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i am currently working on a project which involves the quantification of the wetness in certain areas. However it produces quite different results from what i expected. I'll describe the steps i have taken. To do so i used the ASTER global DEMv2 and clipped it to the parts to be examined. I have to clip the raster, because using the original DEM results in various errors saying "memory allocation failed". ASTER DEM Then i calculated the slope and Catchement area via the SAGA function Catchment Area (Mass-Flux-Method, parameter: flow-spit method area cell). I also tried the Parallel and recursive method, however the results are mostly the same. Then i calculated the Topographic Wetness Index, but the results look very mixed (somewhere between a rainbow and random fuzzy bits) and values are always negative. example of the resulting TWI I expected that the resulting raster should look like and display index values like the following image in the pdf PDF, page 14.

Has someone here experience with the quantification of topographic wetness in an area and could provide a tutorial how to do it right? I would appreciate using SAGA or QGIS, but i also have ArcGis 9 at Hand.

By the way: If i just use the calculate the SAGA wetness index with the clipped elevation data above, SAGA always keeps crashing (i believe because of the large no-data areas, because smaller smarts work)

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Could you please provide a screenshot showing what data you chose when running Topographic Wetness Index? I suspect that you put wrong data as variables to the module. – Vladimir Nov 6 '12 at 10:46
i used the slope and the catchment area as calculated by parallel processing. – Curlew Nov 6 '12 at 10:55
How many rows and cols do you have there? An alternative may be GRASS GIS which handles very large datasets, see – markusN Nov 6 '12 at 15:13
How many rows and cols do you have there? An alternative may be GRASS GIS which handles very large datasets, see – markusN Nov 6 '12 at 15:14
up vote 9 down vote accepted

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


With Catchment Area as input

enter image description here

the results are:

enter image description here

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interesting and nice looking result. Why did you use the srtm elevation data as catchment area? I always used the catchment area as calculated by the various methods in SAGA – Curlew Nov 6 '12 at 10:56
How did you calculate the catchment area? Which method (and maybe parameters) did you use? – Curlew Nov 6 '12 at 12:03
module Catchment Area (Parallel) with only one input - Elevation: srtm. – Vladimir Nov 6 '12 at 12:06
Thank you for the post, Vladimir. Do you know the algorithm that saga uses to calculate catchment area? I would like to do this in ArcMap using the raster calculator, as ArcMap does not have a built in catchment area function. – SoilSciGuy Nov 7 '12 at 18:27
I believe that you are looking for flowaccumliation. Here are directions for calculating CTI in the raster calculator that were posted on the ESRI forum. – Jeffrey Evans Nov 7 '12 at 18:42

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