Watershed Runoff Calculator

Estimate stormwater runoff volume from a watershed based on area, rainfall, and land cover using the rational method.

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acres
inches
C

Runoff Volume

1,357,700 gallons

Runoff (Acre-Feet)4.17 acre-feet
Peak Flow (Rational Method)50.0 acre-in
Water Retained/Infiltrated1,357,700 gallons

Runoff Volume vs Watershed Area

View formula & 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

    • Example Calculation

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

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