The SCS Curve Number (CN) method estimates direct runoff from rainfall. Curve numbers range from 0 to 100, with higher values indicating greater runoff potential. The tables below provide CN values for AMC II (average antecedent moisture condition) by land use and Hydrologic Soil Group (HSG).
SCS Runoff Equation
Runoff (Q):
Q = (P - 0.2S)2 / (P + 0.8S)
for P > 0.2S, otherwise Q = 0
Maximum Retention (S):
S = (1000/CN) - 10 (inches)
Initial Abstraction (Ia):
Ia = 0.2S (traditional) or 0.05S (revised)
Where: Q = runoff depth, P = rainfall depth, S = maximum retention, CN = curve number
Showing 66 of 66 entries
| Land Use / Cover ▲ | HSG A | HSG B | HSG C | HSG D | % Imperv. |
|---|---|---|---|---|---|
Brush (Fair condition) Brush - weed - grass mixture with brush the major element (50-75% ground cover) | 35 | 56 | 70 | 77 | - |
Brush (Good condition) Brush - weed - grass mixture with brush the major element (>75% ground cover) | 30 | 48 | 65 | 73 | - |
Brush (Poor condition) Brush - weed - grass mixture with brush the major element (<50% ground cover) | 48 | 67 | 77 | 83 | - |
Close-seeded Legumes - Contoured (Good) Close-seeded legumes or rotation meadow, contoured, good condition | 55 | 69 | 78 | 83 | - |
Close-seeded Legumes - Contoured (Poor) Close-seeded legumes or rotation meadow, contoured, poor condition | 64 | 75 | 83 | 85 | - |
Close-seeded Legumes - Straight Row (Good) Close-seeded or broadcast legumes or rotation meadow, straight rows, good condition | 58 | 72 | 81 | 85 | - |
Close-seeded Legumes - Straight Row (Poor) Close-seeded or broadcast legumes or rotation meadow, straight rows, poor condition | 66 | 77 | 85 | 89 | - |
Close-seeded Legumes - Terraced (Good) Close-seeded legumes or rotation meadow, terraced, good condition | 51 | 67 | 76 | 80 | - |
Close-seeded Legumes - Terraced (Poor) Close-seeded legumes or rotation meadow, terraced, poor condition | 63 | 73 | 80 | 83 | - |
Commercial/Business (72% impervious) Urban commercial districts with moderate impervious cover | 81 | 88 | 91 | 93 | 72% |
Commercial/Business (85% impervious) Urban commercial districts with high impervious cover | 89 | 92 | 94 | 95 | 85% |
Dirt Roads Unpaved dirt roads | 72 | 82 | 87 | 89 | - |
Fallow - Bare Soil Fallow land with bare soil, no crop residue | 77 | 86 | 91 | 94 | - |
Fallow - Crop Residue (Good) Fallow land with crop residue cover, good hydrologic condition (>20% residue cover) | 74 | 83 | 88 | 90 | - |
Fallow - Crop Residue (Poor) Fallow land with crop residue cover, poor hydrologic condition (<20% residue cover) | 76 | 85 | 90 | 93 | - |
Farmsteads Buildings, lanes, driveways, and surrounding lots on farms | 59 | 74 | 82 | 86 | - |
Gravel Roads Unpaved gravel roads and parking areas | 76 | 85 | 89 | 91 | - |
Industrial (72% impervious) Industrial districts with typical impervious cover | 81 | 88 | 91 | 93 | 72% |
Meadow (Continuous grass) Meadow - continuous grass, protected from grazing, generally mowed for hay | 30 | 58 | 71 | 78 | - |
Newly Graded Areas Newly graded areas with exposed soil (pervious areas only, no vegetation) | 77 | 86 | 91 | 94 | - |
Open Space (Fair condition) Open space, parks, golf courses - grass cover 50-75% | 49 | 69 | 79 | 84 | - |
Open Space (Good condition) Open space, parks, golf courses - grass cover >75% | 39 | 61 | 74 | 80 | - |
Open Space (Poor condition) Open space, parks, golf courses - grass cover <50% | 68 | 79 | 86 | 89 | - |
