Wednesday, July 22, 2009


Here's a simple but very accurate formula for calculating how much water is coming off your roof (and how much water you are losing if you are not harvesting it).

Take the area (2,000 square feet) and multiply it by 8 (amount of rainfall). Take that number and multiply it by 623. Divide that amount by 1000, and you have the amount of runoff in a season. If you also want to figure evaporation, then multiply your answer by .92 or 92% (.08 or 8% is the factor generally used to figure evaporation in a desert environment: 100% -8%=92%).

Thus, a 2,000-square-foot roof, receiving 8 inches of rain per year, will have about 9,710 gallons of runoff. Or, that same area receiving one inch of rain will have 1,246 gallons of runoff. If you want to calculate evaporation, then multiply 1,246 by 92% and that will give you a fairly accurate figure.

No comments:

Post a Comment