Aridity Index

The aridity index is a climatic indicator built from the long-term balance between water supply and water demand. Precipitation represents the supply, while potential evapotranspiration — the amount of water that would evaporate and transpire given the available energy if water were not limiting — represents the demand. Their ratio summarises whether, on average, a climate tends toward surplus or deficit of water.

Several formulations of aridity indices exist, using different inputs and thresholds, but the precipitation-to-potential-evapotranspiration ratio is among the most widely applied because it captures both sides of the water balance in a single, comparable number.

Aridity-index values are grouped into classes that describe increasingly dry climates — commonly hyper-arid, arid, semi-arid, dry sub-humid, and humid. These classes provide an internationally recognised framework for delineating drylands, which cover a large share of the world’s land and support a substantial part of its agriculture and population.

• Indicates whether crops can be grown on rainfall alone or need irrigation
• Helps delineate dryland farming regions and their characteristic constraints
• Signals areas at greater risk of land degradation, salinity, and desertification
• Provides a consistent basis for comparing water availability across regions
• Informs planning for water resources, land use, and drought-adapted systems

A region’s aridity class shapes the whole character of its agriculture, from the crops and varieties that are viable to the importance of irrigation and moisture-conservation practices.

The aridity index is a broad climatic summary and does not capture the timing of rainfall within the year, year-to-year variability, soil water storage, or short-term drought, all of which strongly affect agriculture. Different index formulations and class boundaries exist, so values should be interpreted against the method used. This entry describes the concept rather than giving specific threshold values or classifications for any location.