Watershed Analysis Guide: Delineation, Flow Direction (D8), and Recharge Prioritisation
Rain GEO SAT's Expert workspace ingests high-resolution DEM from ISRO Bhuvan (30 m Cartosat) and NASA NASADEM (30 m SRTM), delineates the contributing watershed for any point, and identifies recharge priority zones using D8 flow direction and accumulation.
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The D8 flow direction algorithm
D8 assigns flow from each cell to the steepest of its 8 neighbours. It is the standard used in ArcGIS Hydrology, QGIS GRASS r.watershed, and Rain GEO SAT's Flow Direction + Drainage Path Engine. Multi-flow direction (MFD, D∞) is available in Expert mode for very flat terrain.
Recharge priority zone mapping
The Expert engine overlays flow accumulation (high value = drainage line), slope (low value = flat), soil texture (sandy/loamy = higher infiltration), and NDVI (bare = higher direct infiltration) to produce a 5-class priority raster from 'excellent recharge zone' to 'runoff dominated'.
- Bhuvan Cartosat DEM — 30 m India coverage
- NASADEM — 30 m global fallback
- Mapbox DEM — 10 m urban tiles where available
- SoilGrids texture — for infiltration classification
- Sentinel-2 NDVI — 10 m vegetation index
Frequently asked questions
- What resolution DEM is needed for a 1 ha plot?
- 10 m or finer. Rain GEO SAT auto-selects the highest resolution available. For urban rooftops, LiDAR (1 m) is optionally supported when uploaded.
Ready to apply this on your site?
Rain GEO SAT calculators use IMD, NASA POWER, Open-Meteo, and CGWB data to deliver engineer-grade rainwater harvesting and groundwater recharge designs.
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