Hydrograph Unit Method Calculator

Example inputs (SCS): Area 2.5 km², excess depth 30 mm, D 0.5 hr, Tc 2 hr, baseflow 0. The sample table below shows early points from the computed hydrograph.

1. Select a method: SCS for quick design, or Custom for measured ordinates.
2. Enter drainage area, excess depth, and excess duration.
3. Add optional baseflow if you need total flow, not just runoff.
4. For SCS, provide Tc or lag. For Custom, paste UH and rainfall increments.
5. Press Calculate hydrograph, then export CSV or PDF.

Design intent and assumptions

This calculator estimates a direct-runoff hydrograph from excess rainfall using the unit-hydrograph concept. It assumes linear response, time-invariant watershed behavior, and consistent excess distribution across the basin. Use it for small to medium catchments where storage is limited or can be represented separately. If infiltration or backwater dominates, results may be optimistic. Record storm source and loss method in design notes.

Key inputs that drive peak flow

Peak discharge is controlled by drainage area, time to peak, and excess depth. For the SCS option, the unit peak is computed as Q_p,u = 484 A / T_p (cfs per inch), then scaled by P_e. As a sensitivity guide, cutting T_p from 3.0 hr to 2.0 hr increases the peak by 50%. Higher excess depth increases volume and lifts recession flows.

Interpreting the hydrograph table

The table reports direct runoff, optional baseflow, total flow, and cumulative volume. Cumulative volume is integrated with the trapezoidal rule and should roughly match the depth–area check: 1 inch over 1 mi² = 2,323,200 ft³. A balance within ±5% is typically acceptable for planning; larger gaps suggest coarse time steps, rounded ordinates, or inconsistent units. Check the tail returns near zero before exporting.

Custom convolution workflow

The custom method convolves rainfall increments with your unit hydrograph on a uniform time step. Choose ordinates with a clear unit basis (for example, cfs per 1 in) and keep the rainfall increments in the same depth unit. A practical Δt is 0.1–0.25 hr for urban sites and 0.25–1.0 hr for rural basins. Shorter steps improve peak timing but create larger tables.

Construction applications

Use the results to support temporary drainage planning, culvert and ditch checks, and detention sizing during phased grading. Compare scenarios by changing excess depth, duration, or Tc to reflect disturbed soil, compaction, or temporary diversions. For conservative design, test a 10–20% lower T_c and a higher excess fraction, then verify outlet constraints separately with hydraulic calculations.

FAQs

What is “excess rainfall” in this tool?

Excess rainfall is the portion that becomes direct runoff after losses. Use runoff depth from your method (CN, losses, or modeling), not the total storm depth.

Which method should I choose?

Use SCS for fast, consistent design hydrographs. Use Custom when you have measured or agency-provided unit hydrograph ordinates and want discrete convolution with your hyetograph.

Why does volume balance show a difference?

Differences come from rounded ordinates, coarse time steps, or mismatched units. Reduce the time step, extend the tail, and confirm depth and area conversions for tighter balance.

Can I include baseflow?

Yes. Baseflow is added to direct runoff to form total flow. Keep baseflow modest and steady; if recession is important, model it separately and add it as a varying series.

What if my unit hydrograph is in m³/s?

Enter ordinates in the same flow unit as you want to see, then select m³/s output. The calculator converts internally for consistency, but your unit hydrograph basis must remain consistent.

Is this suitable for detention routing?

It generates an inflow hydrograph. For detention, route the inflow through your storage–outflow relationship using a routing method (level pool, Modified Puls) and compare peak reductions.