V4 11 Stormwater Report Rev 3 1 26 2023

Chapter 2 Estimating Runoff ‘Technical Release 55 Urban Hydrology for Small Watersheds Table 2-2b — Runoff curve numbers for cultivated agricultural lands Curve numbers for —— Cover description - hydrologic soil group ———— Hydrologic Cover type Treatment ¥ condition A B ce D “allow Bare soil = 7 86 91 94 Crop residue cover (CR) Poor 76 85 90 93 Good m4 83 88 90 Row crops Straight row (SR) Poor R 81 88 91 Good 67 78 85 89 SR+CR Poor 71 80 87 90 Good 64 B 82 85 Contoured (C) Poor 70 a) 84 88 Good 65 B 82 86 C+CR Poor 69 78 83 87 Good 64 74 81 85 Contoured & terraced (C&T) Poor 66 m4 80 82 Good 62 7 78 81 C&T+ CR Poor 65 B 79 81 Good 61 70 17 80 Small grain SR Poor 65 76 84 88 Good 63 75 83. 87 SR+CR Poor 64 B 83 86 Good 60 2 80 84 c Poor 63 74 82 85 Good 61 3B 81 Sd C+CR Poor 62 B 81 8d Good 60 2 80 83 C&T Poor 61 2 7 82 Good 59 70 78 81 C&T+ CR Poor 60 7 78 81 Good 58 69 17 80 Close-seeded SR Poor 66 7 85 389 or broadcast Good 58 2 81 85 legumes or c Poor 64 B 83 85 rotation Good 55 69 78 83 meadow C&T Poor 63 B 80 33 Good 51 67 76 80 ' Average runoff condition, and 1,=0.28 2 Crop residue cover applies only if residue is on al least 5% of the surface throughout the year. 2 Hydraulic condition is based on combination factors that affect infiltration and runoff, including (a) density and canopy of vegetative areas, (b) amount of year-round cover, (c) amount of grass or close-seeded legumes, (a) percent of residue cover on the land surface (good 2 20%), and (¢) degree of surface roughness. Poor: Factors impair infiltration and tend (o increase runoff. Good: Factors encourage average and better than average infiltration and tend to decrease runoff. 2-6 (210-VI-TR-55, Second Ea., June 1986)