An aridity index (AI) is a numerical indicator of the degree of dryness of the climate at a given location. The American Meteorological Society defined it in meteorology and climatology, as "the degree to which a climate lacks effective, life-promoting moisture". Aridity is different from drought because aridity is permanent whereas drought is temporary.[1] 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.
See also: Desert climate.
See main article: Köppen climate classification. 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
R=2 x T
R=2 x T+14
R=2 x T+28
where
T
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.
See main article: Thornthwaite climate classification. In 1948, C. W. Thornthwaite proposed an AI defined as:
where the water deficiency
d
n
In the preparations leading to the 1977 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)[3] and defined as follows:[4]
where
R
P
L
R
L
P
More recently in 1992, the UNEP has adopted yet another index of aridity, defined as:[5]
where
PET
P
PET
P
| Classification | Aridity Index | Global land area | |
|---|---|---|---|
| Hyperarid | AI < 0.05 | 7.5% | |
| Arid | 0.05 < AI < 0.20 | 12.1% | |
| Semi-arid | 0.20 < AI < 0.50 | 17.7% | |
| Dry subhumid | 0.50 < AI < 0.65 | 9.9% |
As this index increases with wetter conditions, some hydrologists refer to this as a humidity index.