NRCS runoff equation with a composite curve number built from land use and hydrologic soil group: runoff depth and volume for a 24-hour design storm. Runs entirely offline; inputs persist in this browser via localStorage.
Use limits: This is the runoff-depth/volume half of TR-55 at average antecedent conditions (ARC II). Peak discharge needs the rest of the method (graphical peak or tabular hydrograph — Tc, rainfall distribution type, pond/swamp factor), or a model. CN values are the standard TR-55 Table 2-2 entries; the local hydrology manual's values and its rules about compositing govern where they differ. Soil groups come from the NRCS Web Soil Survey, not from guessing.
Composite Curve Number
Results
Total area
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Composite CN (area-weighted)
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Potential retention S = 1000/CN − 10
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Initial abstraction Ia = 0.2S
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Runoff depth Q
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Runoff volume
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How the Numbers Are Derived
The runoff equation
Q = (P − Iₐ)² / (P − Iₐ + S), with S = 1000/CN − 10 and Iₐ = 0.2S (all inches). If P ≤ Iₐ, runoff is zero — the storm never exceeds the initial abstraction. Volume = depth × area (1 inch over 1 acre = 3,630 cf ≈ 27,154 gal; 12 acre-inches = 1 acre-ft).
Compositing caveats (the fine print that changes answers)
Area-weighting CN is standard but not always right. TR-55 itself warns that compositing pervious and impervious CNs can misstate runoff when the pervious CN is low or impervious areas are unconnected (drain across lawn before reaching the system) — Figures 2-3/2-4 in TR-55 adjust for that. When precision matters, compute runoff separately per cover and sum the volumes.
ARC/AMC: Table values are average antecedent runoff condition (ARC II). Wet (III) or dry (I) conversions exist but most design criteria just use II — follow the manual.
Low CN + small storm: the method is unreliable when Q computes to a few hundredths of an inch — real-world scatter swamps the estimate.
Where the inputs come from
P: NOAA Atlas 14 Precipitation-Frequency Data Server for the design return period, 24-hr duration.
HSG: NRCS Web Soil Survey — draw the site, read the hydrologic soil group per map unit. Dual classifications (e.g. A/D) use the drained/undrained letter per the drainage condition.
CN: TR-55 Table 2-2 (built into the presets here) or the local manual's table.
References
NRCS TR-55 — Urban Hydrology for Small Watersheds (runoff equation, Table 2-2, composite CN guidance). https://www.nrcs.usda.gov/
NRCS National Engineering Handbook, Part 630 Ch. 9–10 — curve number theory.