Free Tool ยท SCS TR-55 ยท Rational Method ยท NOAA Atlas 14 ยท Civil & Surveying

Hydrology Runoff Calculator

Compute peak discharge and runoff volume for any storm event using the SCS Curve Number method (TR-55) and Rational Method. Full 24-hr unit hydrograph. Pre/post-development comparison. Numbers an engineer can stamp.

Watershed
Storm Event
Land Cover & Soil
Time of Concentration
Rational Method (optional)
SCS Curve Number
Effective CN 61.0
Runoff depth Q 1.079 in (27.4 mm)
Runoff volume 4.497 ac-ft (5,547 mยณ)
Time of concentration Tc 0.16 hr
Time to peak Tp 0.15 hr
Peak discharge (SCS hydrograph) 275.7 cfs (7.808 mยณ/s)
Peak discharge (Rational Method) 1.9 cfs (0.053 mยณ/s)
Unit Hydrograph (time vs flow) Civil
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Frequently Asked Questions

What is the SCS Curve Number method?
The SCS (Soil Conservation Service, now NRCS) Curve Number method from Technical Release 55 (TR-55, 1986) estimates direct runoff depth from rainfall. The CN encodes land cover and soil permeability: CN=98 is fully impervious (paved), CN=30 is dense forest on sandy soil. The runoff equation Q = (P โˆ’ 0.2S)ยฒ / (P + 0.8S) with S = 1000/CN โˆ’ 10 captures the non-linear relationship between rainfall and runoff. It is the standard method for municipal stormwater design in the US.
What is the Rational Method?
The Rational Method Q = CiA estimates peak discharge for small catchments (< 200 acres). C is a runoff coefficient (0.1 for parks, 0.85 for asphalt), i is rainfall intensity in/hr, and A is drainage area in acres. It assumes a single peak discharge โ€” it does not generate a hydrograph. Best suited for inlet and pipe sizing. For larger watersheds or detention pond design, the SCS unit hydrograph method is preferred.
What return period should I use?
Requirements vary by jurisdiction. Municipal storm drains are typically designed for the 10-year event. Detention ponds for the 100-year event. FEMA flood mapping uses the 100-year (1% annual chance) event as the base flood. The contractor Hydrology Report specification for solar/industrial projects typically requires the 100-year/24-hour event from all offsite sources.
How do I get rainfall depth (NOAA Atlas 14)?
NOAA Atlas 14 (hdsc.nws.noaa.gov/pfds/) provides point precipitation frequency estimates for any US location. Enter your latitude/longitude, select the return period and duration. For a 100-year/24-hour event in the US, depths typically range from 3 inches (dry West) to 10+ inches (Gulf Coast). Enter the Atlas 14 value here as your rainfall depth.