NRCS Hydrograph Calculator
Reviewed October 2026 · Civil Systems LLCPre- and post-development runoff, peak flow, and 24-hour hydrographs, computed at 1-minute steps.
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About this calculator
Scope. Built for coursework and for checking hand calculations. Final designs should use TR-20, WinTR-55, or equivalent software and the rainfall data your reviewing agency requires.
Published method Runoff depth from the NRCS curve number equations (TR-55 chapter 2, Ia = 0.2 S). Rainfall timing from the NRCS Type II 24-hour distribution. Hydrograph from the NRCS triangular unit hydrograph (Tp = Δt/2 + 0.6 Tc, qp = 484 A/Tp, base 2.67 Tp), convolved at 1-minute steps so the volume under the hydrograph matches the runoff depth.
Simplification One curve number and one Tc per condition; no baseflow; no channel or reservoir routing. Peaks differ somewhat from TR-55 graphical-method results. TR-55 uses a minimum Tc of 0.1 hr (6 min). Types I, IA, and III will be added once their tables are checked against the NRCS source.
Rainfall timing. The Type II table is interpolated with straight lines between its tabulated times (as coarse as 0.25–0.5 hr near the peak), so intensity is constant within each interval. With a short Tc this shows as short flat steps after the peak. A finer 0.1-hour NRCS table would remove them.
Using the results. The gap between the post- and pre-development peaks is what a detention outlet has to take off the top; the volume difference is what runoff-reduction practices would have to remove. To size and check a pond for this storm, route it in Hydrology Lab experiment 04. The composite CN helper area-weights land covers, CN = Σ(A·CN) / ΣA, rounded to a whole number.