The United States Environmental Protection Agency (EPA) Storm Water Management Model (SWMM) is a dynamic rainfall-runoff-subsurface runoff simulation model used for single-event to long-term (continuous) simulation of the surface/subsurface hydrology quantity and quality from primarily urban/suburban areas. The hydrology component of SWMM operates on a collection of subcatchment areas divided into impervious and pervious areas with and without depression storage to predict runoff and pollutant loads from precipitation, evaporation and infiltration losses from each of the subcatchment. In addition Low Impact Development (LID) and Best Management Practice (BMP) areas on the subcatchment can be modeled to reduce the impervious and pervious runoff. The routing or hydraulics section of SWMM transports this water and possible associated water quality constituents through a system of closed pipes, open channels, storage/treatment devices, ponds, storages, pumps, orifices, weirs, outlets, outfalls and other regulators. SWMM tracks the quantity and quality of the flow generated within each subcatchment, and the flow rate, flow depth, and quality of water in each pipe and channel during a simulation period composed of multiple fixed or variable time steps. The water quality constituents such as water quality constituents can be simulated from buildup on the subcatchments through washoff to a hydraulic network with optional first order decay and linked pollutant removal, Best Management Practice (BMP) and Low Impact Development (LID) removal and treatment can be simulated at selected storage nodes. SWMM is one of the hydrology transport models which the EPA and other agencies have applied widely throughout North America and through consultants and universities throughout the world.
SWMM was first developed between 1969–1971 and has undergone several major upgrades since those years. The major upgrades were: (1) Version 2 in 1975, (2) Version 3 in 1981 and (3) Version 4 in 1988. The current SWMM edition, Version 5, is a complete re-write of the previous Fortran releases in the programming language C, and it can be run under Windows XP, Windows Vista and Windows 7 and with a recomplilation under Unix. The code for SWMM5 is open open source and public public domain code that can be downloaded from the EPA Web Site.