skills/terrain-hydrology/references/authoritative-sources.md

# Authoritative sources

- Last verified: 2026-07-19
- Review cadence: every 6 months
- Refresh triggers: GDAL, GRASS GIS, or WhiteboxTools major release; DEM product update

## Canonical sources

- [GDAL gdaldem documentation](https://gdal.org/en/stable/programs/gdaldem.html) — slope, aspect, hillshade, and color-relief behavior.
- [GRASS GIS r.watershed manual](https://grass.osgeo.org/grass-stable/manuals/r.watershed.html) — flow accumulation, drainage, streams, and basins.
- [WhiteboxTools user manual](https://jblindsay.github.io/ghrg/WhiteboxTools/index.html) — terrain and hydrological processing algorithms.

Record DEM product and date, horizontal and vertical CRS and units, conditioning method, flow algorithm, thresholds, tool versions, and validation controls.