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]
- ^ Middleton and Thomas, 1997. "The World Atlas of Desertification Millennium Ecosystem Assessment (2005a). Climate Change". Chapter 13 in: Ecosystems and Human Wellbeing: Current State and Trends, Volume 1. Island Press.
- ^ a b "Millennium Ecosystem Assessment, 2005. Drylands Systems". Chapter 22 in: Ecosystems and Human Wellbeing: Current State and Trends, Volume 1. Island Press.
- ^ Rodríguez-Iturbe, I. and A. Porporato 2004. Ecohydrology of Water-Controlled Ecosystems: Soil Moisture and Plant Dynamics. Cambridge University Press.