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Classifications of Arid Land Soils 1The definition of aridity used in soil and other classification systems varies depending on the land's intended use and type of management. Climate sufficiently defines aridity for agriculture because irrigation can supplement the water needs of a crop. Type of vegetation better defines aridity for other forms of land management (e.g., forest, range, and wildlife management) that must use more adaptive strategies than irrigation to assure sustainable production. Climatic classifications have evolved at least from the time of early Greek civilization. Early climatic classifications were based on vegetation, which is more apparent than climate. The division between climate and vegetation classifications became more pronounced with the development of climatology and physics as scientific disciplines. Geographers and botanists continue to refine vegetation classifications. These classifications are geographically based using local characteristics of flora, ecology, or physiognomy such as rainforest, grassland, savanna, and desert scrub. Vegetation based classifications are especially useful for identifying aridic environments when soil characteristics override climatic conditions such as in glades and coastal beaches, or when extreme climatic variability is not captured by climatic classifications. Climate classifications range from simple annual precipitation classes to those that incorporate precipitation and its variability, length of humid and growing seasons, precipitation-temperature relationships, evaporation, and precipitation-evaporation relationships. For example, Le Houérou and Popov (1981) classify the soils of intertropical Africa based on average annual rainfall where "Desert" is less than 100 mm/yr, "Very Arid" is between 100 and 200, "Arid" is between 200 and 400, "Semiarid" is between 400 and 600, and "Dry Subhumid" is between 600 and 800. At the other extreme, UNESCO (1979) Man and the Biosphere Program developed a method based on precipitation divided by Penman (1948) evaportranspiration. The UNESCO method produces four classes, "Hyperarid," "Arid," "Semiarid," and "Subhumid." These classes are used in the Global Humidity Zones map and are explained further in its legend. Soil water availability is another method by which to classify arid lands and is use in the USDA (1975) and other soil classification systems. This method is a bridge between climatic and vegetation based systems but is not valid in very cold climates and can be influenced by human activities, such as changes in flooding regime cause by dam construction and management. Footnotes1. Much of this information was summarized from McGinnies, 1988. ReferencesLe Houérou, H.N. and G.F. Popov (1981). An ecological classification of intertropical Africa. Rome: Food and Agriculture Organization of the United Nations. 40 p. McGinnies, W.G. (1988). Climatic and biological classifications of arid lands: A comparison. pp. 61-68. In Arid lands today and tomorrow. E.E. Whitehead, C.F. Hutchinson, B.N. Timmermann, and R.G. Varady (eds.). Boulder, Colo: Westview Press. 1435 p. Penman, M.L. (1948). Natural evaporation from open water, bare soil, and grass. Royal Society of London, Proceedings Series A 193:120-145. Soil Survey Staff (1975). Soil taxonomy. Agriculture Handbook; no. 436. U.S. Department of Agriculture, Soil Conservation Service. 754 p. UNESCO (1979). Map of the world distribution of arid regions.
Accompanied by explanatory note. MAB Technical Notes; no. 7. Paris: UNESCO.
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URL:http://cals.arizona.edu/OALS/soils/classifarid.html
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