There are three methods of harvesting rainwater. Rainwater can be stored in barrels, cisterns or HOGs. Rain barrels are containers designed to capture rainwater runoff from your roof so that you can use it for irrigation in your landscape. Rain barrels are cheap, easy to install and maintain, and can typically store 50 to 100 gallons of water.
Cisterns are larger than rain barrels, and they range from 10 gallons on a small residential site to millions of gallons beneath schools and parks. They can be installed above or below ground. Water from cisterns can be stored until needed and used for irrigation and toilet flushing.
Rainwater HOGs are modular storage systems that and are diagrammed on the next page, however they cost around $315 a piece and can only store 50 gallons of water each.
The advantage of installing a rainwater harvesting barrel for an entire intra-block instead of per individual household is that the cost per gallon of a HOG is $6.30 while the cost per gallon of a communal cistern is only $1.20. While a 2,500 gallon cistern might take up your entire backyard, if twelve households removed their fences, you can use one plot of land for the cisterns and 11 for other activities. Having a 2,500 gallon cistern for 12 families is equivalent to having four water hogs at a fraction of the price.
Rainwater cisterns are ideal for the Richmond District. 1 inch of rainfall on a 2,000 square foot roof generates 1,250 gallons of water that can be reused. San Francisco’s average yearly rainfall has been 20.78 inches. Although we are currently experiencing a drought, water harvesting methods are long-term investments and can still be used when we are not in a drought.
With an average of 20.78 inches of rainfall a year, a single intra-block can store 25,975 gallons of clean water in case of emergencies or for gardening and landscape, or even a communal pool!