Rain Water Harvesting

Rain: a valuable resource and a problem

The Northwest gets a lot of rain in the winter. We get so much that it sometimes causes problems like flooding, sewer overflows, stream erosion, and carrying urban pollution into our waterways. But in the summer we get very little rain (less than Tucson, Arizona). So, it makes sense to conserve. 

Follow these links to learn how to reduce runoff and protect our waterways:

Rain water harvesting usually involves larger cisterns or multiple barrel systems that can store enough water to help water landscapes during our long dry summers. Simple practices like amending soil with compost, mulching, and smart watering are the first steps to storing and conserving water.

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

How much rain water can I catch?

The Puget Sound averages about three feet of rain per year, but two thirds of it falls from November to March. Most areas in the region average less than two inches total rainfall for 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 0.6* x (portion of building footprint).
*One inch of rain falling on a square foot of surface yields approximately 0.6 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 (.6) x (1,400) = 210 gallons
(or less if you’re only gathering from one part of the roof).

Storage, however, is limited to the capacity of your system. Added capacity helps your system weather dry spells, although 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 use all those practices and simple indoor conservation practices (see the Saving Water Partnership) first 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.
Rainfall (inches) 5.6 3.5 3.7 2.7 2 1.6 0.7 .08 1.5 3.5 6.6 5.4

1981-2010, source National Weather Service

You can help solve our winter stormwater runoff problems by connecting a hose to the drain valve on your rain barrel or cistern and running it out into a lawn or landscape. Then just open the drain valve in October, and let the barrel slowly drain out between big storms to slow our runoff all winter. In May, close the drain valve and let your barrel fill up to store water for landscape watering during our dry summers. Learn more about how you can help at RainWise.

Learn more about rain water harvesting systems by following the links below, or search the Web under “rain water harvesting.”

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

Newsletter Updates


Sign up for the latest updates from Public Utilities

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.