Stormwater describes water that originates during precipitation events.

Stormwater is a term used to describe water that enters the sewer system which originates during precipitation events as a result of rain fall or from snowmelt. Stormwater that does not soak into the ground flows on the surface is called surface runoff. In many cases, the excess water flows into surface waterways or is channelled into a combined sewer system or storm sewer system from where it is conveyed for treatment at the wastewater treatment plant or discharged directly into the recipient.

Stormwater may cause problems due to the volume of water, the intensity of the runoff and the potential contaminants that the water is carrying, i.e. water pollution. When runoff flows towards the sewer system, it picks up different contaminants such as petroleum, metal species, pesticides or fertilisers.