Quote from vallstn on 18 August 2026, 17:14

This topic covers catchment delineation in GRASS, SAGA and PCRaster Plugins 

Study Material – https://drive.google.com/drive/folders/1oVQix2ZKmenuD08gPJzJpt7boMabIy9O?usp=drive_link

PCRaster work flow

1. Add the dem to QGIS Interface

2. Clip the dem to aoi file ( to Speed up the operation)

3. Reproject the dem to EPSG 32644

4. Change project crs to EPSG 32644

5. Remove all other layers except reprojected dem

6. Convert the dem to pcraster format – Processing Toolbox > PCRaster > Data management > Convert to PCRaster Format – Select type as scalar

7. Create Flow Direction Map – Processing Toolbox > PCRaster > Hydrological and material transport operations > lddcreate – Select dem.map and don’t change other parameters, enter output name as “flowdirection”

8. Create Channel Network – Processing Toolbox > PCRaster > Hydrological and material transport operations > streamorder

9. Import sample.csv

10. By observing the stream order layer and sample point outlet find out the exact pour point near to sample outlet, and note down the values in “Notepad” as x, y, 1
and save as outlet point.txt

11. Convert the outlet point.txt to pcraster format – Processing Toolbox > PCRaster > Data management > Column file to PCRaster Map – select type as nominal

12. Calculate catchment – Processing Toolbox > PCRaster > Hydrological and material transport operations > catchment – use flow direction.map and outlet.map

Steps for Saga Plugin

1. Import dem, clip to “aoi” and reproject to EPSG:32644, Set Project CRS to EPSG:32644

2. Create filled dem – Processing Toolbox > SAGA Next Gen > Terrain Analysis > Fill Sinks (Wang & Liu) – Select Deterministic 8 from Flow Routing – Disable watershed basins

3. Create Flow accumulation – Processing Toolbox > SAGA Next Gen > Terrain Analysis > Flow Accumulation (One Step) – Select Deterministic 8 from Flow Routing – Disable Specific Catchment Area

4. Import outlet.csv and note the latitude, longitude of the nearest point having high accumulation.

4. Catchment – Processing Toolbox > SAGA Next Gen > Terrain Analysis > Upslope Area – Enter Longitude, latitude in Target x, y coordinate – select dem in elevation field – Select Deterministic 8 from Flow Routing in Method – Enter name for upslope area.

Happy Learning