Desert Zones

geographic areas in which deserts are the predominant natural landscape. Desert zones occur widely in the temperate belt of the northern hemisphere and in the subtropical and tropical belts of both the northern and southern hemispheres. Their common characteristic is aridity. The total annual precipitation is less than 200 mm and in extremely arid regions, less than 50 mm. The moisture coefficient, which reflects the ratio of precipitation to rate of evaporation, varies from 0 to 0.15.

The topography combines uplands, low isolated hills, and inselbergs with structural stratified plains, ancient river valleys, and enclosed lake basins. Relief formation by erosion is weak, and colian landforms are common. Most desert terrain lacks drainage, although it is sometimes intersected by through-flowing rivers, such as the Syr Darya, Amu Darya, Nile, and Huang Ho. There are many dry lakes and lakes whose shape and size change frequently (Lop Nor, Chad, Eyre). Intermittent streams are characteristic, and the groundwater is often saline. The soils are poorly developed, with soluble salts predominating over organic matter in the soil solution. Salt crusts are common.

The vegetation is sparse, usually covering less than half of the soil surface. Under extremely arid conditions it is almost nonexistent. Xerophilic and halophilic communities of Haloxylon, ephemerals, and Artemisia are widely found in Asian deserts. Cactus and creosote bush associations prevail in North American deserts. Although the animal world is fairly diverse, the forms of adaptation to desert conditions are similar. Rodents and reptiles predominate in all deserts. Pasture livestock raising is well developed in desert zones, but agriculture is possible only with irrigation.