SCS Unit Hydrograph Calculator

Calculate peak discharge and generate storm hydrographs using the SCS/NRCS dimensionless unit hydrograph method. Professional-grade hydrology tool for medium-scale watershed analysis.

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 terrain3001.29
Standard conditions4842.08
Steep terrain6002.58

Source: TR-55 (1986), Chapter 3. The standard peak rate factor of 484 applies to average watershed conditions. Adjust for terrain and storage effects.

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