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

    Frequently Asked Questions

    What is a runoff coefficient?

    The runoff coefficient (C) is the fraction of rainfall that becomes surface runoff. A value of 0.5 means 50% of rain runs off and 50% infiltrates or evaporates.

    Why does urbanization increase runoff?

    Impervious surfaces like pavement and roofs prevent water from soaking into the ground. A natural area may have C = 0.15, while a commercial area can be C = 0.90, creating 6x more runoff.

    How does stormwater runoff cause pollution?

    Runoff picks up pollutants including oil, fertilizers, pesticides, sediment, and litter, carrying them directly into streams and lakes without treatment.

    Ready to run the numbers?

    Open Watershed Runoff Calculator