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Drylands are typically characterized by scarcity of water – due to low and irregular rainfall, high temperatures, and high rates of evapo-transpiration – and as a result poor quality of soil. Annual rainfall is very limited and rainfall patterns are also very irregular and unpredictable. Soils tend to be fragile, characterised by significant leaching of nutrients, intensive weathering of minerals, and low natural fertility.

The United Nations Convention to Combat Desertification (UNCCD) - based on the Aridity Index – classifies arid, semi-arid and dry sub-humid areas as drylands. By definition, drylands encompass “areas, other than polar and sub-polar regions, in which the ratio of annual precipitation to potential evapo-transpiration [the aridity index] falls within the range from 0.05 to 0.65.”.

The Convention on Biological Diversity (CBD) and the Millennium Ecosystem Assessment (MEA, 2005) also include hyper-arid zones in their definition of drylands. Following the latter classification, arid lands cover approximately 41 per cent of the earth’s land surface (more than 6 billion hectares) and are inhabited by more than 2 billion people (approximately one third of the world’s population). It is estimated that 18 percent of these drylands are covered by forests and other wooded lands.

Where are drylands found?

As shown in the map below, drylands are widespread all across the globe, including the Mediterranean, Sahara and Sub-Saharan Africa, Central and South Asia, Australian Outback, South American Patagonia and North American Great Plains.  


The world’s drylands and subtypes. Prepared using spatial data from UNEP-WCMC (2007).

last updated:  Thursday, April 2, 2015