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I need the LS grid for an area for RUSLE calculation.
I ran both SAGAs Slope Length from Terrain Analysis - Hydrology and GRASS's r.watershed.
The resulting LS factor grids are completely different. The documentation of both GRASS and SAGA does not describe what calculations these modules are doing. Why are there such stark differences in outputs?
Is SAGA Slope Length output is the same thing as LS factor?
SAGA:
GRASS:
The LS-factor is an important variable to estimate average erosion - annual soil loss (as in RUSLE), and based on experimental data. It becomes greater when the slope is steeper and the effective slope length is shorter. Slope length is the horizontal distance from the ridge to the point where deposition starts to occur. At a given cell, the length to the nearby steepest cell is successively calculated unless all nearby slope is less than half of their own slope.
A suggested overall workflow with a pair of input/output and corresponding SAGA tool are summarized below. As you see, there are many steps and involve several intermediate products.
Slope, Aspect, Curvature tool
Input: DEM
Output: Slope (measured in radian)
Fill Sink (Wang & Liu) tool
Input: DEM
Output: DEM without sink
Catchment Area tool
Input: DEM without sink
Output: Flow Accumulation (Total catchment area)
Flow width and specific catchment area tool
Input: DEM without sink, Flow Accumulation (Total catchment area)
Output: Specific catchment area
LS-factor tool
Input: Slope, Specific catchment area
Output: LS-factor
To speed up the process, you can try Basic terrain analysis tool, which is in the SAGA - Terrain Analysis - Morphometry group. It does all the necessary steps automatically to produce LS-factor.
Catchment area and Flow width and specific catchment area calculations.
LS factortool in the same Hydrology group. You will just need a set of slope raster and specific catchment area raster.Slope Lengthbecause that is the simplest. Are you saying that it is not LS factor? If it is not, then what is it, because its results are very similar to the Bob Hicky method. Where can I find the description of what each SAGA tool is supposed to do. The official docs has nothing about the purpose of each.Basic Terrain Analysistool worked. And yes, you are quite right about the Slope Length being (L). I think both SAGA and GRASS (default setting) calculate LS-factor based on MFD, but I may be wrong,