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I've been working with SAGA GIS to calculate the Topographic Wetness Index of an alpine catchment. I am using the tool Terrain analysis > Hydrology > SAGA Wetness Index because, compared to Terrain analysis > Hydrology > Topographic Wetness Index (TWI), the output is not a one-pixel wide film but a (to me) more realistic broader network of streams.

My input is a DEM (with sinks filled) that was derived from airborne LiDAR data and has a resolution of 1x1m. I don't have a Weights raster which is an optional input. Now my problem is that there are six parameters that shall be chosen before the calculation:

  • Suction (default = 10)
  • Type of Area (default = square root of catchment area; other options total catchment area or specific catchment area)
  • Type of Slope (default = catchment slope, other option local slope)
  • Minimum Slope (default = 0)
  • Offset Slope (default = 0.1)
  • Slope Weighting (default = 1)

Unfortunately, I didn't find much information about them and I am unsure how to decide which ones to use for my site. Both cited publications (Boehner et al 2002 and Boehner & Selige 2006) lack an explanation of these parameters and the SAGA tool itself has hardly any description (see http://www.saga-gis.org/saga_tool_doc/6.3.0/ta_hydrology_15.html).

The latter, for example, says that for Suction "the lower this value is the stronger is the suction effect" but what is this suction effect and how strong should it be in my area?

References:

  • Boehner, J., Koethe, R. Conrad, O., Gross, J., Ringeler, A., Selige, T. (2002): Soil Regionalisation by Means of Terrain Analysis and Process Parameterisation. In: Micheli, E., Nachtergaele, F., Montanarella, L. [Ed.]: Soil Classification 2001. European Soil Bureau, Research Report No. 7, EUR 20398 EN, Luxembourg. pp.213-222. http://eusoils.jrc.ec.europa.eu/ESDB_Archive/eusoils_docs/esb_rr/EUR22646EN.pdf.
  • Boehner, J., Selige, T. (2006): Spatial prediction of soil attributes using terrain analysis and climate regionalisation. In: Boehner, J., McCloy, K.R., Strobl, J. [Eds.]: SAGA - Analysis and Modelling Applications, Goettinger Geographische Abhandlungen, Goettingen: 13-28. http://downloads.sourceforge.net/saga-gis/gga115_02.pdf.
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After lots of calculations and experiments I think I can (at least partly) answer my own question:

  • Suction: according to the source code (Conrad 2014), a suction of 10 is "average", a smaller value, e.g. 5 means a higher suction and a higher value, e.g. 15 means a lower suction.
  • Type of Area: total catchment area is the unaltered version of the area, root of catchment area is the square root of the total catchment area and specific catchment area is the total catchment area divided by the cell size. Square root... leads to lower (drier) results than total catchment area.
  • Type of Slope: catchment slope is the unaltered version of the slope, local slope causes the slope to be calculated for each cell. Local slope gives a more coarse result.
  • Minimum Slope: all values smaller than this value will be set to this value. A smaller value, e.g. 0, leads to higher WI maximum values.
  • Offset Slope: is added to each slope value. No apparent influence on maximum values.
  • Slope Weighting: not sure. In the code, it is used for a power function. d = pow(Suction, Slope_Weight * Slope); but I couldn't make out what influence it has on the results.

I hope this will save someone some time.

Reference:

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