In Southern California, we have a storm drain system and a sewage system that are completely separate. The storm drain system is called the municipal separate storm sewer system (MS4). Two main types of water flow through the MS4: (1) Stormwater, which is rainwater that cannot infiltrate into the ground naturally and instead builds up as it flows over the ground, and (2) dry weather runoff, which originates when it is not raining through activities such as overwatering lawns or washing cars.