Advanced Unit Hydrograph Calculator

Model direct runoff with dimensionless and triangular hydrographs. Compare peak discharge, lag, and base time. Clear outputs support watershed planning and stormwater design decisions.

Calculator Inputs

Responsive calculator grid: 3 / 2 / 1

Enter watershed and storm values below. Results appear above this form after submission.

Choose a standard shape for runoff response.
Total contributing drainage area.
Direct runoff depth from excess rainfall.
Storm block duration used for the unit hydrograph.
Travel delay from rainfall excess to peak response.
Use values above one for slower catchment response.
Controls hydrograph spread. Common triangular value is 2.67.
Smaller steps create smoother ordinate tables.
Reset
Tip: For a classic unit hydrograph, set effective rainfall to 1.0 cm and choose a time step that matches your reporting interval.

Example Data Table

Use these sample scenarios to test the calculator quickly.

Scenario Method Area (km²) Excess (cm) Duration (hr) Lag (hr) Base Factor Approx. Tp (hr) Approx. Qp (m³/s)
Small rural catchment Triangular 45 1.0 2.0 3.0 2.67 4.00 23.40
Mixed land use basin SCS 62 1.2 1.5 4.0 2.67 4.75 32.59
Broad watershed check Triangular 110 0.9 3.0 5.0 3.10 6.50 27.34

Formula Used

1) Runoff Volume

V = A × 10,000 × Pe

A is basin area in km², Pe is effective rainfall in cm, and V is runoff volume in m³.

2) Time to Peak

Tp = (Lag + D / 2) × Ft

Lag is basin lag, D is excess duration, and Ft is the optional time adjustment factor.

3) Triangular Peak Discharge

Qp = 2V / (Tb × 3600)

Tb = Fb × Tp

Fb is the base time factor. The triangular hydrograph preserves the runoff volume exactly.

4) SCS Peak Discharge

Qp = 2.08 × A × Pe / Tp

This form uses A in km², Pe in cm, and Tp in hours. Ordinates follow a standard dimensionless SCS shape.

How to Use This Calculator

  1. Choose either the SCS dimensionless or triangular hydrograph option.
  2. Enter basin area and the effective rainfall depth that becomes direct runoff.
  3. Provide storm excess duration and basin lag time from your hydrologic study.
  4. Adjust the time factor if watershed response is slower or faster.
  5. Set a base time factor for triangular analysis or keep the default value.
  6. Pick a reporting time step, then press Generate Hydrograph.
  7. Review peak discharge, timing values, runoff volume, and the hydrograph table.
  8. Use the CSV or PDF button to export your results.

Frequently Asked Questions

1) What is a unit hydrograph?

A unit hydrograph represents direct runoff caused by one unit depth of effective rainfall over a basin during a specified duration. It helps convert rainfall excess into a runoff response curve.

2) Why does the calculator ask for effective rainfall instead of total rainfall?

Only effective rainfall contributes to direct runoff. Losses such as infiltration, interception, and surface storage should be removed before using this calculator.

3) When should I choose the triangular method?

Use the triangular method for preliminary sizing, quick checks, or when you need a simple runoff shape with direct control over base time.

4) When is the SCS dimensionless option better?

The SCS option is useful when you want a smoother, more realistic hydrograph shape based on a standard dimensionless runoff pattern commonly used in drainage studies.

5) What time step should I use?

Use a time step that matches your reporting interval or routing model. Smaller steps improve curve smoothness but also create larger tables.

6) Why can the integrated volume differ slightly from runoff volume?

The difference comes from numerical integration and the selected time step. Finer sampling usually reduces the volume error shown in the results summary.

7) Can I use this calculator for a classic one-centimeter unit hydrograph?

Yes. Enter 1.0 cm for effective rainfall. The computed ordinates will then represent a one-centimeter unit hydrograph for the selected basin and duration.

8) Does this replace full hydrologic modeling?

No. This tool supports screening studies and design estimates. Detailed projects should still verify basin parameters, losses, routing assumptions, and observed hydrograph data.

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Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.