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Watermark H2O

Located in Santa Cruz serving all of California

Call us at (408) 690-7581
Watermark H2O's Logo

Watermark H2O

Located in Santa Cruz serving all of California

(408) 690-7581

Introduction to Rainwater Harvesting


Rainwater Harvesting

In many places, water conservation is becoming one of the most important environmental issues of the 21st century. Homeowners can reduce water consumption by using in their landscapes a beautiful array of native plants well suited to their region. Another way to conserve water is to harvest the rainfall you receive. Most of us rely on a utility company to supply our water, but early pioneers creatively used roofs and cisterns to collect rainwater for all their household and agricultural uses. With increased concern over water pollution and depletion of the water resources, there has been renewed interest in the self-sufficiency of harvesting rainwater, one of the purest sources of water available.

The most common roofing material used for a rainwater harvesting system is metal because it is smooth, durable and affordable. The catchments area is the same square footage as the footprint of your building, as long as the entire roof has gutters. Gutters should have leaf screens and overhanging tree branches should be pruned back. Be sure all soldering is lead-free. When rainwater is intended for drinking, a roof washer system should be installed that collects and disposes of the first flush of water from the roof.

A variety of cisterns are now commercially available. A cistern should be placed with vehicular accessibility in mind, in a shady area close to the catchments area. A strong foundation is essential to adequately support the weight of the water. Chances are you will need a pump with a pressure head tank to simulate usual water pressure.

Rainfall is free of most water quality hazards and contains almost no dissolved minerals or salts that typically clog plumbing and appliances. Rainfall has a pH slightly below neutral and its acidity can easily be neutralized. You'll want to have your water tested by a certified laboratory to ensure it meets standards for drinking water if you plan to bring it into the domestic water system. Results will help determine if your system will need to employ treatment techniques such as screening, settling, filtering or disinfecting. All of these methods are relatively easy to integrate to your rainwater storage systems.

The design of your rainwater harvesting system should take into account how much water you will use. Look through past utility statements for numbers. Take advantage of low flow plumbing fixtures and water-wise irrigation practices to extend your rainwater holdings. One inch of rain on one square foot of catchments area yields 0.62 gallons. To calculate how much rainwater you can catch each year, multiply the square footage of your roof catchments area by 0.62 gallons by the average rainfall. If your system is not 100 percent efficient at catching the rain, you may want to take this into account in your equation. To determine your daily allotment, divide this figure by 365 days per year. When considering the possibility of drought conditions, cut this figure in half.

In sizing your system, you can determine storage and demand by multiplying the square footage of your roof catchments area by 0.62 gallons by the monthly rainfall, then adding the gallons already in storage and subtracting the monthly demand.

Typically, a rainwater harvesting system will cost much less than drilling a well, and the break-even point may be 15 years out depending on the size of tank you install. However, rainwater is historically reliable and the quality is unsurpassed. A rainwater harvesting system would be an asset to any home, and a healthful investment for an ecologically sustainable community and future.

Reasons To Collect Rainwater

Collecting rainwater makes sense for a variety of economic and environmental reasons:

  • Rainwater is an economical alternative to public water, especially for exterior water uses such as landscape irrigation that require minimal filtration. Although initial equipment installation can be significant, long-term costs are minimal.
  • Rainwater can supplement limited ground water resources. With reduced extraction rates, low-yield ground water wells and springs can last indefinitely. Rainwater can also Supplement surface water resources threatened by rapidly growing municipal water use. Rainwater collection could significantly reduce water extraction rates from rivers during critical summer months, ensuring adequate water remains to support native ecosystems.
  • Rainwater is often the only viable water source in arid regions or on islands where other water sources may be high in salt, limited in availability, or very expensive.
  • Rainwater is low in minerals, so it is ideal for laundry, dishwashing, hair washing, and car washing. Since it contains no chlorine, rainwater is also ideal for filling garden ponds and irrigating sensitive plants.
  • Rainwater is not regulated by municipal water restrictions. During periods of drought, Rainwater can protect investments in landscaping, garden ponds, and swimming pools.
  • Rainwater can cause leaky basements, eroded foundations, overflowing sewers, soil erosion, and water pollution. Collecting rainwater can help these problems while eliminating the need for expensive storm water controls.

The following is very general and a basic introduction to rainwater collection 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 this publication. 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, clothes washers, 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:

  • Requirements per person, eastern states = 500 square feet of roof + 1000 gallons storage
  • Requirements per person, central states = 750 square feet of roof + 2000 gallons storage
  • Requirements 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 10ft 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 rain barrels; a 10ft 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. However with the proper water diversion kits installed on your home, you can increase your capacity of available water for storage depending on what type of system you have installed and your needs.


It's possible to collect rainwater from roofs, parking areas, pavement, lawns, and almost any other surface, but roofs typically yield the best quality water at the lowest cost. The type of roof surface is of little consequence when rainwater is collected for irrigation or other exterior water uses, but when rainwater is collected for interior water uses, it is preferable to use relatively inert materials such as painted metal, terra cotta tile, cement tile, stone, and elastomeric membranes instead of composite shingles, bituminous membranes, and asphalt coatings. However. rooftop debris usually poses a greater water-quality problem than the roofing material, and water from any roof can be treated to drinking-water quality without great expense.

Gutter and downspout sizing for rainwater collection can follow standard practice, although it is preferable to be somewhat conservative to minimize the potential for overflow due to improper installation or settling. Gutter cap systems can be used to reduce the maintenance of pre-filters, but should not be considered as substitutes for pre-filters.

Rainwater systems are most economical when all the rainwater is conveyed to a central site for pre-filtration, storage, and pumping. Piping should be sized using conventional storm water practice which means 4" pipe will suffice for most residential systems but 6" or larger pipe will be required for most commercial systems. A pitch of one-eighth to one-quarter inch per foot is recommended, but this sometimes poses a design challenge because the allowable burial depths of pre-filters and underground tanks are limited. Pipe connections should be watertight to prevent both water loss and infiltration.