Why Not Flush Rooftop Rainwater?

Report Says Roof Runoff Could Solve U.S. Water Woes

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Posted: Wednesday, February 29, 2012 5:00 pm | Updated: 6:48 pm, Wed Feb 29, 2012.

WASHINGTON, D.C. -– As America’s expanding urban areas struggle with major water supply shortages and runoff pollution problems, capturing rainwater from rooftops provides a tremendous untapped opportunity to increase water supply and improve water quality, according to a recent analysis on Capturing Rainwater from Rooftops by the Natural Resources Defense Council.

In its report, NRDC demonstrates the benefits and potential of rooftop rainwater capture, a "green infrastructure" practice that can be used to retain stormwater runoff on-site, by analyzing ways in which eight diverse U.S. cities could incorporate this simple water collection approach. By comparing annual rainfall totals to rooftop coverage, NRDC determined that opportunities exist in each city to capture hundreds of millions of gallons of rainfall every year for reuse. By doing so, residents of these communities would obtain inexpensive onsite water supplies for non-potable uses, such as yard watering and toilet flushing; reduce runoff pollution; and would lower energy costs associated with treating and delivering drinkable-quality water.

"Our analysis shows that solutions to one of America's biggest urban challenges are right in front of us – in this case, literally falling from the sky," said Noah Garrison, lead author of the report and NRDC water policy analyst. "The potential exists for cities throughout the U.S. to capture hundreds of millions or even billions of gallons of rainwater each year from urban rooftops. We encourage policymakers to look closely at the bottom-line benefits of rooftop rainwater harvesting, and consider implementing policies and incentives that generate more momentum for rainwater collection while making the practice more accessible as well."

Specifically, NRDC’s report illustrates opportunities for capturing, treating and supplying harvested rainwater for non-potable purposes in Atlanta, Ga.; Austin, Texas; Chicago, Ill.; Denver, Colo.; Fort Myers, Fla.; Kansas City, Mo.; Madison, Wisc.; and Washington, D.C. Several success stories also demonstrate the effectiveness of rooftop rainwater capture for new construction in New York, N.Y., and redeveloped buildings in Santa Monica, Calif. The total annual volume of rainwater falling on rooftops in these cities alone, if captured in its entirety, would be enough to meet the water supply needs of at least 21 percent to as much as 75 percent of each city’s population.

The report comes as the U.S. Environmental Protection Agency is in the process of updating its national standards for controlling runoff pollution from new development and existing paved areas. NRDC encourages the agency to adopt national standards for on-site stormwater retention that will increase green infrastructure approaches such as rainwater harvesting. As a result, communities can effectively transform polluted runoff flowing to waterways into captured rooftop rainwater used as an on-site water supply resource.

"Urban areas struggling with water supply issues and runoff pollution should look to this report for ideas and encouragement," said Jon Devine, senior attorney in NRDC’s water program.

NRDC encourages cities and states to develop policy options and incentives to encourage more rainwater harvesting. These include:

  • Adopt stormwater pollution control standards that require on-site volume retention.
  • Adopt standards that require or promote rainwater harvesting and/or water efficiency.
  • Review building, health and plumbing codes for barriers to reusing rainwater.
  • Provide incentives for decreasing stormwater runoff and promoting water conservation.
  • Require use of rainwater harvesting on all public properties.

 The NRDC report says water demand in the United States is among the highest in the world, averaging 100 to 165 gallons per person per day — or as much as 4 times more than in some European countries.

"Each day in the Unites States, 44 billion gallons of freshwater are drawn from surface and groundwater sources and delivered by public water systems," the report says. It blames distorted water pricing by publicly owned utilities and an historic availability of fresh water across most of the country for causing America's unquenchable thirst for water. But, it says, many urban areas are already facing serious water shortages as their populations continue to outgrow freshwater supplies.

"Distorted water pricing can lead to unnecessary waste, for example using potable water for non-potable uses such as outdoor landscape irrigation and indoor toilet flushing," the report says. In fact, it says, outdoor water use can constitute 30 to nearly 60 percent of overall domestic demand in some cities.

The average cost of water in the U.S. is $3.53 per 1,000 gallons. "By comparison, a 20-ounce bottle of water selling for $1.50 costs the equivalent of $9,600 for 1,000 gallons — 2,700 times the average cost of tap water," according to the report.

Climate change is predicted to compound the problem as less snow falls in the northern and mountain states, and droughts become more common in the southwest.

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