[GRASS-user] Use of r.watershed

[Hamish]

hydromodeling.com wrote:
> > I have also tried using the -m and -4 flags placed after the
> > r.watershed part.

On modern systems using -m will slow it down a lot for minor memory
savings. Don't use it. I wouldn't bother with -4 either.

> r.terraflow

note as far as output basins go this calculates something different-
sink-watersheds not catchment watersheds. see
  http://thread.gmane.org/gmane.comp.gis.grass.user/21564/focus=21579

But r.terraflow should run in minutes.


> If anyone has used r.watershed on a comparably sized DEM, I would be 
> interested to hear how long it took to run.  By the way my machine
> has a 2.8Ghz P4 processor and 1.25G of RAM.

Yes I have run on something that size, and bigger. It could take up to
two days to complete, but it gets there with nice results. I usually
set it up to run on a friday afternoon and it's done by the time I get
back on monday. Setting the process priority low (ie high renice value)
helps if you leave it running during the week.


To get started, you might try the examples given for the Spearfish
dataset in the help page and on the wiki,

http://download.osgeo.org/grass/grass63/manuals/html63_user/r.watershed.html
  (help page home is in transition- broken link right now)
  http://grass.gdf-hannover.de/wiki/Psmap_flooding_example



Hamish



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