Rainwater Harvesting: Storing Rainwater in Tanks & Cisterns

by Giovanni Castaldo, Santa Cruz Permaculture Design Course participant

This series of blog posts about Rainwater Harvesting provides an overview of some of the key practices. The series is informed and guided by the book Rainwater Harvesting for Drylands and Beyond, Brad Lancaster’s approachable and inspiring work on the topic of rainwater harvesting. This is the third in a series of blog posts by Giovanni Castaldo.

Tanks and cisterns can be used to catch and store rainwater for potable use and washing. The easy calculation mentioned in the prior blog post in this series can be used to get an approximation of how much water can be harvested off of any roof. For example, a 10’x12’ shed (120 square feet) in an area with an annual rainfall of 30 inches (2.5 feet) can catch 300 cubic feet (equal to 2244.16 gallons) of water! Also, for quick reference, about 620 gallons are generated for every 1” of rain and 1,000 square feet of impermeable surface.

Potable Rainwater & Roof Materials

A site’s highest quality rainwater is typically runoff from clean roof materials such as metal, slate, tile, or elastomeric paints approved for rainwater collection systems. This is the most appropriate type of rainwater for storage within cisterns for domestic consumption.

Most rooftop surfaces are just fine for rainwater harvesting. Yet, there are some things to be wary of when it comes to roof materials.

Galvanized Steel & Copper

Galvanized steel roofs can leach zinc into the water, so rainwater captured from this surface that you intend to drink should be checked regularly to make sure that the zinc levels are within the allowable limits for drinking water. In concentrated doses, zinc is an herbicide and as such it might damage plants even at a concentration low enough for the water stored to be considered safe to drink. The same goes with copper roofs.

Asphalt Shingles

With the very common asphalt shingle roofs, the water harvested should not be used to irrigate edibles in the first 3 years after the roof installation. That is how long the adhesives used for the roof’s installation take to off-gas.

Solar Panels

Even though solar panels make a great collection surface, if adhesives were used in their installation it is advisable to test the water collected off the roof for possible toxins.

Wood

Wood shingles are almost always treated with fire retardants, chemicals you should not drink or water your garden with.

Learn more about roofing materials and rainwater harvesting in this article from Blue Barrel Systems.

In general, the roof used for rainwater catchment should be kept clean. Stormwater from dirtier surfaces should go to passive water-harvesting earthworks such as swales, and greywater (from laundry, sinks, showers, etc.) should never be stored in tanks.

Uses of Stored Water

In addition to potable water use, tanks and cisterns can also be used to irrigate a garden, supplementing the landscape with water in especially dry times. This use should complement the foundation of passive water-harvesting earthworks laid throughout the landscape, elongating the watering potential well into the dry season.

Stored rainwater can also be used to support fruit trees and other water-needy vegetation close to the house that provide an oasis-effect around the home.

Photo credit: Penny Mayes

Permits

In California, Assembly Bill 1750 was signed in 2012 allowing any residential, commercial, or governmental landowner to install, maintain, and operate “a rain barrel system, if the system is used only to support water for outdoor, nonpotable uses and is used in compliance with all manufacturer instructions.”

Most simple rainwater catchment systems do not require a permit in California. Plumbing permits (and electrical and building permits, when applicable) are required for catchment storage of 5,000 gallons or more, when the height to width ratio is greater than 2-to-1, or when pumps or a make-up water supply connection is used. If collected rainwater will be used for spray irrigation or indoor potable uses, additional review and permits are needed as well.

The volume limit for one tank does not limit the total volume of water that can be stored off of a roof. You are allowed to daisy-chain together as many tanks as you’d like, so long as they are each less than 5000 gallons.

Learn more about rainwater harvesting for indoor non-potable use in this article by the Resource Conservation District of Santa Cruz County.

This Rooftop Rainwater Harvesting Permit Application & Inspection Checklist for Santa Cruz County contains additional information about local regulations.

Principles of Cistern Systems

In Rainwater Harvesting for Drylands and Beyond, Brad Lancaster lays out 10 principles of cistern systems:

1. Ensure adequate inflow: make sure tanks have proper size gutters, downspouts, and inflow pipes to handle the maximum rainfall intensity.

2. Ensure adequate outflow and use water as a resource: the diameter of the cistern overflow pipe must be equal to the diameter of the cistern’s inflow pipe. The overflow should be directed to another tank or to a mulched/vegetated infiltration basin. If directed to an infiltration basin, the overflow pipe will need to be screened and should drain above mulch, not vegetation, to prevent root entry and clogging.

3. Design the system to collect high quality water. Make sure the water is harvested off of good catchment surfaces, with no contaminations. First-flush systems are nice optional features meant to avoid the material collected in a first rain from making it into the tank.

4. Design a closed system that passively filters itself. A cistern or tank needs to be closed, or screened off from sunlight, insects, and critters. All cistern and downspout openings (inlets and outlets) should be screened. A 45 degree bend prevents direct light to entry into the tank via the downspout. The tank cover will reduce evaporative losses. The outflow pipe needs to be a minimum of 4 inches above the bottom of the cistern to keep the sludge of sediments from being pulled out into the supply pipe. To the same aim, tank inlet should be on the opposite side from the distribution pipe. In this way more of the micro-debris can settle into the sludge layer rather than exit via the faucet.

5. Maintain access to the tank and its interior. Access is needed at all times to check water levels, clean out the tank, and make repairs.

6. Vent the tank. Covered tanks with tight-fitting lids must be vented to prevent a vacuum from forming when large quantities of water are quickly drawn from the tank. The vent needs to be light-proof, critter-proof, and insect-proof.

7. Use gravity to your advantage. The elevation of the catchment surface and the free power of gravity should be used to collect rainwater and distribute it around the site.

8. Make rainwater use convenient. Select a location for the tank that is near both the water source and the destination. This minimizes money and materials needed and helps to maintain water pressure. It should be located within 10 feet of wall or building foundation. It should be easy to walk around the entire above-ground tank to check for and repair leaks.

9. Select and place the cistern so that it does more than store water. It can be a privacy screen, fence, retaining wall, or support pillar for a covered porch.

10. Maintain the system. Inspect and clean the gutters before the rainy season and in times of leaf drop from surrounding trees. Drain the first flush diverter after rainstorms. Check the system periodically to make sure there are no leaks, no broken inlet screens, and no excessive build up of sediments.

Photo credit: Darin McClure

Learn More

Next in the series will be a discussion of berms and swales and how to design them to infiltrate water on your landscape. Stay tuned!

Want to go deeper with all of this? Join us for our Permaculture Design Certificate Course! We spend an entire weekend focused on Regenerating Watersheds & Soils, and part of your design project includes assessing a landscape for rainwater harvesting. Learn more and register today at santacruzpermaculture.com/permaculture-design-course/

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