Watershed Runoff Calculator
Estimate surface runoff depth, volume, and peak discharge using the USDA SCS/NRCS Curve Number (CN) method.
Formulas Used
SCS/NRCS Curve Number Method (TR-55):
- Potential Max Retention: S = (1000 / CN) − 10 [inches]
- Initial Abstraction: Ia = 0.2 × S [inches]
- Runoff Depth: Q = (P − Ia)² / (P − Ia + S) when P > Ia; else Q = 0 [inches]
- Runoff Volume: V = Q × A / 12 [acre-feet]
- Peak Discharge (SCS TR-55): qp = 484 × Ami² × Q / Tc [cfs]
Assumptions & References
- Uses the USDA NRCS (formerly SCS) Curve Number method for direct surface runoff estimation.
- Initial abstraction is assumed as Ia = 0.2S (standard assumption; some studies use 0.05S for urban areas).
- Peak discharge uses the SCS dimensionless unit hydrograph peak rate factor of 484 (valid for average watershed conditions in the US).
- Curve Number (CN) reflects land use, soil group (A–D), and antecedent moisture condition (AMC II assumed).
- Typical CN values: Impervious urban (98), Row crops / Type C soil (82), Woods / Type B soil (60), Meadow / Type A soil (30).
- Time of concentration (Tc) should be estimated using TR-55 sheet flow, shallow concentrated flow, and channel flow methods.
- References: USDA NRCS TR-55 (1986), NRCS National Engineering Handbook Part 630 (2004).