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I am trying to process a DTM by using r.watershed in QGIS 2.14.3, with the GRASS GIS toolbox. It should be the file named: "Number of cells that drain through each cell".

I get some negative values, even if the shape of drainage line seems very correct. I think that the problem might be the number format (like float or integer).

I am working on a .tiff dem, but it was extracted by clipping from a .asc bigger dem.

How can I solve this?

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    if you look at the help it looks like flow accumulations can be negative. have you tried the -a option ('use positive flow accumulation even for likely underestimates')? – Steven Kay Jun 3 '16 at 21:09
  • Ok, thanks!, this has solved the problem! But I think this also means that my dem is too small for covering the whole basin, is it right? – giulio Jun 4 '16 at 7:03
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    Not necessarily. Flow accumulation is negative when some of the flow comes from outside the current region. The neg value is just a reminder that the real flow accum would be larger if you had a larger DEM. But your basin could still be fully covered. The cells with neg flow accumulation could be small sub-basins at the edges, outside your basin of interest. – Micha Jun 4 '16 at 13:51
  • Ok thanks, now I completed the flow acc elaboration. Seems that r.watershed is a very powerful tool, but I needed to control it better. Thanks a lot – giulio Jun 4 '16 at 19:54
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It was solved long ago in the comments, but better as an answer.

Quoting the manual of r.watershed:

Negative numbers indicate that those cells possibly have surface runoff from outside of the current geographic region. Thus, any cells with negative values cannot have their surface runoff [..] calculated accurately.

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