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

[Erin Hanan]

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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