ABSTRACT
Think of arid lands and the mind focuses on the hot, dry desert wastes of the world (Figure 8.1). Dry countries are increasingly demanding worldwide attention, and, as Goudie (1990) notes, one of the biggest environmental issues of the last two decades has been the question of desertification. Dry areas subdivide into extremely or hyper-arid, arid and semi-arid zones. In the former, water is all but absent and the land generally devoid of human habitation. Numerous schemes have been proposed for quantifying the threefold division. Heathcote (1987) suggests that hyper-arid areas receive less than 25 mm of rainfall per year, arid areas between 25 mm/yr and 200 mm/yr and semi-arid regions between 200 mm/yr and 500 mm/yr. In comparison, Great Britain receives, on average, 1,000 mm of rainfall per year and tropical countries, such as parts of south-east Asia, 5,000 mm/yr.
