Understanding global desertification: Biophysical and socioeconomic dimensions of hydrology

Drylands are regions of the globe where the index of aridity (IA)-defined as the ratio of mean annual precipitation (P) to mean annual potential evapotranspiration (PET)-is less than 0.65 (see Chapters 1 and 8). If we restrict IA to the range of 0.05 to 0.65, drylands consist of arid, semiarid, and dry sub-humid regions, which together cover approximately 5.2 billion hectares or 40% of the land area of the world (Table 1). This definition excludes hyper-arid regions of the globe where IA < 0.05, such as the Atacama, Arabian, and Sahara deserts (ca. 0.98 billion hectares or 7.5% of global land area). Based on human land use, ca. 88% of drylands are classified as rangeland, with the remaining 12% used in agricultural production (3% irrigated cropland, 9% rainfed; Table 1). Combined, Asia and Africa contain 64% of all global drylands, dwarfing the amount of dryland area on other continents. In terms of importance, however, these numbers can be somewhat misleading. While Europe contains only ca. 5% of the world's drylands, this represents over 32% of its landmass and is home to 25% of its population. Similarly, Australia contains about 10% of the world's drylands but they cover over 75% of the continent and are home to 25% of its population.