Tutorial: Stream & catchment delineation using GRASS tools
|Site:||IHE DELFT OPEN COURSEWARE|
|Course:||GIS OpenCourseWare for Hydrological Applications|
|Book:||Tutorial: Stream & catchment delineation using GRASS tools|
|Printed by:||Guest user|
|Date:||Tuesday, 19 October 2021, 10:42 AM|
The QGIS Processing Toolbox gives access to many useful GRASS algorithms.
In this tutorial we're going to use the GRASS tools to delineate the Rur river and its catchment.
After this tutorial you're able to:
- create a hydrologically correct DEM by filling the sinks
- calculate flow accumulation
- delineate streams using a flow accumulation threshold
- delineate a catchment of a specified outlet
2. Fill sinks in the DEM
The first step is to fill the sinks in the DEM. Sinks are artificial depressions that trap the water and prevent flow to the outlet.
1. Start QGIS Desktop with GRASS.
2. Add dem.tif to the Layers panel.
3. Open the Processing Toolbox panel: in the main menu go to Processing | Toolbox.
4. In the Processing Toolbox choose to GRASS | Raster (r.*) | r.fill.dir.
5. In the r.fill.dir dialogue choose the DEM layer as Elevation. Keep the default Output aspect direction format as grass. Save the Depressionless DEM as DEM_filled.tif (make sure to choose a GeoTiff) and uncheck the other output layers, because we don't need them.
6. Click Run. Close the dialogue after processing. Processing will take a while. Ignore the red warnings in the log.
More info about the algorithm can be found here.
7. Remove the DEM layer from the Layers panel so we only have the DEM_filled layer there to proceed with.
3. Calculate flow accumulation and flow direction
The next step is to calculate the accumulation of water over the filled DEM.
1. In the Processing Toolbox choose GRASS | Raster (r.*) | r.watershed.
The r.watershed tool has a lot of settings. Check the GRASS manual page for this tool to learn more about the settings of the algorithms.
2. In the r.watershed dialogue choose DEM_filled as Elevation. Set the Minimum size of exterior watershed basin to 500 pixels and check the box to Enable Single Flow Direction (D8) flow. Save the Number of cells that drain through each cell to accumulation.tif and save the Drainage direction to flowdir.tif. Uncheck the other outputs, we don't need them.
3. Click Run. Close the dialogue after processing. Ignore the warnings in red.
The flow direction layer is encoded using the GRASS definition: Drainage is 8 directions numbered counter-clockwise starting from 1 in north-east direction. The value 0 indicates that the cell is a depression area. Negative values indicate that surface runoff is leaving the boundaries of the current geographic region. The absolute value of these negative cells indicates the direction of flow. Therefore we need to convert the flowdir layer to absolute values.
4. In the main menu go to Raster | Raster Calculator.
5. In the Raster Calculator compose the following equation:
( "flowdir@1" < 0 ) * -1 * "flowdir@1" + ( "flowdir@1" >= 0 ) * "flowdir@1"