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r.topidx.1grass - Man Page

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


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

Figure: topographic index map derived from elevation map


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.

See Also

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


Huidae Cho
Hydro Laboratory, Kyungpook National University, South Korea

Based on GRIDATB.FOR by Keith Beven.

Source Code

Available at: r.topidx source code (history)

Accessed: Tuesday May 14 13:40:45 2024

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