Aridity index

An aridity index is a numerical measure of climatic dryness that compares water supply, usually precipitation, with atmospheric evaporative demand. Different formulations use different variables and thresholds.

Potential evapotranspiration: The amount of water that could evaporate and transpire from a well-watered surface under prevailing atmospheric conditions. It represents the atmospheric demand commonly compared with precipitation.
Köppen climate classification: A climate classification system that uses temperature and precipitation patterns to identify climate types. Its desert and steppe classes use precipitation thresholds adjusted for temperature.
Palmer Drought Severity Index: A water-balance index that estimates the severity and duration of meteorological drought. It tracks departures from water balance rather than classifying long-term climatic aridity.
Reference evapotranspiration: The estimated evapotranspiration from a standardized, well-watered reference surface. Choosing this demand measure affects comparability among aridity estimates.
Precipitation: Water that falls from the atmosphere to Earth's surface as rain, snow, sleet, or hail. It is the main measure of water supply in many aridity-index formulations.
Drylands: Regions where precipitation is low relative to potential evapotranspiration, including arid, semi-arid, and dry subhumid zones. Aridity indices are commonly used to delineate these water-limited regions.
Standardized Precipitation Index: A standardized measure of precipitation deficits over a selected accumulation period. It isolates precipitation anomalies without directly representing atmospheric demand.
Atmospheric evaporative demand: The atmosphere's capacity to draw water from surfaces through evaporation and plant transpiration. Its estimation under warming remains central to interpreting changing index values.
UNEP aridity index: A global aridity measure calculated as mean annual precipitation divided by mean annual potential evapotranspiration. This widely used formulation makes the supply-to-demand ratio explicit.
Desertification: Land degradation in arid, semi-arid, and dry subhumid areas caused by climatic variations and human activities. Index trends can help distinguish changing climatic dryness from land degradation.

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