[GRASS-user] r.flow: units for flow accumulation [RESOLVED]

Rich Shepard rshepard at appl-ecosys.com
Mon Oct 22 05:52:47 PDT 2018

On Sat, 20 Oct 2018, Micha Silver wrote:

> I have always found that the best way to do watershed delineation, to get
> streams and flow accumulation is to set the computational region a *bit
> larger* than the basin you are examining, and use a DEM that covers the
> full region. i.e. don't second guess the module. Then all pixels within
> the basin will have positive flow accum values. (Only pixels outside the
> main basin, which are in partially covered, adjacent basins will have
> negative values).
> The final step could be to mask out all areas outside the basin of interest.


   To close this thread: I worked out the details that resolved the issue.

   Briefly, I set the region and mask to the whole basin, used r.grow to
expand the size of the project area by 100m (my assumption of 'bit larger'
with the DEM cell resolution at 1m), set the region to this expanded area
and kept the mask for the project area as input to r.watershed.

   As I wrote before, the area of interest is only 2.1% of the basin and
quite flat (other than a 26m bluff separating the riparian from upland
areas). The resulting accumulation map shows the accumulation areas for
almost all cells to be <25m, which makes sense for this place.

Thanks very much for the help,


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