Pasture - Contoured (Fair) Pasture with contoured grazing management, fair condition | 25 | 59 | 75 | 83 | - |
Pasture - Contoured (Good) Pasture with contoured grazing management, good condition | 6 | 35 | 70 | 79 | - |
Pasture - Contoured (Poor) Pasture with contoured grazing management, poor condition | 47 | 67 | 81 | 88 | - |
Pasture/Range (Fair condition) Pasture, grassland, or range - not heavily grazed (50-75% ground cover) | 49 | 69 | 79 | 84 | - |
Pasture/Range (Good condition) Pasture, grassland, or range - lightly grazed (>75% ground cover) | 39 | 61 | 74 | 80 | - |
Pasture/Range (Poor condition) Pasture, grassland, or range - heavily grazed with no mulch (<50% ground cover) | 68 | 79 | 86 | 89 | - |
Paved Parking Lots, Roofs, Driveways Impervious surfaces including asphalt, concrete, roofs | 98 | 98 | 98 | 98 | 100% |
Paved Roads with Curbs and Storm Sewers Streets and roads with curbs and storm sewers | 98 | 98 | 98 | 98 | 100% |
Paved Roads with Open Ditches Streets and roads with open ditches | 83 | 89 | 92 | 93 | - |
Residential (1 acre lots) Low density residential, lot size 1 acre, 20% impervious | 51 | 68 | 79 | 84 | 20% |
Residential (1/2 acre lots) Medium-low density residential, lot size 1/2 acre, 25% impervious | 54 | 70 | 80 | 85 | 25% |
Residential (1/3 acre lots) Medium density residential, lot size 1/3 acre, 30% impervious | 57 | 72 | 81 | 86 | 30% |
Residential (1/4 acre lots) Medium-high density residential, lot size 1/4 acre, 38% impervious | 61 | 75 | 83 | 87 | 38% |
Residential (1/8 acre lots) High density residential, lot size 1/8 acre or less, 65% impervious | 77 | 85 | 90 | 92 | 65% |
Residential (2 acre lots) Very low density residential, lot size 2 acres, 12% impervious | 46 | 65 | 77 | 82 | 12% |
Roofs (Impervious) Building rooftops - impervious surfaces | 98 | 98 | 98 | 98 | 100% |
Row Crops - Contoured & Terraced (Good) Row crops contoured and terraced, good hydrologic condition | 62 | 71 | 78 | 81 | - |
Row Crops - Contoured & Terraced (Poor) Row crops contoured and terraced, poor hydrologic condition | 66 | 74 | 80 | 82 | - |
Row Crops - Contoured (Good) Row crops planted on contour, good hydrologic condition | 65 | 75 | 82 | 86 | - |
Row Crops - Contoured (Poor) Row crops planted on contour, poor hydrologic condition | 70 | 79 | 84 | 88 | - |
Row Crops - Contoured + Residue (Good) Row crops with residue cover, contoured, good condition | 64 | 74 | 81 | 85 | - |
Row Crops - Contoured + Residue (Poor) Row crops with residue cover, contoured, poor condition | 69 | 78 | 83 | 87 | - |
Row Crops - Straight Row (Good) Row crops planted in straight rows, good hydrologic condition | 67 | 78 | 85 | 89 | - |
Row Crops - Straight Row (Poor) Row crops planted in straight rows, poor hydrologic condition | 72 | 81 | 88 | 91 | - |
Row Crops - Straight Row + Residue (Good) Row crops with crop residue cover, straight rows, good condition | 64 | 75 | 82 | 85 | - |
Row Crops - Straight Row + Residue (Poor) Row crops with crop residue cover, straight rows, poor condition | 71 | 80 | 87 | 90 | - |
Row Crops - Terraced + Residue (Good) Row crops with residue, contoured and terraced, good condition | 61 | 70 | 77 | 80 | - |
Row Crops - Terraced + Residue (Poor) Row crops with residue, contoured and terraced, poor condition | 65 | 73 | 79 | 81 | - |
Small Grain - Contoured & Terraced (Good) Small grain contoured and terraced, good hydrologic condition | 59 | 70 | 78 | 81 | - |
