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number (CN) is used to estimate the relative amount of rainfall that will run off a parcel. 4.2 Cause and Effect To remain fair to all properties, the utility is based on how much a particular parcel contributes to the storm water program needs. The amount of contribution should not be based solely on the amount of rooftops and pavement, or pervious area, on a parcel. The contribution should also consider how much rain actually runs off. Different amounts of rain will produce different amounts of runoff. However, the difference in runoff for a residential property and a commercial site area is not constant for all rainfall depths. Therefore, a rainfall depth should be used in the revenue equation that will result in the various properties paying proportionate amounts. 4.3 Run -Off Equation The revenue equation should not be arbitrarily set based on pre -set factors from another community's ordinance. To be able to demonstrate a rational basis for utility fees, the revenue equation should be based on standard engineering practices. The revenue equation follows the Natural Resources Conservation Services (NRCS - previously SCS) storm water runoff methodology outlined in the SCS National Engineering Handbook," Section 4 - Hydrology (Mockus, 1969). The general runoff equation is: Where: O= (P - 0.2S)2 P + 0.8S Q = Actual Runoff P = Potential Maximum Runoff S = Potential Maximum Retention and initial rainfall abstraction The "S" term can be expressed in terms of the runoff index, or curve number (CN). S = (1000 /CN) - 10 Tables are easily developed for convenient solution. For determining utility fees, P remains constant and CN has been assigned to each of the land use categories. 4.4 How Much Rainfall? The majority of the developed part of most of our communities is residential. Of the nonresidential developed land uses, commercial land use Feasibility Study A- LINOL0602.00 City of Lino Lakes, Minnesota Page 15