards the view that the only effective programmes are those which have the full support of the people. The subsistence farmer cannot afford to respond to philosophical or emotional appeals to care for the soil, and this means that conservation measures must have visible short term benefits to the farmer.

For the subsistence farmer the benefit he would most appreciate might be increased yields per unit of land, or perhaps better production per unit of labour, or perhaps improved reliabi lity of yield. The idea of working together in groups on tasks which require a big labour force is well established in many countries, particularly for planting or harvesting. On a fertile soil with good rainfall it may be sensible to invest a lot of labour or money in sophisticated schemes for controlling the run off, but not in semi arid areas with low and unreliable yields. It follows that attempts to eliminate soil erosion completely may be unrealistic, and that some level of erosion may have to be accepted, and also some risk' of soil conservation measures failing. An example of a realistic approach to the risk of failure are the flood diversion dams built in the people's democratic republic of yemen for spate irrigation schemes. Each end of the diversion is built of stone, or nowadays concrete, with a simple earth centre section. It is accepted that the earth section will be destroyed by big floods but it is cheap to repair or replace thomas 1982.

To upgrade the design and construction so that they could withstand the 25 year flood would increase the construction effort beyond what the farmers can provide. This same approach should be applied to all mechanical conservation programmes in semi arid areas. Many conservation programmes have failed because the technology was inappropraite, or misapplied, or because they did not take account of the social situation and did not involve the people. Heusch 1985 concludes that the large conservation programmes in algeria, morocco and tunisia, from 1950 to 1975, were based on inappropriate technology imported from the totally different conditions of the united states, and the whole effort was a mistake which should not be repeated. This umbrella term can include reduced tillage, minimum tillage, no till, direct drill, mulch tillage, stubble mulch farming, trash farming, strip tillage, plough plant for details see mannering and fenster 1983. In countries with advanced soil conservation programmes, particularly the usa and australia, the concept of conservation tillage is the main theme of the recommendations for cropland, and it is also being taken up quickly in other areas, for example southern brazil.

The application is mainly in mechanized high production farming with good rainfall, or for the control of wind erosion where there is large scale mechanized cereal production. It is less applicable to low input level crop production, or subsistence agriculture. The principles are equally effective in any conditions to maximize cover by returning crop residues and not inverting the top soil, and by using a high crop density of vigorous crops. Conservation tillage also has the advantage of reducing the need for terraces or other permanent struc tures. However there are several disadvantages which hinder the application of conservation tillage in semi arid conditions: · dense plant covers may be incompatible with the well tested strategy of using low plant populations to suit low moisture availability · crop residues may be of value as feed for livestock · planting through surface mulches is not easy for ox drawn planters although there may be no problem with hand jab planters.

One of the reasons for low yields in semi arid areas is the limited amount of moisture available to crop roots. The available moisture will be increased if the rooting depth is increased and it has been shown that in some cases deep tillage can help, for example on the dense sandy soils luvisols in botswana willcocks 1984. Reviewing many studies of experi ments of depth of tillage on alfisols, el swaify finds varied results deep tillage is beneficial for some crops but not all, and on some soils but not all. Also deep tillage requires greater draught power which is usually in short supply in semi arid areas. Ripping or subsoiling can be beneficial, either to increase the porosity of the soil, or to break a pan which is reducing permeability. The deep placement of fertilizer can also be used to encourage more rooting at depth, but again the application of this technique to subsistence farming will be difficult. It includes any farming practice which improves yield, or reliability, or decreases the inputs of labour or fertilizer, or anything else leading towards improved land husbandry, which we have defined as the foundation of good soil conservation.

Sometimes there is a long history of traditional farming and soil conservation practices which have been tested and developed over periods of time which are long enough to include all the likely variations of climate. These traditional practices should give the best long term result, bearing in mind that the farmer's interpretation of 'best' may be based on reliability rather than maximum yield. But the semi arid areas are changing rapidly, and the traditional patterns may be no longer relevant. As jones 1985 says while tradition may incorporate the wisdom of centuries of practical experience, it may also be inappropriate where recent demographic pressures have already compelled changes for instance, the abandonment of bush fallowing or migration onto different types of soil or into more arid areas. There is also the point that the agricultural scientist very often still lacks the recipe for certain success and you cannot require farmers to adopt new practices that are only 50 percent successful. Possible new techniques should have the same basic characteristics as traditional practices, they should be easy to understand, simple to apply, have low inputs of labour or cash, and must show a high success rate i.e. Some of the techniques are: farming on a rade is well established in india swaminathan 1982.cultivations and planting are done on a gentle gradient, sometimes together with graded channel terraces.

Sometimes this may be combined with simple practices to encourage infiltration such as returning crop residues. This seldom provides a complete solution because of the problem of disposal of the surface run off when it does occur. Strip cropping is most useful on gentle slopes, where it may reduce erosion to acceptable levels without any banks or drains. The object may be to improve fertility by the use of legumes or to help control pest or disease. In the semi arid parts of australia a successful practice is to alternate a cereal crop with a free seeding self regenerating annual forage legume such as subterranean clover or medicago. Fallowing is well established and successful in some circumstances but not others. In the drier wheat lands of australia, a bare fallow in summer is used to build up soil moisture before sowing the winter wheat which receives only barely adequate rainfall.

There is a risk of erosion taking place during the summer when high intensity summer thunderstorms fall on the bare soil walker 1982. In east africa, using this method on sloping land has a high risk of erosion pereira et al. 1958 , but on gentler slopes in botswana good results were reported by whiteman 1975.