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Aridity Index
Aridity Index
Aridity Index
Aridity Index
Aridity Index
Microclimatev01.00 credits/per_km2
Responsible:Niccolò Calandri
Layer Aridity Index on real sites
Real Estate UK
Catania
Berlino Quarter
Mina el Cerrejon
Aridity Index
Overview
The Aridity Index (AI) measures how dry or humid an area is by calculating the ratio between mean annual precipitation (P) and potential evapotranspiration (PET). A low value indicates arid conditions — evapotranspiration exceeds precipitation — while a high value indicates humid conditions.
Aridity is one of the most significant factors shaping vegetation distribution, water availability, and ecosystem composition. Areas with very low AI (< 0.03) are classified as hyper-arid, supporting only desert ecosystems with extremely limited biodiversity. Humid areas (AI > 0.65) sustain richer, more productive ecosystems including forests and wetlands.
The platform reports this index as a dimensionless ratio covering the full range from hyper-arid to humid environments.
Classification follows the internationally recognized UNEP (1992) standard:
| Class | AI Range | Description |
|---|---|---|
| Hyper-arid | < 0.03 | Desert; extremely rare precipitation |
| Arid | 0.03 — 0.20 | Semi-desert; sparse xerophytic vegetation |
| Semi-arid | 0.20 — 0.50 | Savanna, steppe; drought-adapted vegetation |
| Dry sub-humid | 0.50 — 0.65 | Grasslands, Mediterranean scrubland |
| Humid | > 0.65 | Forests; water-rich ecosystems |
Calculation Methodology
The Aridity Index is computed as the ratio of mean annual precipitation to mean annual potential evapotranspiration:
where P = mean annual precipitation (mm), PET = potential evapotranspiration (mm, Penman-Monteith method)
Data are sourced from the Global Aridity Index v3.1 (CGIAR-CSI) at approximately 1 km spatial resolution. Historical values use the reference period 1970–2000. Future projections (years ≥ 2040) are based on CMIP6 multi-model ensemble scenarios (SSP1-2.6, SSP2-4.5, SSP3-7.0, SSP5-8.5).
What You See on the Platform
Assessment > Microclimate > "Aridity Index" (Gauge)
Gauge. A semicircular gauge showing the Aridity Index for the site (ROI) and its control area (CA), alongside a vertical color legend.
Purpose: Answers "How dry or humid is this site compared to its surroundings?"
Description: The gauge displays two overlapping arcs — one for the site (ROI) and one for the control area (CA) — along a 0–1.5 scale. Benchmark reference lines indicate target values. A delta badge below the gauge shows the difference between ROI and CA (positive = site more humid than control; negative = site drier). To the right, a vertical color bar marks the position of both values along the aridity gradient. The card spans two columns in the microclimate grid.
0.45Aridity Index
How it's calculated: The displayed value is the mean Aridity Index (P/PET) computed over the site polygon and the control area polygon, using the Global Aridity Index v3.1 raster at ~1 km resolution. The benchmark value is a reference target for the site's climate zone.
Note: This indicator is not inverted — higher values indicate more humid, ecologically better conditions.
Legend:
| Level | AI Range | Color | Meaning |
|---|---|---|---|
| A | 0.75 — 1.5+ | ■ #00A67A | Humid; rich and productive ecosystems |
| B | 0.35 — 0.75 | ■ #00DF80 | Dry sub-humid to humid; diverse vegetation |
| C | 0.15 — 0.35 | ■ #FFD21E | Semi-arid; limited water availability |
| D | 0.05 — 0.15 | ■ #FF8B16 | Arid; scarce precipitation, ecosystem stress |
| E | 0 — 0.05 | ■ #FF367F | Hyper-arid; desert conditions, minimal biodiversity |
Interpretation example:
If the gauge shows ROI = 0.45 and CA = 0.72, the site has a level B aridity index while the control area is in level A. The site is significantly drier than its surroundings — this can signal land use pressures, loss of vegetative cover, or higher evaporative demand relative to the control zone.
Assessment > Microclimate > "Aridity Index" (Highlights Card)
Highlights Card. A compact icon-based summary card in the Highlights section, showing site and control area quality on a 1–5 icon scale.
Purpose: Provides a quick at-a-glance comparison of aridity quality between site and control.
Description: The card displays the title "Aridity" and topic label "E3 | Microclimate". Two rows of icons (1 to 5, site and control) use a custom Aridity SVG icon colored by quality grade. Hover tooltips explain the numeric score.
How it's calculated: The site and control area aridity index values are mapped to a 1–5 icon scale using the same quality ranges as the gauge.
Interpretation example:
If the site row shows 3 icons and the control row shows 4 icons, the site has a slightly drier aridity index than the surrounding landscape — suggesting the site may benefit from vegetation management or review of land cover.
Highlights > "Aridity Index" (Highlights Table Row)
Highlights Table Row. A numeric row in the KPI comparison table within the Highlights section, showing aridity index values for site and control side by side with a quality badge.
Purpose: Allows direct numeric comparison of aridity between site and control with quality grading.
Description: The row is labeled "Aridity Index" and falls under the E3 (Climate/Water) topic group. It shows the site value, the control area value, and a colored A–E grade badge derived from the site value. No unit is displayed (dimensionless ratio).
