Drylands

Drylands are defined by a scarcity of water. Drylands are zones where precipitation is balanced by evaporation from surfaces and by transpiration by plants (evapotranspiration).^[1] The United Nations Environment Program defines drylands as tropical and temperate areas with an aridity index of less than 0.65.^[2] One can classify drylands into four sub-types:

• Dry sub-humid lands
• Semi-arid lands
• Arid lands
• Hyper-arid lands

Some authorities regard hyper-arid lands as deserts (United Nations Convention to Combat Desertification - UNCCD) although a number of the world's deserts include both hyper-arid and arid climate zones. The UNCCD excludes hyper-arid zones from its definition of drylands.

Drylands cover 41.3% of the Earth's land surface, including 15% of Latin America, 66% of Africa, 40% of Asia, and 24% of Europe. There is a significantly greater proportion of drylands in developing countries (72%), and the proportion increases with aridity: almost 100% of all hyper-arid lands are in the developing world. Nevertheless, the United States, Australia, and several countries in Southern Europe also contain significant dryland areas.^[2]

Drylands are complex, evolving structures whose characteristics and dynamic properties depend on many interrelated interactions between climate, soil, and vegetation.^[3]