T he FA O have predicted that by the year 2000 approximately 84 per cent of the world’s total farmland will still rely on rainfall and will yield just over half of all crops (Biswas et al. 1983). (A recent overview of the extent of rainfed cultivation and the potential for expansion was provided by Higgins et al. 1984.) In most developing countries irrigation projects are costing more and more each year. M exico, for example, made great advances with irrigation in the 1950s and 1960s, bu t since the early 1970s due to rising costs has increasingly turned to rainfed farming. One project, the Plan Puebla, a rainfed farming development scheme (begun in 1967), has shown that new crop varieties and cultivation m ethods can quadruple yields of maize and allow diversification into highervalue crops at lower start-up and operating costs than irrigation.