Small Grain - Contoured & Terraced (Poor) Small grain contoured and terraced, poor hydrologic condition | 61 | 72 | 79 | 82 | - |
Small Grain - Contoured (Good) Small grain planted on contour, good hydrologic condition | 61 | 73 | 81 | 84 | - |
Small Grain - Contoured (Poor) Small grain planted on contour, poor hydrologic condition | 63 | 74 | 82 | 85 | - |
Small Grain - Contoured + Residue (Good) Small grain with residue, contoured, good condition | 60 | 72 | 80 | 83 | - |
Small Grain - Contoured + Residue (Poor) Small grain with residue, contoured, poor condition | 62 | 73 | 81 | 84 | - |
Small Grain - Straight Row (Good) Small grain crops in straight rows, good hydrologic condition | 63 | 75 | 83 | 87 | - |
Small Grain - Straight Row (Poor) Small grain crops in straight rows, poor hydrologic condition | 65 | 76 | 84 | 88 | - |
Small Grain - Straight Row + Residue (Good) Small grain with residue cover, straight rows, good condition | 60 | 72 | 80 | 84 | - |
Small Grain - Straight Row + Residue (Poor) Small grain with residue cover, straight rows, poor condition | 64 | 75 | 83 | 86 | - |
Small Grain - Terraced + Residue (Good) Small grain with residue, terraced, good condition | 58 | 69 | 77 | 80 | - |
Small Grain - Terraced + Residue (Poor) Small grain with residue, terraced, poor condition | 60 | 71 | 78 | 81 | - |
Woods (Fair condition) Woods are grazed but not burned, some forest litter covers the soil | 36 | 60 | 73 | 79 | - |
Woods (Good condition) Woods are protected from grazing, litter and brush adequately cover the soil | 30 | 55 | 70 | 77 | - |
Woods (Poor condition) Forest litter, small trees, and brush are destroyed by heavy grazing or regular burning | 45 | 66 | 77 | 83 | - |
Hydrologic Soil Groups (HSG)
Group A low runoff
High Infiltration
Infiltration: 0.3-0.45 in/hr
Typical: Sand, Loamy sand, Sandy loam
Group B moderately low runoff
Moderate Infiltration
Infiltration: 0.15-0.3 in/hr
Typical: Silt loam, Loam
Group C moderately high runoff
Slow Infiltration
Infiltration: 0.05-0.15 in/hr
Typical: Sandy clay loam, Clay loam, Silty clay loam
Group D high runoff
Very Slow Infiltration
Infiltration: 0-0.05 in/hr
Typical: Clay, Silty clay, Sandy clay
Design Guidance
Antecedent Moisture Condition (AMC)
- AMC I (Dry): Use when soils are dry (5-day antecedent rainfall < 0.5" dormant, < 1.4" growing)
- AMC II (Average): Standard design condition (table values)
- AMC III (Wet): Use when soils are saturated (5-day antecedent rainfall > 1.1" dormant, > 2.1" growing)
Composite Curve Number
For watersheds with multiple land uses, calculate an area-weighted composite CN:
CNcomposite = sum(CNi x Ai) / Atotal
Dual HSG Soils
Soils with dual classification (e.g., A/D, B/D, C/D) behave as the first letter when adequately drained and the second letter when undrained. Check local soil surveys for drainage status.
Urban Development
For urban areas, consider the connected impervious area versus total impervious area. TR-55 provides guidance on adjusting CN for unconnected impervious areas that drain to pervious surfaces.
Primary Sources
- USDA SCS TR-55 (1986). Urban Hydrology for Small Watersheds. Tables 2-2a, 2-2b, 2-2c.
- USDA NRCS NEH Part 630 (2004). National Engineering Handbook, Chapter 9 - Hydrologic Soil-Cover Complexes.
- USDA NRCS NEH Part 630 (2004). Chapter 7 - Hydrologic Soil Groups.
- USDA NRCS NEH Part 630 (2004). Chapter 10 - Estimation of Direct Runoff from Storm Rainfall.