[GIS] Topographic Wetness Index

arcgis-desktopqgissaga

Is there any option to SAGA to calculate the Topographic Wetness Index ?
I have a big area and I tried many times with SAGA, the last time I thought the results was ok (before SAGA would crash or give me a empty raster), however I just noticed that for a small part of my study area the TWI raster is empty.

For the last TWI created in SAGA I decided to calculate the slope also in SAGA, but I still have problems with the file. The area with empty cells is near the right margin of the raster.This is how it looks like (black cells are NODATA cells):
enter image description here

I need to recalculate this asap but I can't find any another solution (in arcgis or qgis).

Edit/Update

I've just saw that I have the same problem in other small areas of the image, that are +/- flat. After overlaying with the SRTM, I can see that when the SRTM value is negative that the TWI cells have NODATA values. I guess this is the reason for the error.

Best Answer

You should not be getting null or empty values. How exactly did you try to calculate TWI? In SAGA, module Terrain Analysis - Hydrology -> SAGA Wetness Index should work, so long as all of your input files are correct. i.e slope needs to be in radians, what about sinks?, drainage networks need to be correct, DEM needs to be projected (good discussion on projections here), etc...

The general equation:

TWI/CTI = ln(a / (tan (beta))

where; a=Upland contributing area: [(flow accumulation + 1 ) * (cellsize)]

beta=slope in radians.

References:

Moore, ID., P.E. Gessler, G.A. Nielsen, and G.A. Petersen (1993) Terrain attributes: estimation methods and scale effects. In Modeling Change in Environmental Systems, edited by A.J. Jakeman M.B. Beck and M. McAleer Wiley , London, pp. 189 - 214.
Gessler, P.E., I.D. Moore, N.J. McKenzie, and P.J. Ryan. (1995). Soil-landscape modeling and spatial prediction of soil attributes. International Journal of GIS. 9(4):421-432.

Related Solutions

[GIS] Calculate Topographic wetness index in SAGA

First, you need to calculate at least the slope. F.ex I have the following data: enter image description here

Then put the correct data as variables to the module: enter image description here

And at last you should get the result: enter image description here

UPDATE

With Catchment Area as input

enter image description here

the results are:

enter image description here

Understanding Negative Values in Compound Topographic Index (CTI) / Topographic Wetness Index (TWI)

You should not be seeing negative values in the CTI. Since you did not provide a reproducible example I cannot speculate as to why you are getting incorrect results. The expected range is not limited 1-10. The range will be defined by flow accumulation which is influenced by the size of the basins that are accumulating flow. The index does not rely on washed boundaries so the extent of the DEM should not matter.

The common implementation of CTI can be shown as: CTI = ln(a / (tan (beta)) where; a=Upland contributing area [(flow accumulation + 1 ) * (cellsize)]and beta=slope in radians.

Here are the raster algebra steps to calculate CTI (you will have to translate syntax to your version of ArcGIS):

cellsize=30
fd = flowdirection(dem)
sca = flowaccumulation(fd)
slope = ( slope(dem) * 1.570796 ) / 90
tan_slp = con( slope > 0, tan(slope), 0.001 )
sca_scaled = ( sca + 1 ) * cellsize
cti = ln ( sca_scaled / tan_slp )

Best Answer

Related Solutions

[GIS] Calculate Topographic wetness index in SAGA

Understanding Negative Values in Compound Topographic Index (CTI) / Topographic Wetness Index (TWI)

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