Roof rainwater saves water and helps gardens and fish

By March 23, 2009Uncategorized

This summer Forest Knolls resident John Lerch will use recently collected rainwater stored in three large cisterns to irrigate his vegetable garden.

His irrigation set-up is among 18 rooftop “stormwater harvesting systems” established with the help of a $60,000 Marin Community Foundation grant to a San Geronimo group that is demonstrating ways to conserve water, help fish and recharge groundwater.

“It’s been working great,” said Lerch, who has collected 3,300 gallons of water to feed his artichokes, tomatoes and other plants. “All three tanks are full. I’ll use the water through a garden hose I can attach, and gravity makes the water flow.”

The systems are simple. Rainwater falls on a standard roof, where it is funneled into a downspout. But instead of spilling onto the ground or ending up in a stormwater pipe, water is diverted and stored in a cistern and then can then be used to water gardens.

“It’s a very simple system,” said Paola Bouley, conservation program director at the Salmon Protection and Watershed Network, which is heading the project. “People are hungry for this.”

By storing rainwater for irrigation, water from county reservoirs is saved. In Marin, 33 percent of water demand during the summer is used for landscaping. The county also has a water deficit, which means a dry year would require rationing.

The cistern systems also keep water from rushing off hard services and into creeks. That type of runoff carries sediment that can harm local
fish populations, including endangered coho salmon. Marin’s coho population is at a 15-year low this year.

“Roofs, parking lots, all these impervious surfaces that we have around our homes and business have a direct impact on the streams where the fish are and bring consequences for their survival,” Bouley said.

It does not take long for cisterns to fill. A typical home collects 600 gallons of water from a 1,000-square-foot roof after an inch of rain has fallen. The cisterns are equipped with a valve that releases water as it becomes full.

SPAWN encourages users to create a swail that forms a “rain garden.” As water collects, the water drains through the soil into the natural aquifer, recharging creeks.

A 200-gallon, resin cistern costs about $220; a 300-gallon model costs about $300 and Bouley said a simple 1,500-gallon system can be installed for under $1,000. SPAWN officials hope to get another grant to set up more systems.

“We have two very large cisterns and a smaller one that collects 2,300 gallons of water,” said Julie Vogt, San Geronimo Valley resident. “It’s all rainwater that helps to grow plants. It works very well.”

The water goes to native plants on Vogt’s property; the plants that are being grown by SPAWN, which paid for the cisterns.

Vogt also created a rainwater garden that resembles a creek system, complete with fake salmon. It is fed by water that comes off the roof.

“We used to have a green lawn here, now it holds water that seeps back into the earth naturally,” she said. “It feels like you are doing the right thing.”