Curve Number Runoff Depth (SCS/NRCS)

From a storm rainfall P and a curve number CN (30 to 98, from the land cover and hydrologic soil group), the potential retention S = 1000/CN - 10, the initial abstraction Ia = 0.2 S, and the runoff Q = (P - Ia)^2 / (P - Ia + S). A 5 in storm on CN 80 sheds 2.89 in of runoff (58% of the rain); over 10 acres that is 2.41 acre-ft (785,500 gal) to detain. Because of the Ia threshold, a small storm on a low CN yields no runoff at all. Standard Ia = 0.2 S; a single design storm, not a routed hydrograph. A design aid; the local drainage manual and the engineer of record govern.

S = 1000/CN - 10 (in); Ia = 0.2 S; Q = (P - Ia)^2 / (P - Ia + S) for P > Ia, else 0 (runoff depth, in). Volume = (Q/12) x area(acres) acre-ft.

The NRCS (SCS) Curve Number runoff method (TR-55 Urban Hydrology for Small Watersheds, Chapter 2), by name; verified against the TR-55 runoff figure (P = 5 in, CN = 80 -> Q = 2.89 in).

TR-55 is a free USDA NRCS publication; the curve-number runoff equation is published in it and in every hydrology text.

Estimate. AHJ and licensed professional govern.

Field names used by the API: rainfall_in, curve_number, area_acres, retention_s_in, initial_abstraction_in, runoff_in, runoff_coefficient, runoff_volume_acreft

• Retention S = 1000/CN - 10 (in), CN from land cover and hydrologic soil groupSCS/NRCS TR-55 Ch. 2
• Initial abstraction Ia = 0.2 S (the standard assumption)TR-55
• Runoff Q = (P - Ia)^2 / (P - Ia + S) for P > Ia, else 0SCS runoff equation
• Use runoff depth/volume for detention sizing, one design storm, not a routed hydrographNRCS drainage practice