[GRASS-user] Use of r.watershed

Wed Jan 23 12:28:50 EST 2008

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.

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.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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