Deserts of the World
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The diverse character of deserts has led to a plethora of definitions (see Thomas 1989). The most general definition is that deserts are areas of aridity, and they are usually defined by some measure of water shortage. This is commonly based on the relationship between water gained from precipitation and water lost by evaporation or transpiration (see Cooke et al. 1993). Five major climatic causes of aridity are recognized (Cooke et al. 1993). These are continentality (distance from marine or other water sources), dynamic anticyclonic subsidence, orographic influences, coastal upwelling of cold water associated with cold equatorward-flowing offshore currents and, finally, high reflectivity (albedo) of the desert surfaces themselves. In addition, the enormous variability of all desert characteristics (climate, substrate, vegetation, lithology etc.) makes it difficult to define the boundaries of arid areas and to divide them into smaller typological units. Consequently, the boundaries of arid areas and their typology are usually based on a relatively small number of environmental parameters. As a result, a number of different maps and classifications of deserts are available, some of which are based on climatic data (e.g. Meigs 1953), while others are based on pedological parameters (e.g. Dregne 1976), vegetation patterns (e.g. Shantz 1956), relief (e.g. Thomas 1989) and so on. One of the most widely used schemes (Meigs 1953) is based on Thornthwaite’s (1948) indices of moisture availability (Im): IM= (100S - 60D/PE , where PE is potential evapotranspiration, calculated from meteorological data, and S and D are, respectively, the moisture surplus and moisture deficit, aggregated on an annual basis from monthly data and taking stored soil moisture into account.
KeywordsSandy Desert Monte Desert Australian Desert Intermontane Depression Great Basin Desert
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