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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: enter image description here

GRASS: enter image description here

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  • Are you asking about LS-factor or slope length? If you need LS-factor, SAGA has LS factor tool in the same Hydrology group. You will just need a set of slope raster and specific catchment area raster.
    – Kazuhito
    Commented Dec 21, 2021 at 21:45
  • @Kazuhito what I need is LS factor, that is used in RUSLE. SAGA has multiple tools for LS factor and they all are producing very very different results. I used Slope Length because 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. Commented Dec 21, 2021 at 23:12
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    Yes, they are different. LS factor is based on the Steepness (S) - Slope Length (L) ratio (ref. LS Factor in SAGA (slope units)) while Slope Length (L) itself... ugh... please visit SAGA Team's forum Slope Length.
    – Kazuhito
    Commented Dec 21, 2021 at 23:32
  • @Kazuhito so 1. using Slope Length output for RUSLE is incorrect? 2. Where is the documentation for LS Factor from SAGA as it has many inputs and I am not sure what to provide. As you can see from other questions on the SE this is all very confusing, can you write a detailed answer on this, if you have time? Commented Dec 22, 2021 at 16:52
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    I am glad to know that Basic Terrain Analysis tool 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,
    – Kazuhito
    Commented Dec 23, 2021 at 19:13

1 Answer 1

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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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  • Thanks for the answer. Can SAGA produce LS factor with SFD. Currently, Basic terrain analysis is producing MFD LS Factor. Commented Jan 3, 2022 at 14:19
  • @ar-siddiqui I honestly do not know but I guess it is technically possible. The point is to choose MFD (or FD8 in SAGA terminology) in both of Catchment area and Flow width and specific catchment area calculations.
    – Kazuhito
    Commented Jan 4, 2022 at 11:37

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