Hi Mehebub,

Please see the link: you can see a good discussion about TWI.

https://www.researchgate.net/post/What_is_the_typical_Topographic_Wetness_Index_range_of_values

Hope this will help you to solve the issue.

Truly

Vijith

The Topographic Wetness Index (TWI), also called Compound Topographic Index (CTI), is a steady-state wetness index. In some areas, TWI has been shown in some study areas to predict solum depth (i.e., Gessler et al., 1995). It involves the upslope contributing area (a), a slope raster, and a couple of geometric functions. The value of a for each cell in the output raster (the CTI raster) is the value in a flow accumulation raster for the corresponding DEM. Higher CTI values represent drainage depressions, lower values represent crests and ridges.

Things to consider carefully: FlowAcc and Slope. How you calculate each will strongly affect TWI values. Catchment area (flow accumulation) can be calculated at least 3 ways (i.e., D8, D-infinity, Triangular multiple flow). Slope calculations also can take different forms (i.e., “local slope” of a pixel, max or min slope generated using a small neighborhood around each pixel, etc.). See referenced articles.

CTI = ln(a/tan B)

a = Upstream contributing area in m2, filename = FLOWACC

B = Slope raster, filename = SLOPE

ArcGIS Instructions

1. In Spatial Analyst, create a flow direction raster from a filled, projected DEM (use meters or feet as units).

2. Create flow accumulation raster from flow direction raster. This finds the upslope contributing area for each pixel. Ouput filename = FLOWACC.

3. Create slope raster from the DEM. Ouput filename = SLOPE

4.) In Raster Calculator (Spatial Analyst > Map Algebra > Raster Calculator), create this equation:

Ln((“FLOWACC”*900) / Tan(“SLOPE”))

Note: “FLOWACC” and “SLOPE” are the names of the rasters created in steps 1-3

Dear Mehebub,

The method suggested by Jothibasu is appreciated and I suggest you few pioneer study of this field. I hope it will be helpful for you. If you need further help you can contact me.

1.      Sørensen, R.; Zinko, U.; Seibert, J. (2006). "On the calculation of the topographic wetness index: evaluation of different methods based on field observations". Hydrology and Earth System Sciences 10: 101–112.

2.       Moore, I.D.; Gessler, P.E.; Nielsen, G.A.; Petersen, G.A (1993). "Terrain attributes: estimation methods and scale effects.". In Jakeman, A.J.; Beck, M.B.; McAleer, M. Modelling Change in Environmental Systems. London: Wiley. p. 189 - 214.

3.      Beven, K.J.; Kirkby, M. J. (1979). "A physically based, variable contributing area model of basin hydrology". Hydrolological Science Bulletin 24: 43–69

 visit www.gis4geomorphology.com

Dear Mehebub Sahana,

There is no any toolbox in the ArcGIS for computing TWI. You have to obtain it with other software such as SAGA.

Regards,

Dear Mehebub Sahana,

Please follow the steps which, attached as JPG. to perform your required TWI in ArcGIS platform.

Thank You.