Topographic Wetness Index, Catchment area and Catchment slope (parallel processing)

2014-03-20
2014-03-20
  • Henri Riihimäki

    Hi, all.

    I'm trying to calculate Topographic Wetness Index with SAGA GIS.

    I was wondering how exactly is catchment slope calculated in Catchment Area module (parallel)? I'm having hard time finding any documentation on slope calculation, that would make this clearer to me.

    I'm worried that I'm using wrong type of slope when executing topographic wetness module.
    Most of the literature related to TWI express TWI as Ln(a/tanB) where a is the spesific catchment area and tanB is the local slope. How does catchment slope, calculated here, correspond to this?

     
    Last edit: Henri Riihimäki 2014-03-20
  • Volker Wichmann

    Volker Wichmann - 2014-03-20

    The catchment slope output grid of the Catchment Area (Parallel) module is computed like this: for each cell, the local slope is calculated using the approach of Zevenbergen & Thorne. These slope values are accumulated downslope. Finally, for each cell the accumulated slope values are divided by the derived catchment area of the cell. The unit of the grid are radians.

    The Topographic Wetness Index (TWI) module requires a normal slope grid as input. You can calulate such a slope grid with the Terrain Analysis-Morphometry / Slope, Aspect, Curvature module. Use radians as slope unit as the TWI module expects the slope expressed in radians.

     
  • Henri Riihimäki

    Ok, I expected it to be something like this. Thank you very much for the clarification.

     

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