DrainageCalculators

Hydrograph Convolution Calculator

Calculate runoff hydrographs using unit hydrograph convolution. Applies the principle of superposition to combine rainfall increments with unit hydrograph response per NRCS NEH Part 630.

What This Solves

Generates a runoff hydrograph by convolving (superimposing) a rainfall excess time series with a unit hydrograph using the principle of superposition.

Best Used When

  • You have a unit hydrograph and a rainfall distribution and need the resulting runoff hydrograph
  • You are performing detailed hydrologic modeling with time-varying rainfall
  • You want to see the full flow-over-time response of a watershed to a design storm

Do NOT Use When

Key Assumptions

  • The watershed response is linear (principle of superposition applies)
  • Rainfall excess is uniform over the watershed for each time step
  • The unit hydrograph is time-invariant (does not change during the storm)
  • Base flow is not included in the convolution (added separately if needed)
  • Rainfall excess has already been separated from total rainfall (losses removed)

Input Quality Notes

The quality of the result depends entirely on the unit hydrograph and the rainfall excess hyetograph. Verify that the unit hydrograph time step matches the rainfall increment duration.

Hydrograph Convolution

For educational purposes only. Not a substitute for professional engineering judgment.

Input Parameters

Calculation Mode

Choose convolution or unit hydrograph generation

Select calculation type

Watershed Parameters

Basic watershed characteristics

sq mi

Contributing watershed area

minutes

Computation interval

Unit Hydrograph

Enter UH ordinates as comma-separated values

10 ordinates parsed

Unit hydrograph discharge values (cfs)

in

Depth represented by unit hydrograph

Excess Rainfall

Enter rainfall increments as comma-separated values

4 increments, Total: 2.50 in

Excess rainfall per time step (in)

cfs

Constant base flow to add

Professional Review: For projects requiring regulatory approval or where public safety is involved, have your calculations reviewed and stamped by a licensed Professional Engineer (PE).

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Last verified: February 2026

Disclaimer & Terms of Use

Educational Purpose: The calculations provided by DrainageCalculators.com are for informational and educational purposes only. While we strive for accuracy, these tools do not substitute for professional engineering judgment or designs stamped by a licensed Professional Engineer (PE) where required by law.

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