Jenny,

The inter-subcatchment routing used in SWMM 5 is the same as in SWMM 4.4. That is, if subcatchment I has Q cfs of runoff at the end of time step N which it sends to subcatchment J, then subcatchment J sees an additional "rainfall" of Q/Aj over its area (Aj) during time step N+1. This "runon" to J can't be split in some user-designated fashion between pervious and impervious areas. There is also the capability to use intra-subcatchment routing, where the impervious runoff in a subcatchment can be routed to the pervious sub-area of the same subcatchment (or vice versa). These extensions to SWMM were described nicely in Wayne Huber's article "New Options for Overland Flow Routing in SWMM" in Urban Drainage Modeling, R.W. Brashear and C. Maksimovic, eds., ASCE, Reston, VA, 2001.

Based on suggestions from the SWMM 5 Technical Panel, we are considering extending the intra-subcatchment routing to allow only a user-defined fraction of impervious (or pervious) runoff (say, the fraction covered by roofs) to be routed to the pervious (or impervious) sub-area. This will require making some modifications to the input data forms which are currently contained in SWMM Beta 5A.

Lewis Rossman
rossman.lewis@epa.gov