Aridity index

An aridity index is a numerical value to quantify the degree of climatic dryness at a given location and thus to define arid climatic zones .

description

The worldwide distribution of non-polar arid areas from 1961 to 1990 according to the UNEP climate index: 51 million km² (41% of the land surface), habitat for more than 1/3 of humanity. By the end of the 21st century, the area is expected to grow to 58 million km².

An aridity index can be used to track how the effects of climate change affect local water resources. Although there is agreement about the general location of the arid regions, opinions differ about the methods of determining their exact boundaries. There are differently defined aridity indices. The following are the most common:

Precipitation aridity index

The simplest index is based on rainfall. Those regions are defined as arid in which the mean annual rainfall is less than 250 mm. In semi-arid regions this is 250 to 500 mm. This definition was used by the Intergovernmental Panel on Climate Change .

UNEP aridity index

According to the recommendation of UNEP ( United Nations Environment Program ), the climatic dryness is defined by an aridity index AI, which is obtained by forming the quotient with the value of the annual precipitation to the value of the annual potential evapotranspiration . For the climatic zones of the arid regions, this dimensionless index is less than or equal to 0.65.

Thornthwaite moisture index

The Thornthwaite scheme for non-polar arid areas defines an index for humidity and one for aridity from which a dimensionless moisture index is calculated, weighted. One hundred times the excess water that results in the wet months from the positive difference between precipitation (in cm) minus potential evaporation (in cm) is converted into sixty times the water deficit that results in dry months from the positive difference in evaporation (in cm ) minus precipitation (in cm) is deducted. The intermediate result is divided by the water requirement, which results from the sum of all monthly values of potential evaporation (in cm).

Overview

Definition of the arid climatic zones using various aridity indices
designation	UNEP index	IPCC index [mm]	Thornthwaite Index
hyperarid	<0.05
arid	0.05-0.2	<250	−40 to −60
semi-arid	0.2-0.5	250-500	−20 to −40
subhumid	0.5-0.65

Individual evidence

↑ ^a ^b ^c ^d Maliva, Robert, and Thomas Missimer. "Aridity and drought." Arid lands water evaluation and management. Springer Berlin Heidelberg, 2012. 21–39. ( PDF )
^ ^A ^b NJ Middleton, DSG Thomas: World Atlas of Desertification: United Nations Environmental Program. Arnold, 1992.
↑ ^a ^b Fernando T. Maestre, Roberto Salguero-Gómez, José L. Quero: It is getting hotter in here: determining and projecting the impacts of global environmental change on drylands. In: Philosophical Transactions of the Royal Society of London B: Biological Sciences. 367.1606, 2012, pp. 3062-3075. (on-line)
^ ^A ^b S. Feng, Q. Fu: Expansion of global drylands under a warming climate. In: Atmos. Chem. Phys. 13, 2013, pp. 10081-10094. doi: 10.5194 / acp-13-10081-2013 . ( PDF ; 7 MB)

[Malvia_2012-1] Maliva, Robert, and Thomas Missimer. "Aridity and drought." Arid lands water evaluation and management. Springer Berlin Heidelberg, 2012. 21–39. ( PDF )

[Middleton_1992-2] A ^b NJ Middleton, DSG Thomas: World Atlas of Desertification: United Nations Environmental Program. Arnold, 1992.

[Maestre_2012-3] Fernando T. Maestre, Roberto Salguero-Gómez, José L. Quero: It is getting hotter in here: determining and projecting the impacts of global environmental change on drylands. In: Philosophical Transactions of the Royal Society of London B: Biological Sciences. 367.1606, 2012, pp. 3062-3075. (on-line)

[Feng_2013-4] A ^b S. Feng, Q. Fu: Expansion of global drylands under a warming climate. In: Atmos. Chem. Phys. 13, 2013, pp. 10081-10094. doi: 10.5194 / acp-13-10081-2013 . ( PDF ; 7 MB)