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Rainwater Harvesting in Rain Barrels & Cisterns Design Guidelines

Rainwater Benefits & Uses

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Rainwater harvesting captures, diverts, and stores stormwater runoff for later use. Capturing even a small amount of your roof runoff will have environmental benefits, including reducing demand on your potable water supply and reducing stormwater runoff flowing into storm drains and surface waters. 

Collected rainwater from rooftops may be used for landscape irrigation, fire protection, toilet flushing, and clothes washing. A water cistern or rainwater collection equipment up to 1,500-gallon capacity located on the sides or rear of building and outside of building setback areas, is
eligible for a Counter Determination permit. Larger systems may require Administrative Permits. In addition a permit is required for all indoor and non-potable uses. For permitting questions, contact the PG Community Development Department 831-648-3100.

Rebates are not retroactive
Rebates are not available for projects that have been partly or completely installed. However a new barrel or cistern added to an existing rainwater harvesting system is eligible for a rebate.

Rainwater harvesting systems can range in size and complexity. All systems have basics components, which include a catchment surface, conveyance system, storage, filtration, and overflow.

Is Rainwater Harvesting a Good Idea for My Property?

Rain barrels and cisterns are appropriate for sites with the following characteristics:

  • Roof areas that drain to downspouts that have minimal leaf drop from overhanging tree branches.
  • A level, firm surface to support a rain barrel(s) or cistern to prevent shifting or falling over. A full 55-gallon rain barrel will weigh over 400 lbs. Consider choosing a space that provides an opportunity to ‘scale’ the system, or add more storage capacity over time. 
  • An area on the north or east side of the house, or other area where the rain barrel(s) or cistern can be shaded by vegetation. 
  • A landscaped area away from the building foundation where the rain barrel or cistern overflow and supply outlet can drain by gravity.

How Much Rain Can I Harvest?

The amount of rain you can harvest depends on your roof catchment area, the seasonal rainfall where you live, and your storage capacity. The table below describes the harvesting potential for a 1,000 square foot roof.

 Region
Harvest Potential from an 
Average Storm Event from a 1,000 ft roof 
Annual Harvesting
Potential in an Average Rainfall Year
 Santa Cruz Mountains/Scotts Valley 1,480 gallons  25,000-30,000 gallons
 Santa Cruz/Capitola/Aptos  1,125 gallons  17,000-19,000 gallons
 Watsonville/Carmel Valley  830 gallons  10,000-12,000 gallons
 Monterey/Carmel/Salinas  710 gallons  9,000-13,000 gallons

 

Components of a Rainwater Harvesting System

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Metal roof surface – the cleanest for harvesting rainwater 
Catchment Area
The catchment area is the first point of contact for rainfall. Although it is possible to harvest runoff from parking lots, patios, and walkways, these catchment surfaces have greater levels of debris and contaminants and require more complicated filtration and treatment systems. The best catchment area is a clean roof surface.
 
Roof Materials
The material of the roof is not as important as contaminants that may be on the roof. For landscape irrigation and indoor, non-potable uses, the common asphalt shingle will work fine. If you are constructing a new roof, we recommend a metal roof because they easily shed contaminants. In all cases, it’s important to avoid wooden shingles or metal flashing that contain lead or arsenic. Roofs with cement, clay, or metal surfaces are ideal for harvesting water for a wide variety of uses.

Gutters and Downspouts 
Properly sized and maintained gutters and downspouts are essential to a rainwater harvesting system. 

  • When piping water from a downspout to a cistern, use the same size pipe as your downspout. Do not reduce the size!
  • Strategically locate any new downspouts in an area where the rain barrel or cistern will be most useful.
  • Install a fine mesh gutter guard on gutters to keep leaves and other debris from entering and clogging the gutters. This will reduce the need for cleaning gutters and the rain barrel or cistern.
  • As needed, consult a professional roofer to aid in gutter and downspout installation.
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This gutter is covered by a fine mesh gutter guard to keep debris out.
Courtesy of Gutter Glove
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Installed at a single family residence.
Courtesy of Oona Johnsen

Foundation
Before installing a rain barrel or cistern, prepare the site so that the system will function safely.

