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Chapter 2 Estimating Runoff Technical Release 66 <br />Urban Hydrology for Small Watersheds <br />Table 2 -2b Runoff curve numbers for cultivated agricultural lands It <br />Curve numbers for <br />Cover description <br />hydrologic soil group <br />Hydrologic <br />Cover type <br />Treatment Q/ <br />condition 41 <br />A <br />B <br />C <br />D <br />Fallow <br />Bare soil <br />— <br />77 <br />86 <br />91 <br />94 <br />Crop residue cover (CR) <br />Poor <br />76 <br />86 <br />90 <br />93 <br />Good <br />74 <br />83 <br />88 <br />90 <br />Row crops <br />Straight row (SR) <br />Poor <br />72 <br />81 <br />88 <br />91 <br />Good <br />67 <br />78 <br />85 <br />89 <br />SR + CR <br />Poor <br />71 <br />80 <br />87 <br />90 <br />Good <br />64 <br />76 <br />82 <br />85 <br />Contoured (C) <br />Poor <br />70 <br />79 <br />84 <br />88 <br />Good <br />65 <br />76 <br />82 <br />86 <br />C + CR <br />Poor <br />69 <br />78 <br />83 <br />87 <br />Good <br />64 <br />74 <br />81 <br />85 <br />Contoured & terraced (C &T) <br />Poor <br />66 <br />74 <br />80 <br />82 <br />Good <br />62 <br />71 <br />78 <br />81 <br />C &T+ CR <br />Poor <br />65 <br />73 <br />79 <br />81 <br />Good <br />61 <br />70 <br />77 <br />80 <br />Small grain <br />SR <br />Poor <br />66 <br />76 <br />84 <br />88 <br />Good <br />63 <br />75 <br />83 <br />87 <br />SR + CR <br />Poor <br />64 <br />75 <br />83 <br />86 <br />Good <br />60 <br />72 <br />80 <br />84 <br />C <br />Poor <br />63 <br />74 <br />82 <br />86 <br />Good <br />61 <br />73 <br />81 <br />84 <br />C + CR <br />Poor <br />62 <br />73 <br />81 <br />84 <br />Good <br />60 <br />72 <br />80 <br />83 <br />C &T <br />Poor <br />61 <br />72 <br />79 <br />82 <br />Good <br />69 <br />70 <br />78 <br />81 <br />C &T+ CR <br />Poor <br />60 <br />71 <br />78 <br />81 <br />Good <br />58 <br />69 <br />77 <br />80 <br />Close-seeded <br />SR <br />Poor <br />66 <br />77 <br />85 <br />89 <br />or broadcast <br />Good <br />68 <br />72 <br />81 <br />85 <br />legumes or <br />C <br />Poor <br />64 <br />75 <br />83 <br />85 <br />rotation <br />Good <br />65 <br />69 <br />78 <br />83 <br />meadow <br />C&T <br />Poor <br />63 <br />73 <br />80 <br />83 <br />Good <br />61 <br />67 <br />76 <br />80 <br />r Average runoff condition, and 4 =0.2S <br />8 Crop residue cover applies only if residue is on at least 6% of the surface throughout the year. <br />8 Hydraulic condition is based on combination factors that affect infiltration and runoff, including (a) density and canopy of vegetative areas, <br />(b) amount of year -round cover, (c) amount of grass or close-seeded legumes, (d) percent of residue cover on the land surface (good 2 20%), <br />and (e) degree of surface roughness. <br />Poor. Factors impair infiltration and tend to increase runoff. <br />Good: Factors encourage average and better than average infiltration and tend to decrease runoff. <br />2-6 (210- VI- TR-66, Second Ed., June 1986) <br />