Main algorithm sources are rewritten from GRIDATB.FOR. GRIDATB.FOR is available from http://www.es.lancs.ac.uk/hfdg/topmodel.html. Thanks to Keith Beven. [r.topidx] creates topographic 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) Elevation map need not to be rectangular if study basin was already determined. Non-interesting region may have NULL values. Study basin map can be taken from r.water.outlet then the following r.mapcalc statements will create study basin extracted elevation map. r.mapcalc "belev = if(isnull(basin), basin, elev)"