If by the 'drainage basin' you mean the catchment, then download the
'r.water.outlet' program, which can generate a catchment from a 'aspect'
map generated by 'r.wateshed'
Steve Culshaw
NRA North West
e-mail : nra@cix.compulink.co.uk (use this for general usage)
or sculshaw@cix.compulink.co.uk (Private - only checked at best
biweekly)