[GRASS-user] SRTM r.watershed at multiple scales

Michel Wortmann wortmann at pik-potsdam.de
Wed Aug 21 04:18:26 PDT 2013

Hi jencor808,
I can only recommend using SRTM for subbasin delineations. We are using 
it on a regular basis in our hydrological modelling working group. 
You'll have to play around with thresholds a little. I use different 
thresholds for different areas of my catchment (eg. in mountainous 
regions I'd want smaller subbasins). By masking those areas and run 
r.watershed several times, you can vary your thresholds and then patch 
the different subbasin rasters back together. As the r.watershed 
algorithm often leaves small areas at the edges uncovered, you'll have 
to fill those before or after patching, otherwise you'll have holes in 
your subbasin map. You can also get rid of small subbasins by converting 
your subbasin raster into a vector map and then run
v.clean remove small areas (check manual)

Hope it helps. I have got a python/GRASS script that is station-based 
that does the varying thresholds for you and all the cleaning etc. Let 
me know if you'd want to give that a try.

On 08/16/2013 06:51 PM, jencor808 wrote:
> Hello all~
> I'm working with SRTM data and trying to produce watershed maps in Honduras.
> For most of the country, the threshold seems to work great at 100000, but
> this is not the case on the shore - those watersheds are neglected likely
> due to their smaller scale. Does anyone have experience merging multiple
> scales? Does this make sense? I'd rather not reduce my threshold or the
> whole country because those watersheds seem too small.
> As an aside, what do people think about using SRTM for watershed mapping in
> the first place - given it is a surface model and not a DEM.
> --
> View this message in context: http://osgeo-org.1560.x6.nabble.com/SRTM-r-watershed-at-multiple-scales-tp5072884.html
> Sent from the Grass - Users mailing list archive at Nabble.com.
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