According to Marsh and Kaufman (2013, p. 303) arid regions are characterized by negative moisture balances, i.e., potential evapotranspiration rates permanently exceed precipitation rates. Therefore, most water is immediately transferred back to the atmosphere after a precipitation event. In many arid areas, precipitation rates are so little (sometimes also virtually zero) that actual evapotranspiration rates also become virtually zero. This has very characteristic implications for vegetation, the formation and evolution of soils, and the surface energy balance. Depending on the exact definition of aridity between 17% (Dietz and Veldhuizen 2004) and 41% (UNEP-WCMC, 2007) of the world’s land mass are classified as arid areas.
Plant growth in arid regions is very limited and those plants and the plants that make a living there have adapted to extreme droughts and intensive solar radiation through various physiological adaptations. One example are succulents, which, unlike herbaceous or woody plants, have special storage organs for water in their leaves.
Soils in arid regions only have little soil water and most water movement within the soil (e.g., after a rare precipitation event) is directed upward. This leads to the formation of salt crusts on the soil surface when the rising capillary water evaporates there. This is particularly pronounced in desert areas such as the Sahara or the Atacama in South America.
The surface energy balance is mainly characterized by a very high amount of direct solar insolation since due to the lack of water vapour cloud cover is usually little. At the surface the incoming solar radiation is mostly converted into sensible heat fluxes as no or only little water is available to generate latent heat fluxes (this is the heat required to evaporate water and transform it into its gaseous stage and which is released when the water condenses again).
These conditions therefore also present special challenges for the people living there. Agricultural production is limited and is either only possible at certain times (such as during periodic rainy seasons), or is restricted to a few places with adequate water supply from wells, groundwater sources or rivers. The development of larger fossil groundwater resources - aquifers that were formed in more humid periods of geological history but cannot regenerate under recent conditions - has allowed agricultural production to expand into these areas. However, since these groundwater bodies can hardly reproduce themselves, this form of water use and agricultural production is hardly sustainable.
Figure 1 shows a global map of arid regions produced by the United Nations Environment Program (UNEP). For producing the map an aridity index (AI) was used that takes into account average annual precipitation rates and potential evapotranspiration rates (see http://www.fao.org/dryland-forestry/background/what-are-drylands/en/). Thresholding the AI allowed for classifying the land surface into four different aridity classes. The global desert areas such as the Sahara in North Africa, the Atacama in South America, the Arabian deserts, the inner-continental arid regions of Asia and North America as well as the Australian desert are clearly visible on the map.
Marsh, William, and Martin Kaufman. 2013. Physical Geography - Great Systems and Global Environments. Cambridge University Press.
Dietz, Ton, and Els Veldhuizen. 2004. „The World’s Drylands: A Classification“. In The Impact of Climate Change on Drylands, 19–26. https://doi.org/10.1007/1-4020-2158-5_2.
UNEP World Conservation Monitoring Centre (UNEP-WCMC). 2008. Annual Report 2007. Available online: https://www.unep-wcmc.org/system/dataset_file_fields/files/000/000/214/original/Annual_Report_2007.pdf?1400677478 (accessed latest on 21st November 2020)