How it's calculated: Same P/PET ratio as the gauge. The quality grade A–E is assigned by matching the site value against the platform quality ranges (A ≥ 0.75, B 0.35–0.75, C 0.15–0.35, D 0.05–0.15, E < 0.05).
Interpretation example:
If the row shows site value 0.38 (grade B) vs control 0.51 (grade B), both areas are in sub-humid conditions. The site is slightly drier but within the same quality tier — no immediate ecological concern.
Maps > "Arid Zones" (Map Layer)
Map Layer. An interactive map overlay showing aridity class distribution across the site and a 25 km buffer zone, using a 10-class discrete color palette ranging from hyper-arid (warm earthy tones) to humid (cool blues and greens).
Purpose: Answers "Which parts of this site and surrounding landscape are most arid, and how does aridity vary spatially?"
Description: The map displays the Global Aridity Index as a colored raster, with each pixel classified into one of 10 aridity classes using fixed boundaries (0, 0.03, 0.20, 0.35, 0.50, 0.65, 0.80, 1.00, 1.25, 1.50). Below the map, an "Arid zones" panel lists each aridity class found within the site as scrollable cards showing class name, color, and area in hectares. A version selector allows switching between historical years and future SSP scenarios.
How it's calculated: The raster pixel value (AI ratio) is mapped to one of 10 discrete classes using the fixed boundaries listed above. Color is assigned per class from the fixed colormap. Area for each class is calculated in metric projection for accurate surface measurement.
Legend:
| Class | AI Range | Color | Name |
|---|---|---|---|
| 0 | 0 — 0.03 | ■ #CF7563 | Hyper Arid |
| 1 | 0.03 — 0.20 | ■ #E09053 | Arid |
| 2 | 0.20 — 0.35 | ■ #F3BA41 | Semi-Arid |
| 3 | 0.35 — 0.50 | ■ #FAE038 | Semi-Arid |
| 4 | 0.50 — 0.65 | ■ #D2FA32 | Dry sub-humid |
| 5 | 0.65 — 0.80 | ■ #5DE833 | Humid |
| 6 | 0.80 — 1.00 | ■ #40D168 | Humid |
| 7 | 1.00 — 1.25 | ■ #49B09C | Humid |
| 8 | 1.25 — 1.50 | ■ #458AA1 | Humid |
| 9 | > 1.50 | ■ #3D5894 | Humid |
Interpretation example:
If the map shows most of the site in class 2 (Semi-Arid) with a small patch of class 5 (Humid) near a riparian corridor, the site faces moderate water deficit in most areas — vegetation management near the humid patch can help maintain a moisture refuge for biodiversity.
Data Sources
| Source | Provider | Coverage | Resolution | Period |
|---|---|---|---|---|
| Global Aridity Index v3.1 (CGIAR-CSI) | CGIAR Consortium for Spatial Information | Global | ~1 km (30 arc-sec) | Historical reference 1970–2000; updated 2025 |
| CMIP6 Multi-Model Ensemble (future) | CMIP6 / World Climate Research Programme | Global | ~100 km | 2040–2100, SSP1-2.6 / SSP2-4.5 / SSP3-7.0 / SSP5-8.5 |
Detailed Methodology
The raw raster tile is loaded (ai_v31_yr.tif for historical, aridity_index.tif for future scenarios). A scale factor (1/10,000 for historical, 1 for future) is applied to convert raw pixel values to the dimensionless AI ratio. The data undergoes bilinear resampling to the output projection (EPSG:3857). Negative values are clipped to 0. Water bodies (ESA Land Cover class 80) are masked out.
For the map layer, the AI ratio is classified into 10 discrete classes using fixed boundaries (0, 0.03, 0.20, 0.35, 0.50, 0.65, 0.80, 1.00, 1.25, 1.50). Color is assigned per class. Area for each class is calculated in metric projection for accurate surface measurement.
Future projections use CMIP6 multi-model ensemble scenarios at ~100 km resolution, significantly coarser than the historical 1 km layer.
Technical Notes
- The Global Aridity Index v3.1 is based on historical climate data (reference period 1970–2000); it 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 microclimatic variability (e.g., urban heat effects, topographic shade) may not be captured
- In mountainous terrain, altitudinal precipitation gradients may be underestimated
- Future projections (years ≥ 2040) use a CMIP6 multi-model ensemble at ~100 km resolution — spatial detail is much coarser than the historical layer
- Two classification systems coexist on the platform: the 10-class map colormap (visual) and the 5-level quality scale (KPI scoring) — they use different boundary sets by design
- Water bodies are masked using ESA Land Cover class 80 — no aridity value is reported for lake or river pixels
- Related KPI: Evapotranspiration (ET) measures the water loss component directly; Aridity Index contextualizes it against precipitation to give the full water balance picture
Bibliography
- Zomer, R.J., Xu, J., Trabucco, A. (2022). "Version 3 of the Global Aridity Index and Potential Evapotranspiration Database". Scientific Data, 9, 409. DOI: 10.1038/s41597-022-01493-1
- UNEP (1992). World Atlas of Desertification. United Nations Environment Programme. Edward Arnold, London.
- Trabucco, A., Zomer, R.J. (2019). "Global Aridity Index and Potential Evapotranspiration (ET0) Climate Database v2". Figshare. DOI: 10.6084/m9.figshare.7504448.v6
- Middleton, N., Thomas, D. (eds.) (1997). World Atlas of Desertification. 2nd edition. UNEP/Edward Arnold, London.