SCS Unit Hydrograph Calculator

Enter drainage area, runoff depth, and timing parameters to calculate peak discharge and generate an SCS/NRCS dimensionless unit hydrograph.

Calculate Storm Hydrograph

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

Input Parameters

Watershed Parameters

Enter the watershed characteristics

mi²

Watershed area contributing runoff (mi²)

hours

Time for runoff to travel from most distant point to outlet

in

Runoff depth (precipitation minus losses)

Storm Parameters

Optional parameters for hydrograph generation

hours

Duration of unit excess rainfall (default: Tc/5)

hours

Time interval for hydrograph output (default: Tp/10)

SCS Unit Hydrograph Method Overview

The SCS (NRCS) Dimensionless Unit Hydrograph method generates synthetic hydrographs for storm runoff analysis. Key equations:

  • Peak discharge: qp = K x A x Q / Tp
  • Time to peak: Tp = D/2 + Tlag
  • Lag time: Tlag = 0.6 x Tc
  • Time base: Tb = 5 x Tp

Where:

  • K = Peak rate factor (484 for US units)
  • A = Drainage area (mi²)
  • Q = Excess rainfall depth (in)
  • D = Unit hydrograph duration (hours)
  • Tc = Time of concentration (hours)

Peak Rate Factor Guidelines

Watershed ConditionK (US)K (SI)
Flat/Swampy terrain3000.129
Standard conditions4840.208
Steep terrain6000.258

Source: NRCS NEH Part 630, Chapter 16 (Eq. 16A-6); SI values per FHWA HEC-22 (3rd ed.) Eq. 3-22 (0.00043 × K). The standard peak rate factor of 484 applies to average watershed conditions. Adjust for terrain and storage effects.

Related Calculators

SCS Curve Number

Calculate runoff depth from rainfall

Time of Concentration

Estimate Tc using multiple methods

Rational Method

Quick peak discharge for small areas
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