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)"

