rainwater mini-log


Following is a very general introduction to rainwater collection. Please contact our technical staff for assistance in specific system design for your climate and needs.

RAINWATER AVAILABILITY: Although rainwater can be collected from virtually any surface, bare rooftops generally yield the best quality rainwater with the least treatment. Not all of the rainwater that strikes a roof can be collected: water is lost from evaporation, blowing wind, overflowing gutters, and leaky collection pipes, first-flush devices, and self-cleaning filters. The net collectable rainwater from a bare roof can be roughly estimated as follows:

          collectable rainwater (gallons) = 0.5 x rainfall (inches) x area (square feet)

Monthly and yearly rainfall data for 300 weather regions of the United States, Puerto Rico, and the US Virgin Islands can be found in the table AVERAGE RAINFALL at the end of our Rainwater Handbook (see Documents). As a general observation, in the continental US yearly rainfall averages 10 to 30 inches in the western states, 20 to 40 inches in the central states, and 30 to 50 inches in the eastern states, with widely varying amounts in some mountain and coastal areas such as the Pacific Northwest. Consequently, in terms of roof area, the available annual rainfall would be

          collectable rainwater, eastern states = 15 - 25 gallons per square foot
          collectable rainwater, central states = 10 - 20 gallons per square foot
          collectable rainwater, western states = 5 - 15 gallons per square foot

In the eastern states rainfall is relatively evenly distributed throughout the year; in the western states rainfall is concentrated in the winter months; and in the central states rainfall is concentrated in the summer months. This has important consequences for rainwater system sizing.

SIZING A RAINWATER COLLECTION SYSTEM: On average, Americans use 70 gallons per person per day to operate toilets, showers, clotheswashers, sinks, and other water-using fixtures and appliances. By replacing fixtures and appliances with modern water-efficient versions and repairing leaks, water usage can be reduced to less than 50 gallons per person per day. Comparing demand for water with the availability and pattern of rainwater yields the following very rough “rules of thumb” for rainwater systems used to provide a meaningful percentage of household water demand:

          per person, eastern states = 500 square feet of roof + 1000 gallons storage
          per person, central states = 750 square feet of roof + 2000 gallons storage
          per person, western states = 1000 square feet of roof + 4000 gallons storage

Determining collection and storage requirements for irrigation is more complex because irrigation water usage can be greatly reduced by selecting native plants, or plants that thrive in regions with similar climates. In general, dry-climate plants thrive with one-half inch of rainfall per week, temperate-climate plants with one-inch of rainfall per week, and wet-climate plants with one and one-half inches of rainfall per week. Converting this to gallons:

          irrigation of dry-climate plants (gallons/week) = 0.3 x area (square feet)
          irrigation of temperate-climate plants (gallons/week) = 0.6 x area (square feet)
          irrigation of wet-climate plants (gallons/week) = 0.9 x area (square feet)

As examples, for temperate climate plants such as typical vegetables, ornamentals, and lawn grasses grown in the eastern states, a 10 ft x 10 ft vegetable garden would do well with 0.6 x 100 square feet = 60 gallons per week which could be supplied by a small section of roof feeding a few rainbarrels; a 10 ft x 100 ft strip of ornamentals might need 0.6 x 1000 square feet = 600 gallons per week which could be supplied by a typical house roof feeding a 1000 to 2000 gallon tank, but a quarter-acre (10,000 square feet) of lawn grass could use 0.6 x 10,000 square feet = 6,000 gallons of water per week, a quantity that is beyond the capacity of most rainwater collection systems.

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