Rainwater storage capacity is sized based on two factors: the roof catchment area and your water demand. Roughly, the gallons of water per inch of rain harvested off of a roof area is 0.6 times the roof square footage. Or, you can harvest approximately 600 gallons for every 1000 square feet of roof in every inch of rainfall. This quantity varies slightly based on roof material and pitch. In cistern system design, we recommend sizing your capacity for what you will receive between 2 to 6 inches of rain. As we typically receive around 6 inches of rain in the winter and 6 inches in summer monsoons, sizing towards the 6" rainfall capacity allows for storage of excess rain during the rainy season, and longer use of this water into the dry months. Example 1: A 1,500 square foot flat roof drains to one downspout at the northeast corner of the house.
This downspout receives approximately 1,680 gallons in 2 inches of rain and 5,040 gallons in 6 inches of rain. The customer wishes to use this water to manually water a citrus tree on the east side of the house. For this site, we chose to install two 2,300 gallon steel cisterns, (8' diameter x 7.5' height) for a total capacity of 4,600 gallons. A hanging rain gauge was added to this tank to provide quick visual reference to the amount of water currently stored.
Example 2: A 1,400 square foot pitched tile roof drains to two downspouts. Each downspout on this site receives approximately 750 gallons in 2 inches of rain and 2,250 gallons in 6 inches of rain. This customer has an existing irrigation system with the controller mounted on the north side of the house, but limited space for a large diameter tank. For this system, we chose to install three 1,000 gallon polyethylene cisterns (5' diameter x 7.5' height) for a total capacity of 3,000 gallons. The supply lines from these cisterns are run underground to supply the existing irrigation system.