A municipal separate storm sewer system (MS4) is a system of storm sewers, conveyances, inlets, and similar features that discharge to surface waters of the state. Coverage under an MS4 National Pollutant Discharge Elimination System (NPDES)  permit is required for cities, counties, and other government entities that meet designated population size and/or density criteria.

The Municipal Separate Storm Sewer System (MS4) NPDES stormwater permits issued by KDHE require preparation of a Stormwater Management Program (SMP) document. The SMP outlines how stormwater will be managed to reduce pollution, including required best management practices (BMPs), measurable goals, responsible parties, and compliance activities such as monitoring and reporting. It also addresses regulatory requirements like the six minimum control measures, mapping of the stormwater system, and schedules to ensure ongoing permit compliance.