Watershed Runoff Calculator Formula

Understand the math behind the watershed runoff calculator. Each variable explained with a worked example.

Use the Watershed Runoff Calculator

Formulas Used

Runoff Volume

runoff_volume = runoff_gallons

Runoff (Acre-Feet)

runoff_acre_ft = runoff_gallons / 325851

Peak Flow (Rational Method)

peak_cfs = runoff_coefficient * rainfall_in * area_acres / 1

Water Retained/Infiltrated

retained = area_acres * rainfall_in * (1 - runoff_coefficient) * 27154

Variables

VariableDescriptionDefault
area_acresWatershed Area(acres)50
rainfall_inStorm Rainfall(inches)2
runoff_coefficientRunoff Coefficient(C)0.5
runoff_acre_inDerived value= area_acres * rainfall_in * runoff_coefficientcalculated
runoff_gallonsDerived value= runoff_acre_in * 27154calculated

How It Works

How to Calculate Watershed Runoff

The rational method estimates stormwater runoff based on rainfall, area, and land cover.

Formula

Runoff = Area * Rainfall * Runoff Coefficient

Typical Runoff Coefficients (C)

  • Forest: 0.10-0.20
  • Grass/park: 0.20-0.35
  • Suburban: 0.40-0.60
  • Urban/commercial: 0.70-0.95
  • Parking lot: 0.85-0.95

    • Worked Example

    A 50-acre suburban watershed with 2 inches of rainfall and runoff coefficient of 0.5.

    area_acres = 50rainfall_in = 2runoff_coefficient = 0.5
    1. 01Runoff acre-inches: 50 * 2 * 0.5 = 50 acre-inches
    2. 02Runoff gallons: 50 * 27,154 = 1,357,700 gallons
    3. 03Runoff acre-feet: 1,357,700 / 325,851 = 4.17 acre-feet
    4. 04Retained: 50 * 2 * 0.5 * 27,154 = 1,357,700 gallons

    Ready to run the numbers?

    Open Watershed Runoff Calculator