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I need to find the basin of a small river and this is what I did:

  • I found the drainage direction with r.watershed
  • I used r.water.outlet to find the basin for the coordinates of the water outlet point.

The problem is that this works for some bigger rivers but not for the one I need.

I saw that in the output "stream" of r.watershed my river doesn't appear, could be it the problem? What can I do? Here the images of "accumulation" form r.watershe in which you can see the river and the images of "stream" where there river doesn't appear.

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Are you sure that the drainage point lies exactly on a stream line? Zoom in close to the streams raster (from r.watershed) and get the X-Y coordinates of the center of one pixel on the streams line. Then use those coordinates, together with the drainage direction raster as input to r.water.outlet, and it should work.

If you want to see the actual stream lines in your small basin, you'll need to rerun r.watershed with a lower threshold value.

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  • Thank you very much. The problem was that there wasn't my river in the "stream" output as you can see in the first image I uploaded so I couldn't center the pixel. Using a lower threshold I resolved. Thank you again. – Elena Aragona Aug 25 '20 at 7:59

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