Drylands are characterized by a scarcity of water, which affects both natural and managed ecosystems and constrains the production of livestock as well as crops, wood, forage and other plants and affects the delivery of environmental services. For millennia, drylands have been shaped by a combination of low precipitation, droughts and heat waves, as well as human activities such as fire use, livestock grazing, the collection of wood and non-wood forest products (NWFPs), and soil cultivation. Dryland soils tend to be vulnerable to wind and water erosion, subject to intensive mineral weathering, and of low fertility (due to the low content of organic matter in the topsoil).

The United Nations Environment Programme (UNEP) defines drylands according to an aridity index (AI), which is the ratio between average annual precipitation and potential evapotranspiration; drylands are lands with an AI of less than 0.65. Drylands are further divided, on the basis of AI, into hyper-arid lands, arid lands, semi-arid lands and dry sub-humid lands.

Drylands are found in most of the world’s biomes and climatic zones and constitute 41 percent of the global land area.