The monthly rates and charges are hereby established and determined for any improved premises within the City of Medford, or within any area under contract to be annexed to the City of Medford, for storm drainage system operation and maintenance. The City Council has determined that property not used for single family dwelling purposes is furnished service in proportion to the amount of the property's impervious surface. Each 3,730 square feet of impervious surface is equivalent to that furnished to a single family unit (Equivalent Residential Unit, or ERU). The minimum service charge per tax lot shall be that established for a single family unit.