Skip to main content
Calcimator

Unit Hydrograph Calculator

Calculate the SCS triangular unit hydrograph peak flow, time to peak, and time base from drainage area, lag time, and storm duration.

Inputs

km²
hrs
hrs

Results

Peak Discharge (Qp)

29.71 m³/s/mm

Time to Peak (Tp)

3.5 hrs

Time Base (Tb)9.34 hrs
Time of Concentration5.01 hrs
Recession Time5.84 hrs
Unit Volume499,824 m³
Expected Volume (1mm)50,000 m³
How to Use This Calculator
  1. Enter Drainage Area, Basin Lag Time, and Unit Rainfall Duration.
  2. Review Peak Discharge (Qp) (m³/s/mm) and Time to Peak (Tp) (hrs).
  3. Use Time Base (Tb) (hrs) and Time of Concentration (hrs) to inform your decision.
  4. Use the chart to visualize the results and explore different scenarios by adjusting inputs.

How the result changes with Drainage Area

Drainage AreaPeak Discharge (Qp)Time to Peak (Tp)
5,0002,971.43 m³/s/mm3.5 hrs
17,50010,400 m³/s/mm3.5 hrs
32,50019,314.29 m³/s/mm3.5 hrs
45,00026,742.86 m³/s/mm3.5 hrs

What each input means

Drainage Area
Total contributing drainage area of the watershed in square kilometers
Basin Lag Time
Time from center of excess rainfall to the hydrograph peak (typically 0.6 × Tc)
Unit Rainfall Duration
Duration of the unit excess rainfall pulse in hours

How this is calculated

Formula

Tp = D/2 + Tlag; Qp = 2.08 × A / Tp

Worked example, using the default values

  1. Identify Input Parameters
    Drainage Area = 50, Basin Lag Time = 3, Unit Rainfall Duration = 1 = 3 input(s) provided
  2. Calculate Peak Discharge
    Peak Discharge
    29.71 = 29.71
  3. Calculate Time to Peak
    Time to Peak
    3.5 = 3.5
  4. Calculate Time Base
    Time Base
    9.34 = 9.34
  5. Calculate Time of Concentration
    Time of Concentration
    5.01 = 5.01

Related Calculators

The questions that sit next to this one — chosen by subject, including calculators filed under a different category.

Hydrology

Watershed Area Calculator

Calculate drainage area and morphometric parameters of a watershed from perimeter, shape factor, and longest flow path.

Hydrology

SCS Curve Number

Calculate runoff depth using the USDA SCS (NRCS) Curve Number method. Determine runoff, infiltration, and initial abstraction from precipitation and curve number.

Hydrology

Rational Method Runoff

Calculate peak stormwater discharge using the Rational Method (Q = CIA/360). Estimate runoff from rainfall intensity, runoff coefficient, and catchment area.

More in Environment, Weather & Climate.