Hydrologic Modeling and GRASS

On the informal survey:
I am using GRASS to determine the ln(a/tanB) index for TOPMODEL, in which
raster format is more suitable. I think that a tutorial explaining all that
GRASS can do to manipulate catchments would be helpful. For what I am doing, I
feel that GRASS can accomplish most of what I need, although a simple
statistics module would be handy.
Dan Riker
Duke University