Precipitation seasonality (CV) — 1981-2010 (CHELSA)
This layer maps how unevenly rainfall is spread across the year, averaged over the 1981-2010 reference period.
Source: CHELSA-CMIP6 v2.1 (Karger et al.) Year: 1981-2010 Category: Physical risks · Mitigating services · Climate Chronic Risks · 1981-2010 Coverage: Global Format: Raster grid (~2 km) Used in risk analysis: Yes — gates Mitigating services
What it shows
This layer maps how unevenly rainfall is spread across the year, averaged over the 1981-2010 reference period. Rather than total rainfall, it captures the variability between months — whether precipitation is steady year-round or concentrated in a short wet season. Strong seasonality shapes drought stress, water storage needs and the kinds of ecosystems and agriculture an area can support.
How it is built
The layer comes from CHELSA-CMIP6 v2.1, a high-resolution global climatology developed by Karger and colleagues. The measured variable is the coefficient of variation of monthly precipitation — a standard bioclimatic measure of how much rainfall fluctuates month to month relative to its average. It is derived from the 1981-2010 baseline record and presented as a reference climatology on a fine global grid of roughly 2 km.
How to read it
Higher values indicate stronger seasonality — rainfall heavily concentrated in part of the year, with pronounced wet and dry seasons. Lower values indicate more evenly distributed rainfall across the year. As a baseline layer it describes recent historical climate and is best read alongside the matching future-projection variant.
Class thresholds
The underlying quantity is the coefficient of variation (CV) of monthly precipitation — a standard bioclimatic measure of how much rainfall fluctuates month to month relative to its average. It is dimensionless (a ratio, sometimes read as a percentage): higher CV means rainfall is concentrated into part of the year. On the map it is rendered as a continuous colour ramp from 0 (pale, even year-round rainfall) to 150 (dark, strongly seasonal). For risk scoring the continuous CV is binned into five classes:
| Class | CV of monthly precipitation |
|---|---|
| Very Low Risk | < 29.0 |
| Low Risk | 29.0 – 46.1 |
| Moderate Risk | 46.1 – 61.5 |
| High Risk | 61.5 – 87.9 |
| Very High Risk | ≥ 87.9 |
Separately, a site is flagged on the Mitigating services (Water supply) dimension when the layer value is above 68 — the global 90th percentile of the CV — provided the entity's matching ENCORE "Water supply" pressure is also material.
How these thresholds were set. This is a Darwin in-house classification: the five risk classes are the empirical 20/40/60/80 quantiles of the global CV distribution (computed over 539 geographies, min 8.8 / max 154.2), and the separate proximity cut-off of 68 is the p90 of that distribution. Both are data-driven quantiles of Darwin's own data, not breaks from a published seasonality standard.
Source
CHELSA-CMIP6 v2.1 (Karger et al.), 1981-2010 reference climatology; open licence.
Comparison with the WWF Risk Filter Suite
The WWF Risk Filter Suite does not publish standalone bioclimatic seasonality variables of this kind. This baseline CHELSA layer is a climate-descriptor extension rather than a WWF risk indicator, so it has no direct equivalent in the WWF methodology.
Risk analysis
A site is flagged on a dimension by combining a proximity trigger (this layer) with an activity trigger (the entity's ENCORE pressure/service). Proximity only → Potentially material; proximity and the matching ENCORE pressure/service is material → Very material; neither → Not material.
| Dimension | ENCORE service / pressure | Proximity trigger (this layer) | Activity trigger (entity) |
|---|---|---|---|
| Mitigating services | Water supply (precipitation variability) | Layer value above 68 | “Water supply” pressure ≥ 4; impact ratio ≥ 5%; impact ratio ≥ 5% |
Legend
Symbolised field: Precipitation seasonality (CV)
Generated from darwin/layers/layer-chelsa-precip_seasonality-historical.toml and risk_indicator_pairs.toml (develop).