## 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.