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"