[GRASS-user] Problem with half basins becoming fragmented using r.watershed

[Massi Alvioli]

Hi Erin,

in my experience the small half-basins are not in error, they
appearance depends only on the particular digital elevation
model you are using. And on the threshold you are using in
r.watershed, of course. Did you tray scanning a wide range
of thedholds to make sense of your results?

A similar issue was discussed in details in these papers of ours,
along with possible solutions to the problem of small basins.
We actually discuss slope units, which, essentially, are half basins
obtained from r.watershed with different theshold values
in different regions of a study area:

https://doi.org/10.5194/gmd-9-3975-2016
http://2018.geomorphometry.org/Alvioli_Marchesini_Guzzetti_2018_geomorphometry.pdf

the method presented in the papers is based on the software
r.slopeunits, available here:

http://geomorphology.irpi.cnr.it/tools/slope-units

and one can get simple half basins by ignoring the option "cleansize",
or specifying from value for "cleansize", which
would remove small slope units form the final map using a few different methods.

hope this helps.


Massi

2018-07-13 19:06 GMT+02:00 Erin Hanan <ejhanan at gmail.com>:
> Hello,
>
> I am new to the list; please let me know if my question should be formatted
> differently.
>
> I have been running r.watershed on a large (>400 km2) basin using a 10 m
> resolution dem. I am finding that it produces some half basins that are
> either fragmented or very small (i.e., just a few pixels). This seems to
> happen mostly in areas bordering the stream, though I do sometimes get half
> basins that are also not connected to the stream. I'm attaching a zoomed in
> screen shot that illustrates some very strange sliver and fragmented basins
> that are bordering larger, more appropriately delineated half basins. In
> this case, the inappropriate basins all border the stream.
>
> I have tried running r.watershed using GRASS6.4 and GRASS7. I have also
> tried using SFD, MFD with a range of convergence values, and the -4 flag.
> These options produced different half basin delineations, but the problem
> with sliver basins and basin fragments has not gone away.
>
> I also checked the dem values for the inappropriate half basins, and they
> are very similar to the values for neighboring normal basins. So at this
> point, I'm not sure how to fix the problem. Any advice from the GRASS
> community would be greatly appreciated.
>
> Thank you,
>
> Erin
>
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