I am using the GrassGIS r.stream.extract extension to delineate a stream network with the options 'mexp' (Montgomery exponent) and 'd8cut' (Use SFD above this threshold). It works perfect so far and is a great improvement over r.watershed. My problem is that if I use r.stream.basins afterwards I cannot extract half basins (left and right site of a river) as it was possible in r.watershed. I wonder if there is a tool with whom I can do that? r.stream.extract delivers a flow direction map and the stream network.

Best Regards

1 Answer 1


What you refer to as half basins are often called hillslopes. I'm not aware of any tool in GRASS GIS that can extract these features from a stream network and a DEM, although it is possible that I have overlooked the correct tool. However, if you are open to using alternative open-source GIS then I can recommend the hillslopes tool in Whitebox GAT, for which I am a developer.

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The tool takes a raster streams file and a D8 flow pointer grid as input and outputs a raster for which each cell is assigned to the unique hillslope draining either to a particular side of a stream segment or to a stream head. I've simply vectorized the raster polygons and streams file in the image above for visualization purposes.

enter image description here

If it turns out that you require a GRASS based solution, just let me know in a comment and I'll delete my answer, otherwise, I hope this helps to solve your problem.

  • Excellent answer! I also cannot find a similar tool in GRASS. Regardless, I would keep this answer in as it provides a very clear solution for another software which could be of use to others.
    – Joseph
    Jan 14, 2015 at 15:45
  • It works perfect with my data from GRASS after I changed the flow direction map. Thank you very much and I will try to check out WhiteboxGIS a little more as soon as possible.
    – MonteCarlo
    Jan 14, 2015 at 17:20
  • @Joseph Thanks very much. I was a little surprised when I looked into it that there wasn't a similar tool in GRASS because that's where I thought I had got the idea many years ago to make the tool in what was then TAS. Oh well. Jan 14, 2015 at 21:10
  • @MonteCarlo I'm glad that it worked. I'm sorry, I had thought about the fact that you'd probably need to redo your D8 flow pointer raster afterwards (since GRASS likely uses a different ordering scheme than Whitebox) but didn't remember to add it into the answer. Regardless, I'm glad that it was able to be of use to you. Jan 14, 2015 at 21:12

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