Catchment area calculation in QGIS

The method is similar to any other GIS method. The steps are as follows; the main step is highlighted in bold fonts. It is up to you to beautify the map.

  1. Get the DEM files (for eg. from https://earthexplorer.usgs.gov/)
  2. Clip it if necessary. This can reduce the computation time.
  3. Reproject the lat/long DEM file to metric base e.g. UTM (this is essential for any area based analysis).
    Raster>Projection> Wrap
  4. Make sure the projection coordinate used in DEM and your data frame matches.
  5. Fill the projected DEM for any inconsistent data.
    Processing toolbox>SAGA>Terrain analysis(hydrology)>Fill sinks (wang & liu)
  6. Calculate Strahaler order for the filled raster. This is necessary to know the river lines. The intake location should lie exactly on the river line shown by Strahaler .
    Processing toolbox>SAGA>Terrain analysis(channels)>Strahaler order
  7. Delineate the catchment using upslope command. Choose the X,Y coordinate lying on the line calculated by Strahaler .
    Processing toolbox>SAGA>Terraine analysis>Terrain analysis(hydrology)>Upslope area
  8. Using Raster to vector convert the delineated raster to vector.
    Raster>Conversion>Raster to vector
  9. Go to attribute table, add new field with formula “area”. The area is calculated in square meters.

 

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