The stormwater user fee charge is based on the amount of impervious surface on a parcel. Impervious surfaces are typically manmade features on a property that cannot absorb rainfall including rooftops, driveways, sidewalks, parking lots, etc.
The square footage of impervious surface on a property has a direct correlation to the stormwater runoff characteristics for the parcel including runoff volume, velocity, discharge rate and pollutant concentration. Increased runoff volume, velocity, discharge rate and pollutant concentration place a greater demand on the City drainage system to safely and efficiently convey runoff from the parcel.
As such, one can deduce that larger parcels with greater amounts of impervious area will be charged a higher user fee based on the relative demand placed on the City's drainage system and the increased service level provided to that parcel.