Suppose your watershed has 80% impervious area but runoff calibration suggests 20% impervious hence hinting that only ¼ of the impervious area is directly connected. Now if you set %Impervious in line H1 of RUNOFF to 20, what happens to the rest of the actually impervious area? Will it be treated by SWMM as pervious area and so the same infiltration, detention storage, and manning n values as impervious area will be used for it? What about the quality (build-up wash-off coefficient of this "undefined" portion of the watershed? The puzzling thing is that I know that in SWMM/RUNOFF the runoff from one surface does not flow over another surface.

Thanks for considering this simple question,
Kaveh Zomorodi, Ph.D.
kzomorodi@nier.org