GRASS GIS – Using SAGA or GRASS Tools for RUSLE LS Factor in QGIS

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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:
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GRASS:
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Best Answer

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.

  1. Slope, Aspect, Curvature tool

    Input: DEM

    Output: Slope (measured in radian)

  2. Fill Sink (Wang & Liu) tool

    Input: DEM

    Output: DEM without sink

  3. Catchment Area tool

    Input: DEM without sink

    Output: Flow Accumulation (Total catchment area)

  4. Flow width and specific catchment area tool

    Input: DEM without sink, Flow Accumulation (Total catchment area)

    Output: Specific catchment area

  5. 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.

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