[GRASS-user] Use of r.watershed

Wed Jan 23 13:19:45 EST 2008

On Wednesday 23 January 2008, hydro at hydromodeling.com wrote:
> Thanks for pointing out the excerpt from the manual.  So, based on
> that it seems that r.watershed will take longer to run than the
> ArcHydro tools.  How much longer? I guess I will just let it run and
> see if I can find out.  Then I will run r.terraflow and compare the
> differences between ArcHydro, r.terrafow and r.watershed.
>
> 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.

Here is a small comparison I did a while back on watershed delineation, 
comparing r.watershed, r.terraflow, and basin from Arc/Info:

http://casoilresource.lawr.ucdavis.edu/drupal/node/284

note that it is a little sparse on the details, and could use some better 
figures. see code examples for explanation.

cheers,

Dylan

> Quoting M S <mseibel at gmail.com>:
> > 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.h
> >tml
> >
> >
> > 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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-- 
Dylan Beaudette
Soil Resource Laboratory
http://casoilresource.lawr.ucdavis.edu/
University of California at Davis
530.754.7341