This layer shows a composite surface runoff risk index (0-1) that highlights where rainwater tends to concentrate and flow off the land instead of infiltrating. It combines three datasets computed on Google Earth Engine: flow accumulation from MERIT Hydro, impervious surface fraction from Resource Watch (30 m), and terrain slope from NASA SRTM (30 m). High values (warm colours) mark flat, sealed, water-converging areas; low values (green) mark permeable or steep terrain where water drains or infiltrates.
A continuous composite index (0-1) identifying land where surface water accumulates on impervious, flat ground. All computation runs server-side on Google Earth Engine; only the final 30 m raster is downloaded.
| Code | Provider | Resolution | Year |
|---|---|---|---|
MERIT/Hydro/v1_0_1 (band upa) | MERIT Hydro | ~90 m | static |
| projects/resource-watch-gee/cit_039_impervious_surfaces_30m_mosaic | Resource Watch | 30 m | static |
| USGS/SRTMGL1_003 | NASA/USGS SRTM | 30 m | 2000 |
Each input is normalized to [0, 1] and the three are multiplied: risk = normalize(log10(flow_acc)) × (impervious/100) × (1 - slope/45). Flow accumulation is log10-compressed (capped at log10 = 6), impervious surface fraction is scaled from its 0-100% range, and slope is inverted so flat terrain (0°) scores 1 and terrain at or above 45° scores 0. The product is projected to Web Mercator at 30 m scale and clipped to the area of interest.