Background Definition and characteristics
Drylands are generally defined as lands with limited rainfall. Mainly their dryness is due to the negative balance between precipitation and evapotranspiration rates. Drylands are thus been defined in terms of water stress as areas where mean annual precipitation (P) is less than half of the potential evapotranspiration (PET). According to the FAO (1993), drylands are agroclimatic zones having short growing periods, which is defined as the period when both water and temperature permit crop growth. So drylands are zones falling between 1-74, 75-119 and 120180 growing days representing arid, semi-arid and dry sub humid lands, respectively.
Drylands are characterised by low (100-160 mm annually) erratic and highly inconstant and unreliable rainfall levels. Precipitation is low concentrated during short periods, resulting much of the rainfall to be lost in evaporation and the usual intensity of storms ensures that much of the rainfall runs off in floods. Fragile environments and unpredictable drought and floods are common features of drylands ecosystems.