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Semiarid Regions
Definition
The semiarid regions of the world are defined as transition zones between arid and sub humid belts. Semiarid regions are also defined as areas where precipitation is less than potential evaporation, characterized by high temperatures (30-45oC) in the hottest months.
Arid/Semiarid agricultural areas (WDR 2008)
Characteristics
Arid and semiarid areas account for one third of the earth's surface land area. ICRISAT (1998) estimates that semiarid areas, especially within the tropics, cover most parts of the developing nations in the world including Latin America, most parts of sub-Saharan Africa, large portion of Eastern and Southern Africa and parts of India and South East Asia.
Semiarid regions are characterized by low and restricted precipitation because of the inability of moisture bearing winds to penetrate into, and cool down within such regions. Also, zones of high pressure may prevent entry of the winds resulting into meteorological phenomenon associated with little or lack of rainfall in these areas.
Almost all precipitation in arid and semiarid regions occurs as rain, except in higher altitudes where dew and hoar frost can form due to great differences between day and night temperatures.
Semiarid lands of the world are also characterized by basins of closed drainage due to inability of the surface waters to maintain themselves against evaporation. Both transpiration and evaporation are high because abundant heat energy is supplied to change the limited amounts of liquid water into water vapor either directly or through biological processes thus maintaining the heat balance of the area.
Semiarid areas have been classified according to amount of rainfall received, duration of the wet period and the dominant vegetation. The wet period refers to a period during which rainfall is higher than evapotranspiration and is usually distributed over a 3-4 month period in a year. The vegetative cover is characterized mainly by shrubs, scrubs and grass.
