r.watershd & topo index

[Bahram Saghafian]

Here is what the man page says about negative values in accumulation file:

          Negative numbers indicate that those cells possibly have
          surface runoff from outside of the current geographic region.
          Thus, any cells with negative values cannot have their
          surface runoff and sedimentation yields calculated accurately.

The overflow message MAY be due to large number of cells. Try -m option.
It is very slow however.

B.S.