[GRASS-user] Use of r.watershed

[M S]

If you are in a relatively flat area, there needs to be consideration given
to the algorithm (to my knowledge there is no "choice" in the ArcGIS suite)
used to do the most important thing:  delineate the watershed basin.  Since
the question was asked about differences, this may apply if your terrain is
relatively flat.

>From the r.watershed manual, "If watershed divides contain flat to low
slope, *r.watershed* will generate better basin results than *r.terraflow*.
(*r.terraflow* uses the same type of algorithm as ESRI's ArcGIS watershed
software which fails under these conditions.)"
http://download.osgeo.org/grass/grass63/manuals/html63_user/r.watershed.html


Mark

On Jan 23, 2008 11:42 AM, <hydro at hydromodeling.com> wrote:

> Thanks.  I will try and change my threshold value this evening.
>
> A little background, I have completed this phase of the project at
> "work" using ArcHydro and it took about half a day total to run
> through the tools (fill, flow direction, flow accumulation, etc.).  I
> am trying to now do the same project at home using open source tools
> as a means of learning grass and as a comparison.  I wonder if the
> r.watershed algorithms differ enough from the ArcHydro ones to make
> execution times very different?  Any thoughts?
>
> Quoting Charles Ehlschlaeger <c.ehlschlaeger at insightbb.com>:
>
> > r.watershed will take a long time for a 8,600x10,300 cell DEM under any
> > circumstance. Your threshold value (400) might also be slowing things
> down.
> >
> > How large should your smallest exterior watershed basin be? If your grid
> > cells are 10mx10m, your basins will be 400x10mx10m = 40,000 m^2: not
> that
> > large. The larger the th value, the faster the program will run.
> >
> > Sincerely, chuck
> >
> > Chuck Ehlschlaeger, Geospatial Science Consultant
> > PERTAN GROUP, http://pertan.com/
> > c.ehlschlaeger at insightbb.com, 309-255-2727
> > http://chuck.ehlschlaeger.info/
> >
> >
> > -----Original Message-----
> > From: grass-user-bounces at lists.osgeo.org
> > [mailto:grass-user-bounces at lists.osgeo.org] On Behalf Of
> > hydro at hydromodeling.com
> > Sent: Wednesday, January 23, 2008 8:52 AM
> > To: Charles Ehlschlaeger
> > Cc: grass-user at lists.osgeo.org
> > Subject: RE: [GRASS-user] Use of r.watershed
> >
> > Chuck,
> >
> > Thanks for your reply.  The DEM has 8,627 rows and 10,270 columns.
> > The command line statement I used is:
> >
> > r.watershed el=fill th=400 ac=fac ba=basins st=str
> >
> > I have also tried using the -m and -4 flags placed after the r.watershed
> > part.
> >
> > Thanks,
> >
> > Matt
> >
> > Quoting Charles Ehlschlaeger <c.ehlschlaeger at insightbb.com>:
> >
> >> How many rows and columns are in the input map? Also include the exact
> >> command line statement you use to run the program.
> >>
> >> Sincerely, chuck
> >>
> >> Chuck Ehlschlaeger, Geospatial Science Consultant
> >> PERTAN GROUP, http://pertan.com/
> >> c.ehlschlaeger at insightbb.com, 309-255-2727
> >> http://chuck.ehlschlaeger.info/
> >>
> >>
> >> -----Original Message-----
> >> From: grass-user-bounces at lists.osgeo.org
> >> [mailto:grass-user-bounces at lists.osgeo.org] On Behalf Of
> >> hydro at hydromodeling.com
> >> Sent: Wednesday, January 23, 2008 7:15 AM
> >> To: grass-user at lists.osgeo.org
> >> Subject: [GRASS-user] Use of r.watershed
> >>
> >> Hello All,
> >>
> >> I am new to GRASS but have been using GIS (mainly ESRI products) for
> >> quite a few years.  I am trying to learn how to use GRASS to carry out
> >> hydrology related analyses.  Currently, I am trying to use the
> >> r.watershed command to delineate the basins for a watershed.  My
> >> question is, how long would you expect it to take to run?  I am
> >> wondering if I am doing it wrong or if it just takes longer than I
> >> would expect.  The DEM is about 170MB TIFF before I import it to
> >> GRASS.  I have tried to run it a few times using different options,
> >> each time letting it run for 3 - 4 hours before killing it. I have
> >> tried both the -m and -4 flags.  Any input would be appreciated.
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