  • Find or create a level location near the downspout on which to place the rain barrel or cistern.
  • Cisterns can be installed on grade on compacted earth, concrete or stone paver foundation. An engineered foundation is required for large cisterns.
  • Install pea gravel around your rain barrel or cistern so that rainwater will not erode the edges of the foundation.
  • Maintain clear access to the rain barrel outlets and cleaning access points.

 

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Cistern Overflow to Rain Garden
Landscaped with California Native Plants

Rainwater Overflow
The overflow pipe should be the same size as the rainwater inlet pipe. 

  • Overflow pipes must be screened with maximum 1/16” mesh material to prevent animals and mosquitos from entering.
  • When designing the overflow path, remember that in heavy storms rain barrels and cisterns will overflow. A 1,000-sq.-ft. roof will produce about 600 gallons of runoff during a storm that produces a depth of 1 inch of rain.
  • Allow overflow to drain to a vegetated area of your landscape. Rain gardens, dry creek beds and swales allow you to integrate overflow into a beautiful landscape feature that creates habitat and infiltrates water into your landscape so that you won’t have to irrigate as much.
  • Ensure that areas receiving overflow do not have standing water for more than 48-hours. Always direct overflow away from your home’s foundation.
 
Required and Optional Filtration to Keep Water Clean

Required Pre-Tank Debris Screens
A debris excluder or screen with a maximum 1/16” mesh is required to keep large debris and mosquitoes from entering the barrel or cistern. Many cisterns and barrels are manufactured with debris screens pre-installed on inlets.

Optional Roof Washer or First Flush Device 
Leaves, twigs, sediment, and animal waste are common in runoff, especially at the beginning of a storm (“first flush”). This debris can result in clogging and encourage bacterial growth. A first flush diverter helps remove debris and contaminants by directing the first few gallons of runoff from the roof to landscaping, away from the rain barrel or cistern. 
Required Post-Tank Sediment Filtration
Rainwater harvesting systems used for drip irrigation and indoor, non-potable uses such as toilet flushing and clothes washing are required to have a 100 micron sediment filter installed on rainwater outlet piping.

Disinfection
Disinfection options include ultraviolet light, ozone, and chlorine. Disinfection is required for potable uses of rainwater such as showering, cooking and drinking water.
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Debris screen and first flush downspout diverter installation
Courtesy of The City of Oakland

 

Design Considerations for Rain Barrels and Small Cisterns 

When installing rain barrels and cisterns, consider the following criteria unless otherwise instructed by the municipality.

  • If irrigating edible landscapes, consider pipes that meet FDA food grade standards.
  • There shall be no direct connection of any rain barrel or cistern and/or rainwater collection piping to any potable water pipe system. Rainwater systems shall be completely separate from potable water piping systems. 
  • Place the bottom of a rain barrel at a higher elevation than the landscape, to use gravity flow.
  • The low water pressure from a small rain barrel will not operate in-ground sprinkler or low-volume devices. Consider using laser cut drip tubing. 
  • All rain barrels and cisterns should have a screen to ensure mosquitoes cannot enter. 
  • If the water is not needed for irrigation during the rainy season, consider releasing the water to a vegetated area between storms, so the barrels will be empty to catch rain from the next storm. This will help protect your watershed by reducing the quantity and speed of water entering local creeks during storms. Install a spigot and laser cut drip tubing to allow the rain barrel or cistern to slowly drain between storms. You can store the water captured towards the end of the rainy season to irrigate your garden in the dry season. 
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Daisy-chained system
Courtesy of Acterra

 

Operation and Maintenance

After installing your rain barrel or cistern, follow these tips for long-term safety and functionality. 

  • Regularly check the gutters and gutter guards to make sure debris is not entering the rainwater harvesting system.
  • Inspect the screens on the rain barrel or cistern prior to the wet season to make sure debris is not collecting on the surface and that there are not holes allowing mosquitoes to enter the rain barrel. Inspect screens more frequently if there are trees that drop debris on the roof.
  • Clean the inside of the rain barrel once a year (preferably at the end of the dry season when the rain barrel has been fully drained) to prevent buildup of debris. If debris cannot be removed by rinsing, use vinegar or another non-toxic cleaner. Use a large scrub brush on a long stick, and avoid actually entering the rain barrel. Drain wash water to landscaping.

BASMAA are acknowledged for providing text and formatting used in this fact sheet.