What is Water Quality Volume and Why Should You Care?
Water Quality Volume (WQV) determines the volume of stormwater runoff that needs to be managed to improve water quality. This is essential for environmental management, urban planning, and regulatory compliance.
By understanding and calculating WQV, you can develop strategies to mitigate pollution and protect local aquatic ecosystems.
How to Calculate Water Quality Volume
The formula for WQV:
[\text{WQV} = \text{Precipitation Depth} \times \text{Watershed Area} \times \text{Runoff Coefficient}]
Where:
- WQV is the Water Quality Volume (acre-inches or cubic meters)
- Precipitation Depth is the design storm rainfall depth
- Watershed Area is the drainage area contributing runoff
- Runoff Coefficient is the fraction of rainfall that becomes runoff (0-1)
Calculation Example
Calculate WQV for a 10-acre site with 1 inch of precipitation and a runoff coefficient of 0.6.
[\text{WQV} = 1 \times 10 \times 0.6 = 6 \text{ acre-inches}]
To convert to cubic feet:
[6 \text{ acre-inches} \times 3,630 = 21,780 \text{ cubic feet}]
Runoff Coefficients by Surface Type
| Surface Type | Runoff Coefficient |
|---|---|
| Concrete/Asphalt | 0.85 - 0.95 |
| Rooftops | 0.75 - 0.95 |
| Gravel | 0.40 - 0.60 |
| Lawns (flat) | 0.10 - 0.25 |
| Lawns (steep) | 0.25 - 0.35 |
| Natural Forest | 0.05 - 0.25 |
Applications
- Site Development: Size retention ponds and bioswales
- Stormwater Permits: Meet regulatory requirements
- Environmental Protection: Reduce pollutant loading to waterways
- Flood Management: Understand site runoff characteristics