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*Average Runoff Condition. Ia = 0.2S <br />Source: Soil Conservation Service TR-55 <br />SCS Curve Numbers (CN) <br />Land Use Description Hydrologic Soil Group <br />Residential A B C D <br />Average lot size: <br />1/8 acre or smaller 77 85 90 92 <br />1/4 acre 61 75 83 87 <br />1/3 acre 57 72 81 86 <br />1/2 acre 54 70 80 85 <br />1 acre 51 68 79 84 <br />2 acre 46 65 77 82 <br />1/8 acre or smaller 77 85 90 92 <br />Paved parking lots and roofs 98 98 98 98 <br />Streets and roads: <br />Paved with curbs 98 98 98 98 <br />Gravel 76 85 89 91 <br />Dirt 72 82 87 89 <br />Commercial and business areas 89 92 94 95 <br />Industrial districts 81 88 91 93 <br />Open spaces, lawns, and parks: <br />Good condition 39 61 74 80 <br />Fair condition 49 69 79 84 <br />Fallow 77 86 91 94 <br />Row crops 72 81 88 91 <br />Page 1 of 1SCS Curve Numbers (CN) <br />06/08/2014mk:@MSITStore:C:\Program%20Files\Autodesk\AutoCAD%202015\C3D\HHApps\Hyd...