[GRASS-user] LS faktor in r.watershed

[Stefan Luedtke]

Thanks a lot, that helped and made my day. 

Cheers, 

Stefan

On Wed, 2012-12-12 at 21:52 +0100, Markus Metz wrote: 

> On Wed, Dec 12, 2012 at 10:16 AM, Stefan Luedtke
> <sluedtke at gfz-potsdam.de> wrote:
> > Dear friends,
> >
> > I am working with the r.watershed module in order to compute the LS Faktor
> > for (R)evised-Universal-Soil-Loss-Equation. For this option in the module, I
> > have to set the threshold for the minimum size of exterior watersheds. There
> > are many options out there to compute the LS-factor, but I could not find
> > the paper/report/.. that describes the algorithm used in GRASS.
> >
> > Revised Universal Soil Loss Equation for Western Rangelands (Weltz et al.
> > 1987).
> >
> > I want to understand why I need to set this threshold if I want r.watershed
> > to compute the LS -factor. So if somebody could help me with:
> >
> > a) the reference, so I can check by my self.
> > or
> > b) a short explanation why this parameter must be set.
> 
> The LS and S factor calculation is only done for cells that are not
> streams. I guess the reason is that soil loss does not apply to
> streams (rivers). Streams in turn are determined using the threshold
> option: all cells receiving more than threshold accumulated surface
> flow are regarded as streams. A reasonable threshold depends on 1) the
> resolution of the DEM, 2) the characteristics of the area under
> consideration, 3) the context of the analysis.
> 
> >
> > By the way, I tested r.watershed with several thresholds (resulting in 1 big
> > subbasin up to several hundreds) but the results for the Ls factor hardĺy
> > changed.
> 
> Because it should have changed only for cells that are streams / not
> streams depending on the threshold.
> 
> HTH,
> 
> Markus M
> 
> >
> > Thanks in advance,
> >
> > Stefan
> >
> >
> > _______________________________________________
> > grass-user mailing list
> > grass-user at lists.osgeo.org
> > http://lists.osgeo.org/mailman/listinfo/grass-user
> >
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