Name

ARIDITY INDEX (1)

Brief definition

An index of the average water available in the soil, defined as the ratio between mean annual precipitation (P) and mean annual evapotranspiration (ETP). It is a critical environmental factor affecting the evolution of natural vegetation and therefore rain erosivity by considering rainfall and air temperature.

Higly degraded and desertified area under semiarid climatic conditions and high aridity index in the island of Lesvos (photo by C. Kosmas)

Unit of measure

Numeric value

Type of Indicator

 State

Objective

The primary objective is the identification of desertification prone areas, with particular reference to degradation of soil and water resources. It is used for the definition and mapping of ESAs.

Importance with respect to desertification

The atmospheric conditions that characterize a desert climate are those that create large water deficits, that is, potential evapotranspiration is much greater than the precipitation. The aridity index classifies the type of climate in relation to water availability. The higher the aridity index of a region the greater the water resources variability and scarcity in time, the more vulnerable the area to desertification. This indicator is part of a set of tools to identify and mitigate land degradation, used by many of the Annex IV Focal Points. Along with the soil loss index and the drought index, it contributes to producing a scale of the state of health of soil and water resources and consequently to the elaboration of development strategies compatible with the resources available in a given area.

International Conventions and agreements

The UNCCD emphasizes that combating desertification must be tackled within the general framework of actions to promote sustainable development.

Secondary objectives of the indicator

Within the ESA model to investigate the individual processes linked to land degradation and desertification and defining desertification risk of an area.

Definitions and basic concepts

The Aridity index (1) can be estimated by the Bagnouls-Gaussen index (BGI) using the following equation:
n
BGI = S (2ti - Pi)*k
i=1
where: ti is the mean air temperature for month i in 0°C, Pi is the total precipitation for month i in mm; and k represents the proportion of month during which 2ti - Pi >0.

Aridity index can be also defined as the ratio between mean annual precipitation (P) and mean annual evapotranspiration- (ETP) calculated with the Penman formula.

Benchmarks Indication of the values/ranges of value

The aridity index defined as the ratio P/ETP has been proposed by the UNCCD to identify areas prone to using the following three classes:

• >0.65
• 0.5-0.65
• <0.5

Each class corresponds to many climatic zones (humid, dry sub-humid; arid and semi-arid respectively) and a different score is assigned to each. According to the UNCCD desertification cannot exists without a certain level of aridity (between 0.05 and 0.65).

The following benchmarks have been defined for the definition of ESAs based on the estimation of the Bagnouls-Gaussen index (BGI):

BGI<50, 50-75, 75-100, 100-125, 125-150, >150

Methods of measurement

The aridity index can be easily calculate using long term monthly meteorological data such as rainfall, air temperature, wind speed, sunshine duration, relative humidity, etc.

Limits of the indicator

The quality of the indicator depends on the number and distribution of meteorological monitoring stations over an area. Since meteorological parameters are very variable in time and space, the stations have to be evenly distributed across an area covering the whole range of climatic conditions.

Linkages with other indicators

Rainfall, Slope aspect, Vegetation cover, Water availability, Fire risk, Rainfall seasonality, Soil salinity index

Data required to calculate the indicator

Data required for the BGI is monthly rainfall and air temperature averaged for a period of at least 30 years. Data required for EPT are air temperature, relative humidity, sunshine duration, and wind speed.

Data sources

Necessary data are usually available and accessible and the cost/benefit ratio is reasonable.

Availability of data from national and international sources

Data can be obtained from various regional, national or international institutions involved in the collection and the elaboration of meteorological data.

Main institutions responsible

Agricultural University of Athens

DGF - Direcçao-Geral das Florestas. Ministèrio da Agricultura, do Desenvolvimento Rural e das Pescas.

Dirección General de Conservación de la Naturaleza (DGCN), Secretaría de Medio Ambiente, Ministerio de Medio Ambiente, Spain.

Other contributing organizations

Universities of Lisbon, Murcia, Basilicata, Amsterdam, Leeds

Other references

Brandt, J., and Thornes, J., 1996 Mediterranean Desertification and Land Use. J. Wiley & Sons, Chichester, England, 554.

Kosmas C., Ferrara A., Gerontidis, St. Bellotti B., Detsis V., Faretta S., Mancino G., and Pisante, M. 1999. A comparative analysis of the physical environment of two Mediterranean areas threatened by desertification. Mediterranean M. 127-148.

Conacher, A., and Sala, M., 1998. Land Degradation in the Mediterranean Environments of the World: Nature and Extent, Causes and Solutions. J. Wiley & Sons, Chichester, 491 p.

http: //www.dgf.min-agricultura.pt.

Desertification Indicators for the European Mediterranean Region (Enne, G. and Zucca,C. 2000)

Mapa de Aridez (Servicio Meteorológico Nacional, Spain).

Contacts Name and address

Agricultural University of Athens
Dr Constantinos Kosmas
email: lsos2kok@aua.gr

Direcçao-Geral das Florestas,
Av.a Joao Crisotomo, n. 28-5.
1069-040 Lisboa- Portugal,
email: dgf.web@mail.telepac.pt

Leopoldo Rojo Serrano
Dirección General de Conservación de la Naturaleza,
Ministerio de Medio Ambiente
Gran Vía de San Francisco 4,
28005 Madrid (Spain)
email: LRojo@mma.es