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I am interested in using r.watershed as integrated in QGIS for a course I teach, but am noticing discrepancies in the flow accumulation (facc) rasters and resulting stream networks that I get from QGIS-GRASS (r.watershed) compared with ArcGIS. The r.watershed stream network is discontinuous (see blue-green cells below), compared with the continuous network of ArcGIS (red cells below; red layer plotted beneath the blue one). The facc grids were generated using the same DEM filled with ArcGIS, and both use a threshold of 500 cells for streams. The facc values from QGIS-GRASS also show high values surrounded by lower values at the discontinuities, indicating what should be a sink, but ArcGIS does not see them as sinks on this filled DEM, and filling the DEM again does not change the result. I am using QGIS V 3.12.2 (see image below for full version information).

I have seen other answers to filling in a discontinuous stream network (e.g. here), but don't agree that you should have to paste over these gaps with any further raster-vector processing, since facc values should always decrease going downstream.
I also find that the r.water.outlet function on the same facc grid does not give a correct watershed boundary for some pour points (but does fine on others).

Is there an easy solution, and/or is this discontinuity in r.watershed a common issue? Thank you!

The original DEM and the resulting facc rasters can be found here: https://drive.google.com/open?id=17_RPXoyR7IVTo7K6UQ_92yWEAMny6a0i&authuser=tbiggs%40sdsu.edu&usp=drive_fs

Stream network from r.watershed(blue-green) and ArcGIS (red).

Software versions

  • Note, I made a typo " facc values should always decrease going downstream." should be " facc values should always increase going downstream." Thanks! – Trent Biggs Aug 11 at 17:21
  • Are you sure your raster "dem_arcgisfill" is actually filled ? (Did you use the "Fill sinks (wang & Lui) tool on your raster in QGIS ?). I can't use the function r.watershed right now because of an issue with grass on my computer but I've used the tool "catchment area" instead and the flow accumulation raster looks like the one you've created with ArcGis – wanderzen Aug 11 at 21:14
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This is related with the option of flow direction. It seems you performed the r.watershed command without enabling Single Flow Direction, so the default is Multiple Flow Direction.

enter image description here

How does it affect the stream network creation is unknown for me, I never work with that part, but running the command with the Single Flow Direction option gives me no gaps in the accumulation maps

Blue is MFD and red is SFD. enter image description here

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  • I'm in precisely the same situation!, with QGIS, Grass and SAGA.The flow direction is a very important process that will determine your result, there are many flow direction algorithms, with different conceptualizations simple flow direction, multiple flow directions, flow tracing. I recommend you to try also with SAGA GIS is more direct than GRASS, in my case, I have noticed some differences but not significant. – Luis Perez Sep 23 at 15:03

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