Drought-challenged U.S. communities are overlooking what could be a major source of relief: stormwater, which generates more water annually than is stored in lakes Mead and Powell, the largest reservoirs in the West.
But Colorado and other states with laws against collecting stormwater are likely to miss out on its potential.
Heavy rains produce some 59.5 million acre-feet of water annually, according to “Untapped Potential: An Assessment of Urban Stormwater Runoff Potential in the United States,” released last month by the Pacific Institute, a water research think tank based in Oakland, California.
Lakes Powell and Mead store some 49.4 million acre-feet, according to the U.S. Bureau of Reclamation.
That 59.5 million acre-feet of stormwater is roughly 93% of the water used by all U.S. cities and industry in 2015, according to the Pacific Institute. An acre-foot serves about two to three U.S. households for a year.
But because this source has never been fully analyzed or developed, it is not yet widely used.
“Our results indicate that there is a vast potential for stormwater capture all across the country,” said Bruk Berhanu, a lead author of the study and senior researcher in water efficiency and reuse at the Pacific Institute.
With climate change and warming, streamflows are projected to decline in Colorado and elsewhere in the coming years, and there is increasing pressure to find new sources and better use existing water supplies.
“As communities in the West face increasing strain on their water supplies, planners have been looking at strategies that use an ‘all of the above’ approach,” Berhanu said. “We aren’t suggesting stormwater could cover all of our future water supplies, but they can help fill the gap between our current water supplies and projected demands.”
Estimated annual urban stormwater runoff by state
Source: Pacific Institute, “Untapped Potential: An Assessment of Urban Stormwater Runoff Potential in the United States”
But use of stormwater comes with conditions. It would require major new facilities to capture, store and treat it if it is to be used for drinking water. If too much is captured, it could reduce water available for the environment, according to the report.
And in some places, such as Colorado, the practice isn’t allowed.
Under what’s known as the Prior Appropriation Doctrine, water users with the oldest, or most senior water rights, get their water first, even if their diversion point lies farther downstream than someone else’s. And stormwater, once it reaches the stream, becomes part of someone’s water right. If larger amounts were captured, it could jeopardize other water rights already in place.
The City of Aurora, and others, have actively worked for decades to find new ways to make their water supplies stretch further, but stormwater capture is not one of them.
“What works in some states, does not work in Colorado,” said Greg Baker, a spokesman for Aurora Water, referring to the legal prohibitions against the practice.
Could that change? Possibly.
Colorado has taken major strides in recent years to re-examine how water that falls from the sky may be collected and used in ways that don’t harm neighbors downstream. In 2009, for instance, the state passed a law that opened the door to rainwater harvesting in some rural areas and then in 2016 allowed homeowners across the state to use rain barrels to capture small amounts of water for use on gardens and lawns.
That state also created a pilot program to encourage more research. The Dominion Water and Sanitation District in Douglas County, to date, has been the only water district to participate in the pilot, according to Andrea Cole, Dominion’s general manager. Soon it may be able to legally capture rainwater when, later this year, it will ask a state water court to approve collecting rainwater commercially to serve parks and other public spaces in Sterling Ranch, one of the most water-efficient residential developments in the state.
To get to this point, Dominion spent 15 years tracking how much rain fell on the development before anything was built, and tracking how much more water was generated after new homes and roads were built and the water began falling on roofs and other solid surfaces, instead of the soil.
“In Colorado, water is precious, so every last drop is accounted for in somebody’s system. … But when you change the land from an open prairie to a development, the water no longer [sinks] into the soil, or makes its way to nearby streams,” Cole said.
Measuring the water has and will continue to be a meticulous process, she said.
“We can only capture that water [that falls on] Sterling Ranch. … If it is outside the ranch, we have to allow it to go back to the stream,” Cole said.
Sterling Ranch sharply limits outdoor water use, so lawns are scarce. The plan is to use the rainwater for parks and gardens so that homeowners with little of their own grass have a place to play and relax, Cole said.
The Pacific Institute’s Berhanu said he is hopeful that the new report will generate more interest in developing stormwater to help fill looming gaps in water supplies.
“In a state like Colorado, we would hope that this information builds the case for revisiting those policies and making adjustments to enable more stormwater capture,” Berhanu said.
The potential is there, Cole said.
“We are the first out of the chute, and being the first is always scary. But people are watching to see what we can get through water court,” she said. “Once there is a [legal] water right for it, we are going to see new developments trying to use this.”
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