Drylands are defined by a scarcity of water. Drylands are zones where precipitation is balanced by evaporation from surfaces and transpiration by plants (evapotranspiration). The United Nations Environment Program defines drylands as tropical and temperate areas with an aridity index of less than 0.65. The drylands can be classified into four sub-types: dry sub-humid lands, semi-arid lands, arid lands, and hyper-arid lands. Some authorities consider Hyper-arid lands as deserts (United Nations Convention to Combat Desertification) 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.
Drylands are complex, evolving structures whose characteristics and dynamic properties depend on many interrelated links between climate, soil, and vegetation.
The livelihoods of millions of people in developing countries depend highly on dryland biodiversity to ensure their food security and their well-being. Drylands, unlike more humid biomes, rely mostly on above ground water runoff for redistribution of water, and almost all their water redistribution occurs on the surface Dryland inhabitants' lifestyle provides global environmental benefits which contribute to halt climate change, such as carbon sequestration and species conservation. Dryland biodiversity is equally of central importance as to ensuring sustainable development, along with providing significant global economic values through the provision of ecosystem services and biodiversity products. The UN Conference on Sustainable Development Rio+20, held in Brazil in June 2012, stressed the intrinsic value of biological diversity and recognized the severity of global biodiversity loss and degradation of ecosystems.
- 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.
- 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.
- Puigdefabregas, J (October 1999). "Scales and processes of water and sediment redistribution in drylands: results from the Rambla Honda field site in Southeast Spain". Earth-Science Reviews. 48: 39. doi:10.1016/S0012-8252(99)00046-X.
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