DrainageCalculators

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.

What This Solves

Generates the SCS/NRCS dimensionless curvilinear unit hydrograph for a watershed, providing a detailed runoff response to a unit of rainfall excess.

Best Used When

  • You need a detailed runoff hydrograph for medium-scale watershed analysis
  • You are developing inputs for detention basin routing or downstream flood analysis
  • You want the standard NRCS curvilinear hydrograph shape rather than the simplified triangular version

Do NOT Use When

Key Assumptions

  • The dimensionless unit hydrograph shape follows NRCS NEH Part 630 Chapter 16 ratios
  • Peak flow occurs at the dimensionless time ratio t/Tp = 1.0
  • The hydrograph shape is fixed — only the peak magnitude and timing scale with watershed characteristics
  • Watershed lag time is 0.6 * time of concentration
  • The peak rate factor (PRF) of 484 applies (standard for average conditions)

Input Quality Notes

Watershed area, time of concentration, and curve number are the key inputs. Ensure Tc is computed with methods appropriate for the watershed size. For urban watersheds, consider reduced Tc due to improved channels and storm sewers.

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.

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

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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