United States Environmental Protection Agency
The Stormwater Management Model (SWMM) was first produced in 1971 by the US EPA. Currently this model is undergoing its fifth major rewrite. SWMM is a dynamic rainfall-runoff simulation model that can be used for single event or long-term (continuous) simulation of runoff quantity and quality from primarily urban areas. The SWMM model is freely available for download and comes with a comprehensive user manual to guide users through the modelling process. Although SWMM comes with national level data, managers have the option of inputing their own data into the model. This allows for more detailed anaylses to completed when the appropriate data is readily available.