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Stormwater Runoff Coefficients: SCS curve number
cn open space good A to cn res 1 2ac B
Rational Method runoff coefficients (C values) by land use/surface type and return period, plus SCS/NRCS curve numbers (CN) by hydrologic soil group (A/B/C/D) and land cover/condition — from ASCE MOP 77, FHWA HEC-22, USDA-NRCS TR-55, and NEH-630
| method type | record id | C max (dimensionless) | description | land use | return period yr (yr) | standard | surface type | unit |
|---|---|---|---|---|---|---|---|---|
| SCS curve number | cn open space good A | 39 | Open space/parks — good cover (>75% grass), HSG A | open space | 2 | USDA-NRCS TR-55 Table 2-2a; USDA-NRCS NEH-630 Ch.9 | good cover | dimensionless |
| SCS curve number | cn open space good B | 61 | Open space/parks — good cover (>75% grass), HSG B | open space | 2 | USDA-NRCS TR-55 Table 2-2a | good cover | dimensionless |
| SCS curve number | cn open space good C | 74 | Open space/parks — good cover (>75% grass), HSG C | open space | 2 | USDA-NRCS TR-55 Table 2-2a | good cover | dimensionless |
| SCS curve number | cn open space good D | 80 | Open space/parks — good cover (>75% grass), HSG D | open space | 2 | USDA-NRCS TR-55 Table 2-2a | good cover | dimensionless |
| SCS curve number | cn pasture good A | 39 | Pasture/grassland — good condition, HSG A | agriculture | 2 | USDA-NRCS NEH-630 Ch.9 Table 9-1 | pasture | dimensionless |
| SCS curve number | cn pasture good B | 61 | Pasture/grassland — good condition, HSG B | agriculture | 2 | USDA-NRCS NEH-630 Ch.9 | pasture | dimensionless |
| SCS curve number | cn pasture good C | 74 | Pasture/grassland — good condition, HSG C | agriculture | 2 | USDA-NRCS NEH-630 Ch.9 | pasture | dimensionless |
| SCS curve number | cn pasture good D | 80 | Pasture/grassland — good condition, HSG D | agriculture | 2 | USDA-NRCS NEH-630 Ch.9 | pasture | dimensionless |
| SCS curve number | cn res 1 2ac A | 54 | Residential 1/2-acre lots (25% impervious), HSG A | residential | 2 | USDA-NRCS TR-55 Table 2-2a | 1 2 acre lot | dimensionless |
| SCS curve number | cn res 1 2ac B | 70 | Residential 1/2-acre lots (25% impervious), HSG B | residential | 2 | USDA-NRCS TR-55 Table 2-2a | 1 2 acre lot | dimensionless |
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