NAME

       r.topidx  - Creates topographic index map from elevation raster map.

KEYWORDS

       raster, hydrology

SYNOPSIS

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

   Flags:
       --overwrite
           Allow output files to overwrite existing files

       --verbose
           Verbose module output

       --quiet
           Quiet module output

   Parameters:
       input=name
           Input elevation map

       output=name
           Output topographic index 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 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.

SEE ALSO

       r.topmodel, r.water.outlet, r.mapcalc

REFERENCE

       Moore, I.D., R.B. Grayson, and A.R. Ladson, 1991. Digital terrain modeling:  A  review  of
       hydrological, geomorphological, and biological applications. Hydrol. Processes 5:3-30.

AUTHORS

       Main algorithm sources are rewritten from GRIDATB.FOR.
       Thanks to Keith Beven.

       GRASS port by Huidae Cho
       Hydro Laboratory, Kyungpook National University, South Korea

       Last changed: $Date: 2011-11-08 03:29:50 -0800 (Tue, 08 Nov 2011) $

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       © 2003-2013 GRASS Development Team