ABSTRACT
There is no single agreed definition of the term drylands. Two of the most widely accepted definitions are those of FAO and the United Nations Convention to Combat Desertification (UNCCD, 2000). FAO has defined drylands as those areas with a length of growing period (LGP) of 1–179 days (FAO, 2000a); this includes regions classified climatically as arid (Plate 1), semi-arid and dry subhumid. The UNCCD classification employs a ratio of annual precipitation to potential evapotranspiration (P/PET). This value indicates the maximum quantity of water capable of being lost, as water vapour, in a given climate, by a continuous stretch of vegetation covering the whole ground and well supplied with water. Thus, it includes evaporation from the soil and transpiration from the vegetation from a specific region in a given time interval (WMO, 1990). Under the UNCCD classification, drylands are characterized by a P/PET of between 0.05 and 0.65. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781849773744/65c2c63a-5adc-4ca3-89cb-094580b38b8d/content/figu_1_C.jpg" xmlns:xlink="https://www.w3.org/1999/xlink"/> A view of dryland (with village in the background) in the Sahel, southern Niger (P. Cenini) https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781849773744/65c2c63a-5adc-4ca3-89cb-094580b38b8d/content/plate_1_C.jpg" xmlns:xlink="https://www.w3.org/1999/xlink"/>
