Even though we live in the arid Southwest, Santa Fe receives an average of 14.2 inches over 66 days of rain each year. Surprisingly, a single quarter acre of land in the city can harvest over 100,000 gallons of water annually! How can you calculate the amount you can harvest from your roof? Read on…
Calculating Rainfall for Harvesting
1) Using a measuring tape, measure your roof from one edge to the next. You can do this on the ground since the building perimeter has the same dimensions. If you have drawing plans with the measurements, that’s one less step. Write these measurements down. For example, your roof may be 40 feet x 60 feet.
2) Now multiply to find the total square footage of your roof. Multiply 40 feet times 50 feet and you’ll get 2,000 square feet. That’s your total catchment area.
3) Time for a little more math. An inch of rainfall on a square foot of surface area yields .623 gallons. So, multiply .623 gallons by the number of surface square feet of your roof. In our example it would be .623 x 2,000, which equals 1,246. So for each inch of rainfall, your roof could collect 1,246 gallons of rainwater.
4) Since Santa Fe receives an average of 14.2 inches of rain per year, multiply your roof’s yield per inch of rainfall (1,246 gallons) x 14.2 (Santa Fe’s annual average rainfall) and you get 17,693.2 gallons. That’s a lot of water that can be harvested and used intelligently to water your landscape, provide drinking water, or be stored in cisterns for a variety of uses.