The dynamic climatology of drylands

Surprisingly, given that the critical and unifying variable for dryland environments is a shortage of water on a seasonal or longer-term basis, there has been a long-standing difficulty in determining their geographical extent (Beaumont, 1989; Wallen, 1967), though it is generally estimated that hyper-arid, arid and semi-arid lands in total cover a third of the Earth’s land surface (UNEP, 1992; see Fig. 1.1). The absence of significant moisture is manifest in the characteristics of the soils, vegetation and topography. Consequently, Oliver (1973) and Nir (1974) have suggested ways of identifying arid lands by a variety of non-climatic criteria. Straightforward classical approaches create regionalizations using isopleths of climatic elements with respect to associations with vegetation and agricultural conditions, such as the 250 mm rainfall limit as the arid boundary (Oliver, 1981). In contrast, indexing methods delimit regions with differing levels of aridity by the application of objective standard formulas.