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Rain Catchment

SF Water Facts:

San Francisco’s total annual water demands: approx. 92 million gallons a day (33.5 billion gallons per year ) (SFPUC –2000)

SFPUC’s Water System Improvement Program and Recylced Water Master Plan hopes to save 1.5 billion gallons per year

Average San Francisco resident uses 65-73 gallons per day, with the majority used in the bathroom! (SFPUC – 2002)


Using Water

Gallons Per Use

No. of Times Per Day


Taking a bath




Taking a shower




Flushing a toliet




Washing hands or face




Getting a drink




Brushing your teeth




Washing clothes




Watering Outside




Using a dishwasher




Washing dishes by hand










Figure 1. John Woodwell, "Water Efficiency in Your Home," 1991

Brad Lancaster’s Facts from his Book:

Estimating Rainfall Runoff UsingRules of Thumb

Rough rule of thumb for calculating rainfall runoff volume on a catchment surface:

You can collect 600 gallons of water per inch of rain falling on 1,000 square feet of catchment surface. On the really big scale: You can collect 27,000 gallons of water per inch of rain falling on 1 acre of catchment surface.

Possible Volume of Runoff from a Roof or Other Impervious Catchment Area:

catchment area (ft2) x rainfall (ft) x 7.48 gal/ ft3 = maximum runoff (gal)


(length (ft) x width (ft)) x rainfall (ft) x 7.48 gal/ft3 = maximum runoff (gal)

(45 ft x 25 ft) x (22 in ÷ 12 in/ft) x 7.48 gal/ft3 = maximum runoff (gal)

1,125 ft2 x 1.84 ft x 7.48 gal/ft3 = 15,484 gal

15,484 gal = runoff (42 gallons per day)

Pounds of pressure per square inch of water per foot of height = 0.43 psi/ft

Weight of Stored Water = stored water (gal) x 8.34 lb/gal = weight of stored water (lb)

Links to resources and information:

www.harvestingrainwater.com - Brad Lancaster’s site, information from his book, calculations, equipment resources

www.oaec.org/waterinstitute - Occidental Arts & Ecology Center’s riviting educational program on water

www.rainwaterharvesting.org - rainwater supersite from New Dehli, lots of examples and detailed diagrams for large systems

www.rainwaterharvesting.com - site from Sri Lanka with other resources and studies on rainwater harvesting, detailed project analysis

www.wonderwater.net - business in Mt.Shasta that does professional cistern sales and installation (installed $20k system in SF)

www.watertanks.com - business in Windsor that sells everything you need for home or agricultural size tank/cistern systems

www.epa.gov/enviro/wme/ - EPA site with detailed information about AIR, LAND, and WATER in your community

www.sfwater.org - SFPUC site with links to information, education, tours, and free water conservation tools