NRCS Hydrograph Calculator
Reviewed October 2026 · Civil Systems LLCPre- and post-development runoff, peak flow, and 24-hour hydrographs, computed at 1-minute steps.
Calculate time of concentration
Enter a flow path; use 0 length to omit a segment.
Run the design storm
Check the 1-, 10-, and 100-year storms using the same pond design.
At the hydrograph cursor time. Remaining bar space represents rainfall yet to come.
Route the post-development storm
Adjust the pond and outlet to meet the release target.
Screening model: square floor, level-pool routing and simplified free-flow outlets. Tailwater, barrel capacity and emergency spillway flow are not modeled; overtopping is flagged.
How does the pond change the storm?
Compare the same storm before development, after development, and after pond routing.
The chart shares the pond animation time. Drag on the chart or use the pond slider to explore.
Data table and CSV
Show the calculation
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 selected NRCS 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). Select Type I, IA, II, or III to apply the same storm to both conditions.
Rainfall timing. All four storm types use the WinTR-55 v2.00 mass curves at 0.1-hour (6-minute) intervals, as distributed by HydroCAD. Cumulative rainfall is interpolated linearly at 1-minute calculation steps. Intensity is the average over each published 6-minute interval; smaller steps may remain in short-Tc hydrographs. No smoothing is applied.
Using the results. Compare pre- and post-development peaks and runoff volumes, then route the storm through the pond to check the peak release and storage. The composite CN helper area-weights land covers, CN = Σ(A·CN) / ΣA, rounded to a whole number.