Water purification shock (NDR)
Horizon-resolved projection of the change in nitrogen-retention capacity, from the IMAGE-GNM river-N nutrient model. This layer family backs the Water purification ecosystem service of the Nature Stress Test.
Source: IMAGE-GNM (Global Nutrient Model, river N) Year: 2015 baseline → 2035 / 2050 / 2080 Category: Scenario · Ecosystem service (ES-direct, Tier-2) Coverage: Global Format: Raster grid (COG) Used in risk analysis: Yes — supplies the Water purification service shock in the Nature Stress Test; not used in present-day materiality scoring.
What it shows
This layer family projects how the capacity of the landscape to retain nitrogen — and so to purify water before it reaches a watercourse — is expected to change under climate-and-land-use scenarios, relative to a 2015 baseline.
It is an ES-direct (Tier-2) layer: it carries an ecosystem service shock, not an ecosystem component shock. Where a natural-asset layer (Habitats, Soils, Species, Atmosphere, Water) is projected onto services through the ENCORE dependency matrix, an ES-direct layer is read straight into the service it names. When this layer resolves for the requested scenario and horizon, the Nature Stress Test takes the Water purification degradation from the layer itself and bypasses the ENCORE projection; if it does not resolve, Water purification falls back to being projected from its underlying components (chiefly Water and Soils).
How it is built
IMAGE-GNM routes nitrogen through soils, groundwater and the river network. The layer scores in-stream nitrogen concentration per cell, and the shock is the relative change in that concentration (%) versus the 2015 baseline.
Sign convention: positive = worse. More nitrogen in the water means the purification service is failing, so the value rises as the service degrades. This is a pressure/state layer, like PM2.5 and water stress — the opposite of the service/abundance layers (LUH2, MSA, GLOBIO-ES), whose values fall as the service degrades. The stress-test engine normalises both families before computing loss.
Why concentration and not retention. In-stream retention is the "supply-pure" quantity and was tested first, but it is nearly scenario-invariant: under high-input scenarios retention scales with the extra load, so the retained fraction barely moves (SSP1 +6.8 % vs SSP5 +7.4 %). The degradation signal lives in the concentration (SSP1 +3.3 % vs SSP5 +11.7 %), which is also the directly interpretable metric.
| Layer | Scenario | Horizon | Provenance |
|---|---|---|---|
NDR_shock_ssp1_2035 | SSP1 (optimistic) | 2035 | Real IMAGE-GNM band |
NDR_shock_ssp1_2050 | SSP1 (optimistic) | 2050 | Real IMAGE-GNM band |
NDR_shock_ssp1_2080 | SSP1 (optimistic) | 2080 | Extrapolated from the 2050 band |
NDR_shock_ssp3_2035 | SSP3 (pessimistic) | 2035 | Real IMAGE-GNM band |
NDR_shock_ssp3_2050 | SSP3 (pessimistic) | 2050 | Real IMAGE-GNM band |
NDR_shock_ssp3_2080 | SSP3 (pessimistic) | 2080 | Extrapolated from the 2050 band |
Horizon provenance. IMAGE-GNM publishes 5-yearly bands, so 2035 and 2050 are real model slices. The model does not publish a band at 2080: that horizon is extrapolated from the 2050 band. Treat 2080 Water purification as a directional trajectory rather than a modelled projection.
Scenario forcing. The pessimistic member of this family is driven by SSP3-RCP6.0, the forcing IMAGE-GNM publishes for the regional-rivalry pathway. The natural-asset layers of the same stress test use SSP3-RCP7.0, and the application labels the pessimistic scenario SSP3-7.0 throughout. The narrative is the same regional-rivalry storyline; the radiative forcing behind this particular service model is the slightly lower one.
How to read it
Values are the projected change in river nitrogen concentration, in percent versus 2015. More positive = greater projected loss of the water-purification service at that location; values near zero indicate stability; negative values indicate projected improvement in water quality. Read scenario and horizon together — ssp3_2080 is the pessimistic pathway at the most distant (and extrapolated) horizon.
Class thresholds
The underlying quantity is a change-of-condition flux: relative change in in-stream N concentration, in % vs the 2015 baseline, capped at ±100 %. The layer is rendered as a continuous symlog diverging gradient rather than discrete risk classes, over the endpoints −100 / 0 / +100. The ±100 endpoints are an in-house symmetric display choice, not published thresholds.
Map rendering is being corrected. The colour ramp and the "high NDR shock zone" proximity flag on this layer were configured for the service/abundance convention (red at −100, flag below −30 %), which is inverted relative to the data described above. The stress-test results are unaffected — the engine normalises the sign (darwin#6873) — but until the layer definition is updated, the map colours and the zone flag for this layer read backwards. This section will be updated with the corrected ramp.
Source
IMAGE-GNM — the Global Nutrient Model coupled to the IMAGE integrated-assessment model (PBL Netherlands Environmental Assessment Agency), river-nitrogen component. Scenario slices at 5-yearly bands.
Comparison with the WWF Risk Filter Suite
This is a forward-looking, horizon-resolved scenario layer and has no equivalent in the WWF Risk Filter Suite, which publishes present-day indicators only. Treat it as a Darwin extension that projects an ecosystem-service trajectory WWF does not itself produce.
Legend
Symbolised field: Water purification shock (%)
Generated from darwin/layers/layer-ndr-shock-ssp{1,3}-{2035,2050,2080}.toml (develop).