Large-scale irrigation schemes have been linked primarily to perennial surface water resources notably in Egypt, Nigeria, Mali, Mauritania and Senegal. However, since the 1960s, the rise of drilling and pumping technology has permitted the development of large groundwater-dependent schemes. They are found across all zones and include high-value cash and export cropping and intensive vegetable and fruit cropping. Patterns of water use vary greatly, but often it is not used efficiently; and there have been significant economic and environmental ramifications from excessive drawdown of nonrecharged aquifers, and from excessive irrigation that has led to rising groundwater tables with soil salinization and sodication problems.

Small-scale irrigated systems occur in many places across the region and, although they may not be important individually (in terms of numbers of people involved or in the amount of food and other crops produced), they are a significant element in the survival of people in dry areas. Such systems develop along small perennial streams and at oases, or are built where flood and spate irrigation is feasible, as well as around boreholes. The major crops are mixed cereals and vegetables. These locations (where water is available) always provide a focal point for socio-economic activity, but intense local competition for limited water resources between livestock owners and farmers is becoming increasingly evident. The hatching in Figure 3.1 denotes areas with substantial small-scale irrigation.

The irrigated farming system is thus quite complex. In many cases, irrigated cropping is combined with rainfed cropping or animal husbandry. It is also possible to distinguish between full and partial water control. Crop failure is generally not a problem, but livelihoods are vulnerable to water shortages, scheme breakdowns and deteriorating input/output price ratios. Major constraints include iron toxicity problems, scarcity and quality of water resources in dry regions and excessive water in humid zones.

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