For residential and nonresidential development, the basis for the fee shall be the calculated increase of developed site runoff expressed in acre feet determined for a one-hundred-year storm (six inches of rain in twenty-four hours).

The difference between developed and un-developed property shall be calculated utilizing the runoff coefficients published by the Sonoma County water agency. The proportion of vegetated area to total area may be used to calculate runoff coefficients for single and two-family residential projects when the director of public works/city engineer deems it appropriate and more accurate. (Ord. 1653 NCS §2, 1986; Ord. 1530 NCS §1 (part), 1982.)