Aridity index

An aridity index (AI) is a numerical indicator of the degree of dryness of the climate at a given location. A number of aridity indices have been proposed (see below); these indicators serve to identify, locate or delimit regions that suffer from a deficit of available water, a condition that can severely affect the effective use of the land for such activities as agriculture or stock-farming.

Historical background and indices

At the turn of the 20th century, Wladimir Köppen and Rudolf Geiger developed the concept of a climate classification where arid regions were defined as those places where the annual rainfall accumulation (in centimetres) is less than R/2, where:

* R=2 imes T if rainfall occurs mainly in the cold season,
* R=2 imes T+14 if rainfall is evenly distributed throughout the year, and
* R=2 imes T+28 if rainfall occurs mainly in the hot season.

where T is the mean annual temperature in Celsius.

This was one of the first attempts at defining an aridity index, one that reflects the effects of the thermal regime and the amount and distribution of precipitation in determining the native vegetation possible in an area. It recognizes the significance of temperature in allowing colder places such as northern Canada to be seen as humid with the same level of precipitation as some tropical deserts because of lower levels of potential evapotranspiration in colder places. In the subtropics, the allowance for the distribution of rainfall between warm and cold seasons recognizes that winter rainfall is more effective for plant growth that can flourish in the winter and go dormant in the summer than the same amount of summer rainfall during a warm-to-hot season. Thus a place like Athens, Greece that gets most of its rainfall in winter can be considered to have a humid climate (as attested in lush foliage) with roughly the same amount of rainfall that imposes semi-desert conditions in Midland, Texas, where rainfall largely occurs in the summer.

In 1948, C. W. Thornthwaite proposed an AI defined as:

AI_T = 100 imesfrac{d}{n}

where the water deficiency d is calculated as the sum of the monthly differences between precipitation and potential evapotranspiration for those months when the normal precipitation is less than the normal evapotranspiration; and where n stands for the sum of monthly values of potential evapotranspiration for the deficient months (after Huschke, 1959). This AI was later used by Meigs (1961) to delineate the arid zones of the world in the context of the UNESCO Arid Zone Research programme.

In the preparations leading to the UN Conference on Desertification (UNCOD), the United Nations Environment Programme (UNEP) issued a dryness map based on a different aridity index, proposed originally by Mikhail Ivanovich Budyko (1958) and defined as follows:

AI_B = 100 imesfrac{R}{LP}

where R is the mean annual net radiation (also known as the net radiation balance), P is the mean annual precipitation, and L is the latent heat of vaporization for water. Note that this index is dimensionless and that the variables R, L and P can be expressed in any system of units that is self-consistent.

More recently, the UNEP has adopted yet another index of aridity, defined as:


where PET is the potential evapotranspiration and P is the average annual precipitation (UNEP, 1992). Here also, PET and P must be expressed in the same units, e.g., in milimetres. In this latter case, the boundaries that define various degrees of aridity and the approximate areas involved are as follows:

ee also

*Climate classification
*Savory brittleness scale


* Budyko, M. I. (1958) "The Heat Balance of the Earth's Surface", trs. Nina A. Stepanova, US Department of Commerce, Washington, D.D., 259 p.
* Huschke, Ralph E. (1959) "Glossary of Meteorology", American Meteorological Society, Boston, Second printing-1970.
* McIntosh, D. H. (1972) "Meteorological Glossary", Her Majesty's Stationery Office, Met. O. 842, A.P. 897, 319 p.
* Meigs, P. (1961) 'Map of arid zone', in L. D. Stamp (Editor) "A History of Land Use in Arid Regions", UNESCO Arid Zone Research, Publication XVII, Paris, 388 p.
* UNCOD Secretariat (1977) "Desertification: Its causes and consequences", Pergamon Press, 448 p.
* UNEP (1992) "World Atlas of Desertification".

Wikimedia Foundation. 2010.

Look at other dictionaries:

  • aridity index — a number indicating how much more precipitation could be lost by evapotranspiration if it were available than is actually lost at a given location. * * * …   Universalium

  • aridity index — a number indicating how much more precipitation could be lost by evapotranspiration if it were available than is actually lost at a given location …   Useful english dictionary

  • Palmer Drought Index — The Palmer Drought Index, sometimes called the Palmer Drought Severity Index and often abbreviated PDSI, is a measurement of dryness based on recent precipitation and temperature. It was developed by meteorologist Wayne Palmer, who first… …   Wikipedia

  • Climate — For other uses, see Climate (disambiguation). Worldwide Climate Classifications Climate encompasses the statistics of temperature …   Wikipedia

  • Evapotranspiration — (ET) is a term used to describe the sum of evaporation and plant transpiration from the earth s land surface to atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and… …   Wikipedia

  • Nightingale — This article is about the bird. For other uses, see Nightingale (disambiguation). Nightingale  …   Wikipedia

  • Geography of Kerala — Kerala is prone to several natural hazards, the most common of them being landslides, flooding, lightning, drought, coastal erosion, earthquakes, Tsunami, wind fall and epidemics [Yashodharan et al., (2007)] .LandslidesThe highlands of Kerala… …   Wikipedia

  • Climate classification — Leslie Holdridge s Life Zone Classification system is essentially a climate classification scheme. Climate classification systems are ways of classifying the world s climates. A climate classification may correlate closely with a biome category,… …   Wikipedia

  • Savory brittleness scale — The Savory brittleness scale is a measure of the year round distribution of humidity in a particular environment. It was developed by Allan Savory a Zimbabwean biologist.A rainforest would be 1 on the scale and a desert would be 10.The scale… …   Wikipedia

  • Übereinkommen der Vereinten Nationen zur Bekämpfung der Wüstenbildung — Das Übereinkommen der Vereinten Nationen zur Bekämpfung der Wüstenbildung in den von Dürre und/oder Wüstenbildung schwer betroffenen Ländern, insbesondere in Afrika (englisch: United Nations Convention to Combat Desertification in those Countries …   Deutsch Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.