Learn how to figure out roof rain runoff with this catchment area calculation formula, to calculate catchment capacity and water holding needs.
Catchment Area Calculation Formula
If you’re using a roof to harvest rain, the size of your catchment area is the square footage of the building’s footprint. Each square foot of collection area should yield 0.6 gallons of water per inch of rainfall, although some water will be lost to evaporation and leakage, which is factored in by an efficiency factor of 0.8.
To determine your roof’s water-harvesting potential:
- Multiply the square feet of collection area by 0.6 gallons (per square foot of area).
For our sample calculation, we used Austin, Texas, which receives an annual average rainfall of 32 inches. From 2,500 square feet of collection area, about 38,400 gallons of water per year, or 105 gallons per day, can be harvested.
To more accurately determine how much storage capacity you need and what kind of demand your system can withstand, a monthly balance calculation is necessary.
