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Note: A new GRASS GIS stable version has been released: GRASS GIS 7.8, available here.
Updated manual page: here


r.topidx - Creates a topographic index (wetness index) raster map from an elevation raster map.


raster, hydrology, wetness, topographic index


r.topidx --help
r.topidx input=name output=name [--overwrite] [--help] [--verbose] [--quiet] [--ui]


Allow output files to overwrite existing files
Print usage summary
Verbose module output
Quiet module output
Force launching GUI dialog


input=name [required]
Name of input elevation raster map
output=name [required]
Name for output topographic index raster map

Table of contents


r.topidx creates topographic index (wetness index), ln(a/tan(beta)), map from elevation map


a: the area of the hillslope per unit contour length that drains through any point,

tan(beta): the local surface topographic slope (delta vertical) / (delta horizontal).

Input maps may have NULL values. For example, if you have a MASK for a watershed (basin map from r.water.outlet), the following command will create a masked elevation map (belev):

r.mapcalc "belev = if(isnull(basin), basin, elev)"

r.stats -Anc prints out averaged statistics for topographic index.


Running the command below will generate the topographic index map shown in the figure:
g.region raster=elevation -p
r.topidx input=elevation output=topidx_elevation
r.topidx example
Figure: topographic index map derived from elevation map


r.topmodel, r.water.outlet, r.mapcalc
Known issues


Cho, H., 2000. GIS Hydrological Modeling System by Using Programming Interface of GRASS. Master's Thesis, Department of Civil Engineering, Kyungpook National University, Korea.

Moore, I.D., R.B. Grayson, and A.R. Ladson, 1991. Digital Terrain Modeling: A Review of Hydrological, Geomorphological, and Biological Applications. Hydrological Processes 5:3-30.


Huidae Cho
Hydro Laboratory, Kyungpook National University, South Korea

Based on GRIDATB.FOR by Keith Beven.

Last changed: $Date$


Available at: r.topidx source code (history)

Note: A new GRASS GIS stable version has been released: GRASS GIS 7.8, available here.
Updated manual page: here

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