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!).
So it’s hard to store enough water from spring rains to last long for summer irrigation. You need large cisterns or multiple rain-barrel systems 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.
How much rain water can I catch?
Puget Sound averages about 3 feet of rain per year, but 2/3 of it falls from November to March. Most areas in the region average less than 2 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, you would solve the following:
- 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. 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 use all those practices and simple indoor water conservation practices before investing in big rainwater collection systems.
Seattle Area Average Monthly Rainfall
1981-2010, source National Weather Service
Learn more about rain water harvesting systems
King County Rain Barrel Information and Sources – has factsheets and suppliers to help you find or build a system.
Rainwater Harvesting for Beneficial Use (pdf) – from the Seattle Department of Planning and Development, provides a good overview of larger systems for indoor uses, along with code and design requirements.
Rainwater Harvesting and Connection to Plumbing Fixtures (pdf) – from the Seattle & King County Public Health Department describes plumbing code requirements for indoor use of rain water.
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.
Green Home Remodel – Roofing (pdf) – has useful information on roofing choices, see the “Rainwater Harvest” section.
Texas Manual on Rainwater Harvesting (pdf) – is the standard reference for professionals on designing rainwater harvesting, storage, and reuse systems.
Arizona Cooperative Extension Rainwater Harvesting (pdf) – also has information on designing larger home systems.
American Rainwater Catchment Systems Association – links to other resources for design professionals, and current news on rainwater harvesting around the US.