What I'm asking about is not specifically a GRASS issue. I'm helping a Master's student use a hydrologic model, where I have written some GRASS scripts to generate one of the required input files. It's called the 'connectivity' file because it identifies how the basin grids are connected to each other (based on flow direction and flow accumulation). It works pretty well.
The problem is this; the watershed in his study region includes a large shopping mall that is outside the natural drainage, but through the miracles of site development, stormwater now flows into the watershed of interest. Of course, the entire mall area is impervious (parking lots and rooftops).
I was thinking that the mall area could be recoded as a slightly sloping surface into the natural watershed, with a contrived 'channel'. Any thoughts on how we might do this. To be sure, a different model (SWMM) would have been better (my original suggestion), but we're past that point.