Learn about harvesting roof rain and calculating catchment capability.
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:
1. Multiply the square feet of collection area by 0.6 gallons (per square foot of area).
2. Take this total times 0.8 (the efficiency factor).
3. Multiply the total from Step 2 by your area's annual average rainfall (in inches). You can find the average precipitation in your area by visiting the National Oceanic and Atmospheric Administration. This data chart gives monthly and yearly averages of data collected for specific cities during the past 30 years.
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. The Texas Guide to Rainwater Harvesting gives step-by-step instructions on figuring this calculation.
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