Aridity index
Encyclopedia
An aridity index 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.
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
, where:
where
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:
where the water deficiency
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 
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:
where
is the mean annual net radiation (also known as the net radiation balance), 
is the mean annual precipitation, and 
is the latent heat of vaporization for water. Note that this index is dimensionless and that the variables 
, 
and 
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
is the potential evapotranspiration and 
is the average annual precipitation
(UNEP, 1992). Here also,
and 
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:
Historical background and indices
At the turn of the 20th century, Wladimir KöppenWladimir Köppen
Wladimir Peter Köppen was a Russian geographer, meteorologist, climatologist and botanist. After studies in St. Petersburg, he spent the bulk of his life and professional career in Germany and Austria...
and Rudolf Geiger
Rudolf Geiger
Rudolf Oskar Robert Williams Geiger was a German meteorologist and climatologist....
developed the concept of a climate classification
Köppen climate classification
The Köppen climate classification is one of the most widely used climate classification systems. It was first published by Crimea German climatologist Wladimir Köppen in 1884, with several later modifications by Köppen himself, notably in 1918 and 1936...
where arid regions were defined as those places where the annual rainfall accumulation (in centimetres) is less than
, where:-
if rainfall occurs mainly in the cold season, -
if rainfall is evenly distributed throughout the year, and -
if rainfall occurs mainly in the hot season.
where
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
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
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
Midland, Texas
Midland is a city in and the county seat of Midland County, Texas, United States, on the Southern Plains of the state's western area. A small portion of the city extends into Martin County. As of 2010, the population of Midland was 111,147. It is the principal city of the Midland, Texas...
, where rainfall largely occurs in the summer.
In 1948, C. W. Thornthwaite
C. W. Thornthwaite
Charles Warren Thornthwaite was an American geographer and climatologist. He is best known for devising a climate classification system, published in 1948, that is still in use around the world, and also for his detailed water budget computations of potential evapotranspiration...
proposed an AI defined as:
where the water deficiency
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 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
United Nations Environment Programme
The United Nations Environment Programme coordinates United Nations environmental activities, assisting developing countries in implementing environmentally sound policies and practices. It was founded as a result of the United Nations Conference on the Human Environment in June 1972 and has its...
(UNEP) issued a dryness map based on a different aridity index, proposed originally by Mikhail Ivanovich Budyko (1958) and defined as follows:
where
is the mean annual net radiation (also known as the net radiation balance), is the mean annual precipitation, and is the latent heat of vaporization for water. Note that this index is dimensionless and that the variables , and 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
is the potential evapotranspiration and is the average annual precipitationPrecipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...
(UNEP, 1992). Here also,
and 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:| 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% |
See also
- Climate classificationClimate classificationClimate classification systems are ways of classifying the world's climates. A climate classification may correlate closely with a biome category, as climate is a major influence on biological life in a region...
- Savory brittleness scaleSavory brittleness scaleThe Savory brittleness scale is used to describe the annual distribution of humidity in a particular environment. It was developed by Allan Savory, a Zimbabwean biologist. The scale is used in land management because the distribution of humidity affects the way that land, particularly when...
- AridificationAridificationAridification is the process of a region becoming increasingly dry. It refers to long term change rather than seasonal variation.It is often measured as the reduction of average soil moisture content....
- DesertificationDesertificationDesertification is the degradation of land in drylands. Caused by a variety of factors, such as climate change and human activities, desertification is one of the most significant global environmental problems.-Definitions:...