Watershed Runoff Calculator — Formula
## 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
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
Esempio Risolto
A 50-acre suburban watershed with 2 inches of rainfall and runoff coefficient of 0.5.
- Runoff acre-inches: 50 * 2 * 0.5 = 50 acre-inches
- Runoff gallons: 50 * 27,154 = 1,357,700 gallons
- Runoff acre-feet: 1,357,700 / 325,851 = 4.17 acre-feet
- Retained: 50 * 2 * 0.5 * 27,154 = 1,357,700 gallons