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Chapter 2 Estimating Runoff Technical Release 55 Urban Hydrology for Small Watersheds Table 2-2c Runoff curve numbers for other agricultural lands II Curve numbers for -------- Cover description hydrologic soil group ---- Hydrologic Covertype condition A B C D Pasture, grassland, or range—continuous forage for grazing. 2✓ Meadow—continuous grass, protected from grazing and generally mowed for hay. Brush—brush-weed-grass mixture with brush the major element. 31 Woods—grass combination (orchard or tree farm). ;;i Woods. fy Poor 68 79 86 89 Fair 49 69 79 84 Good 39 61 74 80 — 30 58 71 78 Poor 48 67 77 83 Fair 35 56 70 77 Good 30V 48 65 73 Poor 57 73 82 86 Fair 43 65 76 82 Good 32 58 72 79 Poor 45 66 77 83 Fair 36 60 73 79 Good 30V 55 70 77 Farmsteads—buildings, lanes, driveways, — 59 74 82 86 and surrounding lots. 1 Average runoff condition, and I. = 0.25. 2 Poor. <50%) ground cover or heavily grazed with no mulch. Fair. 50 to 75% ground cover and not heavily grazed. Good.• > 75% ground cover and lightly or only occasionally grazed. 3 Poor. <50% ground cover. Fair. • 50 to 75% ground cover. Good.• >75% ground cover. 4 Actual curve number is less than 30; use CN = 30 for runoff computations. B CN's shown were computed for areas with 50% woods and 50% grass (pasture) cover. Other combinations of conditions may be computed from the CN's for woods and pasture. 6 Poor., Forest litter, small trees, and brush are destroyed by heavy grazing or regular burning. Fair: Woods are grazed but not burned, and some forest litter covers the soil. Good: Woods are protected from grazing, and litter and brush adequately cover the soil. (210 -VI -TR -55, Second Ed., June 1986) 2-7