I have a 2,000 square foot home in Austin and another in El Paso. How much water can I expect to collect at each location in a year?
In theory, a rainwater harvesting system can collect approximately 0.62 gallons of water per square foot of roof area, per inch of rainfall. In practice, however, there is always some loss due to first flush, evaporation, splash-out, overshoot from gutters, and possible leaks. Most installers use an efficiency of about 75 to 85 percent for the system. For your Austin home, using a collection rate of 0.62, a system efficiency of 0.85, and an average annual rainfall of 32 inches, you can expect to collect about 34,000 gallons of rainwater per year (0.62 x 0.85 x 2,000 x 32 = 33,700 gallons per year). Using these same parameters for the El Paso location but with an average annual rainfall of 8.5 inches, the amount of water you can expect to collect will be only about 9,000 gallons (0.62 x 0.85 x 2,000 x 8.5 = 8,950 gallons).
