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Slow it down, spread it out, soak it in: Collecting rainwater keeps it from going down the drain

January 30, 2010
Nanette Londeree

We've been pounded and pummeled the last few weeks - nature doing her winter thing with forceful wind, pelting rain, thunder and lightning, even a bit of hail. Water is everywhere - streaming off rooftops, sheeting down driveways, turning curbside gutters into turbulent little rivers and storm drains into veritable whirlpools.

With our Golden State in the midst of a multiyear drought, it seems like such a waste - all that fresh water literally going down the drain. And as it travels, the water picks up myriad pollutants that wend their way to San Francisco Bay or the ocean, dumping a variety of unwanted materials into those delicate ecosystems. Pouring down rain may not seem like the time to think about conserving water and protecting the environment, but it's actually a good time to assess how to better utilize the precious liquid, and, rather than letting it funnel down rainspouts and head right for storm drains, finding ways to capture it, slow it down, spread it out and let it soak in.

First, a bit about our water. Marin's fresh water supply comes from rainfall collected in reservoirs during our rainy season (late fall to mid-spring). The rainwater is treated to potable standards, as defined by the U.S. Environmental Protection Agency (EPA) Safe Drinking Water Act, by one of the municipal suppliers and delivered to customers for use indoors and outdoors. Once used, it either goes to the sewer system (general indoor uses), is absorbed into the ground (irrigation and other outdoor uses) or travels to a storm drain that ultimately flushes out to the bay or ocean (runoff).

In an undisturbed natural environment, about 50 percent of rainwater infiltrates into the ground, 40 percent evaporates or is taken up by plants and about 10 percent runs off the surface. Runoff can easily be double or triple that amount in developed areas with their increased percentage of hard, impervious surfaces (roof tops, driveways, patios, sidewalks, streets and parking lots). That's a lot of water heading right down the proverbial drain. As Marin is dependent on the cycles of nature to provide us with an adequate supply of fresh water, it makes sense to maximize our ability to capture and use this valuable resource.

Rainwater harvesting (or rain gardens as they're commonly known in much of the country) work by collecting, holding, moving and infiltrating rainwater runoff from hard, impenetrable surfaces to another location within an individual site. They can augment irrigation supplies, help improve drainage problems, increase the amount of water that filters into the ground to recharge groundwater supplies, protect creeks and streams from pollutants, provide valuable habitat for birds, butterflies and beneficial insects, all while enhancing the beauty of yards and neighborhoods.

The age-old practice of collecting and storing rainwater in some sort of holding vessel for later use is becoming increasingly popular across the country given the cost and availability of water. There are all kinds of options available, from simple barrels connected to a downspout, to large capacity tanks with associated materials for distribution. And it's easy to capture a lot of water - in an optimum scenario, an inch of rainfall collected off a 1,000-square-foot surface could capture 600 gallons of water.

Slowing the movement of water during periods of intense flow can be accomplished with a variety of different design elements (berms, swales, shallow depressions) that hold water on the landscape so it can soak into the ground. Designs can look like traditional landscaping, employing a variety of trees, shrubs, flowers and other plants that tolerate wet and dry conditions. Beneath the plants, layers of mulch, soil and sometimes gravel, catch excess water and slowly disperse it through the ground and back into the water table.

Building a rain garden in your own yard is probably the easiest and most cost-efficient thing you can do to reduce your contribution to storm water pollution. The EPA, in partnership with the U.S. Botanic Garden, initiated a program to showcase green techniques aimed at minimizing the impact of storm water runoff. In 2008, San Francisco established permitting requirements for rainwater harvesting systems, encouraging its reuse for non-potable applications. Marin Municipal Water District is sponsoring a one-year rainwater harvesting pilot project, affectionately named by program leader SPAWN, the "10,000 Rain Gardens Project," that will provide technical and educational assistance to customers who want to install harvesting systems or just learn more about the practices. It will feature on-site technical advice, on-line educational materials, workshops and demonstration sites.

"Homeowners can observe and understand how rainwater moves on their property while it's raining," offers Paola Bouley, SPAWN conservation program director, finding ways to redirect rainwater from downspouts to rain gardens or swales where it will have a chance to infiltrate into the soil. "Start simply and keep adding over time to keep more and more rainwater on your property," she says. "Even if we can't keep all of the rainwater that falls on our property out of the storm drains every little bit helps, and if everyone did even a little bit to reduce storm water runoff, we could collectively have a huge impact."


- Marin Municipal Water District:

- SPAWN's "10,000 Rain Gardens Project":

- San Francisco Rainwater Harvesting:

- "Rain Gardens, A How-to Manual for Homeowners":

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