private land draining into public land

[Chuck Ehlschlaeger]

To find ALL land draining into public land:

1) run r.watershed, saving the aspect and accumulation output maps.  

2) Using the accumulation map, find the cells in the public land that have the
greatest accumulation.  If any public land accumulation values are negative,
you may have land from off your elevation map draining into you public lands.

3) Determine the UTM coordinates of cells with large accumulation values at
the edge of public and private land where the water is flowing OFF the public
lands.

4) For each large accumulation, run r.water.outlet using the aspect map from
r.watershed and the UTM coordinates of the large accumulation.  All non-zero
output from r.water.outlet from any of accumulation cells that are on private
lands drain onto the public lands.

chuck

P.S.  You could run r.water.outlet for EVERY cell on public lands, garenteeing
you grab every private land cell, but that would take even more effort.