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?
- 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.