[GRASS-user] Repeated r.watershed runs

[Ken Mankoff]

Hi List,

I'm trying to determine flow past a drainage basin outlet. The complicating
factor is that I need to do this each day for 40 years. If I do
"r.watershed" ~14,000 times I'll get the results, but it will take 3 days.
It seems that r.watershed is likely calculating many things each time
through the loop. Is there a more efficient way to this? A flag to
r.watershed that isn't documented? Something with time-series?

Alternatively, because I only need the flow at the outlet, I could
calculate the basin, not route the flow, and instead sum the values in the
basin. I assume this would take seconds or minutes rather than days. In
this case I'm not sure of the best way to define the basin. I tried doing
r.water.outlet upstream from the outlet, but I think this uses SFD, which
means the basin may be significantly underestimated.

I also tried inverting/flipping the DEM and then running r.watershed with
convergence=1, and a flow equal to 0 everywhere except 1000 at the outlet
(now the source due to the inversion) to see where it flooded upstream (now
downstream due to the inversion). This didn't seem to work... because
basins are filled and flow routes to the edge of the DEM, I could not pick
out the

Any advice how to either a) efficiently route 14,000 FLOW rasters over 1
DEM or b) determine the full basin will be much appreciated.

Thanks,

    -k.
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