Arid lands are characterized by low rainfall and high evapotranspiration. Depending on definition, they are also referred to as drylands, or alternatively, as one category within drylands. For this purpose, arid lands are described in their broader definition as drylands. Drylands encompass arid, semi-arid, and sub-humid zones where average annual rainfall is lower than total evapotranspiration (classified using the aridity index). Drylands generally exclude true deserts, such as the Sahara, which are considered hyper-arid and are of low productive potential and consequently low population density. Drylands cover 41 percent of the earth's land surface and support more than one-third of its population. Every continent contains drylands that often cover extensive areas. However, they are most extensive in Africa and Asia. Australia, the United States, Russia, ...

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