Calculate stormwater runoff depth using the SCS (NRCS) Curve Number method. Includes runoff calculation, composite CN for multiple areas, and CN lookup tables. Free online calculator for drainage engineers.
Runoff Calculation: Calculate runoff depth (Q) from rainfall depth (P) and curve number (CN) using the SCS equation.
Composite CN: Calculate area-weighted average curve number for watersheds with multiple land uses or soil types.
The SCS (now NRCS) Curve Number method estimates direct runoff from storm rainfall. The basic equation is:
Where:
| Land Use / Cover | Group A | Group B | Group C | Group D |
|---|---|---|---|---|
| Commercial (85% imp.) | 89 | 92 | 94 | 95 |
| Residential (1/4 acre) | 61 | 75 | 83 | 87 |
| Lawn (good condition) | 39 | 61 | 74 | 80 |
| Pasture (fair) | 49 | 69 | 79 | 84 |
| Woods (fair) | 36 | 60 | 73 | 79 |
| Paved (impervious) | 98 | 98 | 98 | 98 |
Source: USDA TR-55 (1986), Table 2-2. Values shown for AMC II (average conditions).
Dormant season with less than 0.5" of rainfall in prior 5 days. Lowest runoff potential.
CNI = 4.2 CNII / (10 - 0.058 CNII)
Normal conditions. This is the standard condition used in TR-55 tables.
Standard CN values from tables
Growing season with more than 2.1" of rainfall in prior 5 days. Highest runoff potential.
CNIII = 23 CNII / (10 + 0.13 CNII)
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