Rainwater Harvesting

Northwest rain: too much in the winter, too little in summer

The Northwest gets a lot of rain in the winter, so much it causes problems like flooding, sewer overflows, stream erosion, and polluted runoff into our waterways. But all summer we get very little rain (less than Tucson, Arizona).

It’s hard to store enough rainwater to last long enough for summer irrigation. You need large cisterns or multiple rain-barrels to store enough water to make any difference. Simple practices like amending soil with compost, mulching, and smart watering are the first steps to storing and conserving water.

Using rainwater is easy and Seattle residents can get a great deal on rain barrels.

Rain barrels and cisterns help reduce stormwater runoff and protect our waterways. Follow these links to learn about several ways you can help.

How much rainwater can I catch?

Puget Sound averages about 3 feet of rain per year, but 3/4 of it falls from October to March. Most areas in the region average less than 3 inches total rainfall for June, July, and August.

To determine the amount of rain your roof catches, multiply your home’s width by its length (in feet) to estimate its footprint. Then estimate the portion of this area that drains to the downspout you’ll be using to catch your rain.

This formula will give a rough estimate of how much rain you can catch:

  • Rain caught (gallons) = (inches of rain) x 0.623* x (portion of building footprint)

    *One inch of rain falling on a square foot of surface yields approximately 0.623 gallons of water.

For example, if your home’s footprint is 1,400 square feet, and you want to know the amount of water that comes from a ¼ inch (.25”) rain event:

  • Rain caught (gallons) = (.25) x (.623) x (1,400) = 218 gallons (or less if you’re only gathering from one part of the roof)

Storage is limited to the capacity of your system. Added capacity helps your system be prepared for dry spells. Practically though, most homeowners don’t have room to store the thousands of gallons they use in landscape irrigation through our dry summers, and the large cisterns to do it would take a very long time to pay back. Capacity and cost are directly related: decide how much you want to spend on storage. Natural Yard Care practices like building soil with compost and mulching, choosing low-water use plants, and Smart Watering practices all have much shorter paybacks—and grow healthier lawns and gardens too. So, you may want to choose to use all those practices and simple indoor water conservation practices before investing in big rainwater collection systems.

Seattle Area Average Monthly Rainfall

Month Jan. Feb. March April May June July Aug. Sept. Oct. Nov. Dec.
5.18 4.04 4.03 2.61 1.83 1.61 0.52 0.74 2.08 4.01 5.41 5.36

2012 – 2022, source Ag Weather Net—Washington State University—UW Station

Related links

RainWise rebates for installing rain gardens and cisterns. See other tips for reducing runoff, like amending soil with compost and planting trees in Managing Stormwater at Home. (PDF)

Links to other sites

Saving Water Partnership - A variety of water conservation information
King County Rain Barrel Information and Sources - Factsheets and suppliers to help you find or build a system.
Rainwater Harvesting for Beneficial Use (PDF) - A good overview of larger systems for indoor uses, along with code and design requirements. From the Seattle Department of Planning and Development.
Rainwater Collection in Washington State - Outlines the Department of Ecology’s 2009 policy decision allowing rainwater collection and reuse systems, and has many useful links.
Research indicates rainwater is safe for irrigating vegetables – read the recent research from Australia and WA Dept. of Ecology.
Texas Manual on Rainwater Harvesting (PDF) - The standard reference for professionals on designing rainwater harvesting, storage, and reuse systems.
American Rainfall Catchment Systems Association - Links to other resources for design professionals, and current news on rainwater harvesting around the US.

Public Utilities

Andrew Lee, General Manager and CEO
Address: 700 5th Avenue, Suite 4900, Seattle, WA, 98104
Mailing Address: PO Box 34018, Seattle, WA, 98124-5177
Phone: (206) 684-3000

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Seattle Public Utilities (SPU) is comprised of three major direct-service providing utilities: the Water Utility, the Drainage and Wastewater Utility, and the Solid Waste Utility.