Watershed Runoff Calculator
Estimate stormwater runoff volume from a watershed based on area, rainfall, and land cover using the rational method.
Runoff Volume
1,357,700 gallons
Runoff Volume vs Watershed Area
公式
## 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
计算示例
A 50-acre suburban watershed with 2 inches of rainfall and runoff coefficient of 0.5.
- 01Runoff acre-inches: 50 * 2 * 0.5 = 50 acre-inches
- 02Runoff gallons: 50 * 27,154 = 1,357,700 gallons
- 03Runoff acre-feet: 1,357,700 / 325,851 = 4.17 acre-feet
- 04Retained: 50 * 2 * 0.5 * 27,154 = 1,357,700 gallons
常见问题
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.