r.topidx - GRASS manual
Author: GRASS Development Team
Description: r.topidx: Creates a topographic index (wetness index) raster map from an elevation raster map.
Keywords: raster, hydrology, wetness, topographic index

## NAME

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

## KEYWORDS

raster, hydrology, wetness, topographic index

## SYNOPSIS

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

### Flags:

--overwrite
  Allow output files to overwrite existing files
--help
  Print usage summary
--verbose
  Verbose module output
--quiet
  Quiet module output
--ui
  Force launching GUI dialog

### Parameters:

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

## DESCRIPTION

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

- 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 raster mask set for a watershed (using 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.

Lat/long projects are not supported. If data is not projected, please create a new project with a projected coordinate reference system and reproject the data there. Please run r.topidx from that project.

## EXAMPLE

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

## REFERENCES

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

## AUTHORS

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)

Latest change: Thursday Nov 06 22:27:54 2025 in commit: 298b38614712934179355ea767403d6bfea8f5f9

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© 2003-2026 GRASS Development Team, GRASS 8.5.1dev Reference Manual