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Roof-Reliant Landscaping™ Step 11C: Sizing Your Cistern: Part III – Understanding Normalcy

by Nate Downey

Now that you have determined the square footage of your collection area, the next step in sizing your cistern is to estimate the amount of precipitation that you might collect in a given time period. It is important to note that there is a significant distinction between “average” and “normal” when discussing the amount of precipitation your location receives in a year.

Although average annual precipitation data is easy to find for most municipalities and counties throughout the state, the concept of average precipitation is misleading in New Mexico. It is actually normal for a location to get 20 percent less precipitation than the average annual precipitation figure. This is because occasional wet years skew the average.

Take the example of Albuquerque from 1996 through 2005. The table below shows that Albuquerque received an average of 9.09 inches of precipitation during this 10-year period (which is 0.43 inches more than its historic average of 8.66 inches). Albuquerque received less than the average annual rainfall during five years of this period, and during three of those years it received only about 70 percent of the 10-year average.

Historical weather data shows that a normal year receives about 80% of the precipitation that an average year receives. The equation below describes this concept in simple arithmetic terms. Here, average precipitation is multiplied by 80% in order to determine the most likely amount of precipitation for a location in a given year.

Average precipitation x 0.80 = “Normal” precipitation

For Albuquerque, using the historic average of 8.66 inches of average precipitation results in this equation:

The table below shows how to convert inches of rainfall into gallons per square foot. For every inch of rainfall, 0.62 gallons of water can be harvested from every square foot of roof surface. Rounding the “normal” year in Albuquerque to 7.0 inches reveals that every square foot of roof catchment surface will predictably result in 4.34 gallons of collected rainwater.

Finally, in order to finally estimate the amount of precipitation that can potentially be collected from a roof in a normal year, multiply the total square footage by the number of gallons per square foot. For a 1,800-square-foot roof in Albuquerque, this translates into less than 8,000 gallons of water. The calculation looks like this:
1,800 (square feet of catchment area) x 4.34
gallons = 7,812 gallons

In summary, your catchment area multiplied by the number of gallons per square foot in a normal year equals the total number of gallons you can expect to collect off of your roof during a normal year.

10/12/2015 | (0) Comments