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 at cix.compulink.co.uk (use this for general usage) or sculshaw at cix.compulink.co.uk (Private - only checked at best biweekly)