Aridity Index

Microclimatevv0

Responsible:Niccolò Calandri

Layer Aridity Index on real sites

Real Estate UK

Aridity Index

Catania

Aridity Index

Berlino Quarter

Aridity Index

Mina el Cerrejon

Aridity Index

Overview

The Aridity Index (AI) is the ratio between mean annual precipitation (P) and potential evapotranspiration (PET): AI = P / PET. A low value indicates an arid area (evapotranspiration exceeds precipitation), while a high value indicates a humid area (precipitation exceeds evapotranspiration).

Aridity is a determining factor for vegetation distribution, water availability and ecosystem composition. Areas with a very low aridity index (<0.03) are classified as hyper-arid and host desert ecosystems with extremely limited biodiversity. Humid areas (AI > 0.65) support richer and more productive ecosystems.

Calculation Methodology

Data are sourced from the Global Aridity Index v2 (CGIAR-CSI) at approximately 1 km resolution. The AI is calculated as the ratio between mean annual precipitation (WorldClim/CHELSA) and potential evapotranspiration calculated using the Penman-Monteith method.

The classification adopted follows the UNEP (1992) definition, the international standard for aridity classification:

Class	AI Range	Description
Hyper-arid	< 0.03	Desert; extremely rare precipitation
Arid	0.03 – 0.20	Semi-desert; sparse xerophilous vegetation
Semi-arid	0.20 – 0.50	Savanna, steppe; drought-adapted vegetation
Dry sub-humid	0.50 – 0.65	Grasslands, Mediterranean scrub
Humid	> 0.65	Forests, water-rich ecosystems

Unit: dimensionless (ratio of mm precipitation to mm evapotranspiration)

Data Sources

Code	Name	Provider	Resolution	Availability
`WRD_ARIDX_22`	Global Aridity Index v2	CGIAR-CSI	1000 m	2022

Quality Range and Levels

Indicator	Unit	Range	Inverted
`aridity`	—	[0, 0.05, 0.15, 0.35, 0.75, 1.5]	No

Not inverted: higher values indicate more humid areas with greater water availability.

Level	AI Range	UNEP Class	Interpretation
E (Critical)	< 0.05	Hyper-arid	Desert; minimal biodiversity and water availability
D (Poor)	0.05 – 0.15	Arid	Semi-desert; sparse xerophilous vegetation
C (Moderate)	0.15 – 0.35	Semi-arid	Savanna/steppe; drought-adapted vegetation
B (Good)	0.35 – 0.75	Dry sub-humid to humid	Grasslands, scrub, open forests
A (Excellent)	> 0.75	Humid	Rich forests; high water and ecological availability

Technical Notes

The Global Aridity Index v2 is based on historical climate data (reference period 1970–2000)
The index does not capture seasonal variability: areas with medium-high AI may have intense dry seasons
The ~1 km resolution is adequate for landscape-scale analysis; local micro-climatic variability may be missed
In mountain contexts, the resolution may underestimate altitudinal precipitation gradients

References

Trabucco, A., Zomer, R. (2019). "Global Aridity Index and Potential Evapotranspiration Climate Database v2". CGIAR-CSI GeoPortal.
Middleton, N., Thomas, D. (eds.) (1997). "World Atlas of Desertification". 2nd edition. UNEP/Edward Arnold, London.

Quality Ranges and Levels

aridity

ECritical

DLow

CModerate

BGood

AExcellent

0 → 0.04

0.05 → 0.14

0.15 → 0.34

0.35 → 0.74

0.75 → 1.5

Critical0 – 0.04

Low0.05 – 0.14

Moderate0.15 – 0.34

Good0.35 – 0.74

Excellent0.75 – 1.5

Data Sources & References

Sources

WRD_ARIDX_22

Methodology

Ratio between mean annual precipitation and potential evapotranspiration (Penman-Monteith method). Source: Global Aridity Index v2 (CGIAR-CSI, ~1 km, reference period 1970–2000). UNEP classification: < 0.03 hyper-arid, 0.03–0.20 arid, 0.20–0.50 semi-arid, 0.50–0.65 dry sub-humid, > 0.65 humid.