In order to estimate the amount of gallons harvested from each RWH system, we utilized a simple function. The factors of this equation include a conversion factor, the total area of a rooftop and the average rainfall in inches. A RWH system can collect an estimated 0.62 gallons of water per square foot (11). The product of this equation equates to the total gallons of water collected, assuming no water is lost between the catchment area and the storage tank. The formula is shown below:
G = 0.62 (A) (R)
where G is the total gallons harvested, 0.62 is the conversion factor, A is the catchment area in sq ft, and R is the average rainfall in inches.
where G is the total gallons harvested, 0.62 is the conversion factor, A is the catchment area in sq ft, and R is the average rainfall in inches.
However, because water loss is evident in the process, the outcome of this equation is not completely accurate. Factors like splashing, wind interference, overflow and evaporation all contribute to water loss, and these factors amount to a 15% reduction in water collected (11). The losses are included in the following equation:
G actual = 0.85 (G)
Using the average rainfall per month of Harris County as well as the estimated rooftop area of the UH Student Canter, Alumni Center and Recreational Center, an estimate of total gallons harvested can be obtained with the equations above. The following table shows these estimates:
According to UH’s Utility and Sustainability Coordinator Paul Brokhin, the University of Houston pays a set rate of $4.20 per thousand gallons. Using this information, we obtained an estimate of the value of the water, and the following graph illustrates a monthly breakdown of savings from each water harvesting system.
By constructing a rooftop water collection system on the Student Center, Alumni Center and Campus Recreation & Wellness Center, the University of Houston can save an estimated total of $51,616.25 per year. After obtaining this estimate, we were able to calculate how long it will take to offset the total installation cost. View our findings by clicking the link below.