I have derived Topographic Wetness Index (TWI)/CTI with ArcGIS 10.3 using 30m DEM resolution. Earlier, it was propagating negative values which were mainly confined to ridges. I have restructured the 'Raster Calculator' expression as below and got rid of negative values.

Ln(("Flow_acc"+ 1 ) * 900/"Tan_SlopeRad")

The below process diagram would acquaint you about steps I have taken to arrive at the positive integer values. Process Flow Diagram

and the resultant image clearly indicates values having 'No Data'. These values are mainly confined to Zero Degree Slope within River. Resultant TWI Image

Now, How would I deal with values having ''No Data'???


Zero slope will lead to an infinitely high TWI value, as described in the answer to a similar question (Getting Negative Topographic Wetness Index (TWI) values in SAGA GIS?). This is what leads to the NoData values you see. As far as I know, TWI is used as an indicator for wetness conditions in soils, so probably looking at locations right within a river bed won't make sense anyway.

  • as such “TWI calculates the capacity of water accumulation of each pixel in a watershed using a DEM” (Beven & Kirkby,1979). Subsequently, it is evident in my case that pixels having No Data values are actually containing Zero Gradient and surrounded by higher Z values. So, according to the function of the TWI there are more chances of water accumulation there. Especially, wetlands, ponds etc are the places having less slope gradient than surrounded terrain. Seeing 'No Data' pixel values right center of the river really doesn't make sense. – Ben Mar 17 '16 at 4:37
  • I have solved this problem by inserting the following expression – Ben Mar 23 '16 at 10:11
  • Ln(("Flow_acc"+ 1 ) * 30/"Tan_SlopeRad") and secondly removed sinks from my 'Flow Direction Raster' using WhiteBox and export into ArcGIS and then derived Flow Accumulation Raster. In WhiteBox, Sinks are efficiently removed. Resultently, there were no more 'No Data' values in my TWI raster – Ben Mar 23 '16 at 10:14

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