[GRASS-user] LS faktor in r.watershed

Markus Metz markus.metz.giswork at gmail.com
Wed Dec 12 12:52:08 PST 2012

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.


Markus M

> Thanks in advance,
> Stefan
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