NEW YORK — Green roofs can be a cost-effective way to keep water from running into sewer systems and causing overflows, Columbia University researchers have found. Stuart Gaffin, research scientist at Columbia’s Center for Climate Systems Research, has been supervising a green rooftop monitoring project at Con Edison’s Learning Center in Queens, N.Y., home to 21,000 plants on a quarter acre. He estimates the green roof retains 30 percent of precipitation, allowing plants to release the water as vapor, rather than overflowing the city’s sewage systems.

"The information we are collecting is invaluable in helping us determine the costs and benefits of green infrastructure projects,” Gaffin said. “Without solid data from experiments like this, it is impossible for us to know which projects are the best options for protecting the environment.”

If New York City’s 1 billion square feet of roofs were transformed into green roofs, it would be possible to keep more than 10 billion gallons of water a year out of the city sewer system, according to the study. The city, like other older urban centers, has a combined sewer system that carries stormwater and wastewater. The system often reaches capacity during rains and must discharge a mix of stormwater and sewage into New York Harbor, the Hudson River, the East River, and other waterways.

Columbia formed its research partnership with Con Edison in 2008; the partners saw the green roof and an adjoining white roof as an outdoor laboratory for environmental research. Gaffin’s team found last year that the green roof and white roof save energy and reduce urban air temperatures.

“When we built our green roof we were confident that researchers from Columbia would gain important knowledge about protecting the environment,” said Saddie Smith, vice president for facilities for Con Edison. “Three years later, it’s clear that our project has helped us understand how roofs can save energy, cool the atmosphere, and prevent stormwater runoff.”

The researchers used instrumentation to measure sunlight and other forms of energy entering and leaving the green roof. That data allowed them to calculate the amount of energy leaving the roof in the form of water vapor.

The study concluded that, based on the price of building and maintaining a green roof, it costs as little as two cents a year to capture each gallon of water.