YUMA, Ariz. -- Along the dusty hem of the U.S.-Mexico border, the Yuma Desalting Plant has come to symbolize the region's unquenchable thirst.

The $211 million plant is surrounded by a mosaic of farm fields, a few dormant and brown, many astonishingly green against the desert flatlands, brought to life by Colorado River water diverted via an extensive irrigation system that has helped make Yuma -- as a sign near downtown says -- "the winter lettuce capital of the world."

On a recent 112-degree afternoon, pigeons flitted across an empty open-air treatment tank at the idle water plant and a hawk brought in years ago to control the pigeons kept watch from a tower.

The Yuma Desalting Plant was built in 1992 to treat saline farm runoff to help the United States comply with treaty obligations for delivering good-quality Colorado River water to Mexico, but the plant has stood idle for most of its 20-year existence. Photo courtesy of Bureau of Reclamation.

And this is how the plant has passed most of its 20 years. Built in 1992 to treat saline agricultural return flows from an irrigation system that waters fields in the next valley, the plant was aimed at helping the United States comply with treaty obligations to deliver 1.5 million acre-feet of good-quality Colorado River water to Mexico. There are 325,851 gallons in an acre-foot, a year's supply for a family of four.

But a flood damaged the plant soon after its construction, and the 1990s turned out to be banner years for Colorado River flows, diluting the salty runoff and making the plant unnecessary.

"No sooner did they have the plant than they had some of the fattest water years ever on the river," said Doug Hendrix, a spokesman for the federal Bureau of Reclamation.

The 21st century has brought prolonged drought and dire climate change predictions for the Southwest, prompting new interest in the plant from the region's cities. Several studies, including one by the U.S. Center of the Stockholm Environment Institute last year, predict shrinking water supplies in the Southwest over the coming decades (Land Letter, Feb. 24, 2011).

So after a brief test run in 2007, Reclamation decided two years ago to give the plant a workout, running it at one-third capacity for almost a year, ending in March 2011.

By all accounts, the trial run was successful, treating 29,000 acre-feet of irrigation runoff ahead of schedule. Reclamation had allowed 12 to 18 months for the run to accommodate potential down time for maintenance, but the operation ran smoothly and achieved the agency's treatment goal six weeks early (Land Letter, April 7, 2011).

"The plant never shut down," plant manager Mike Norris said. "It performed much better than anticipated."

While the amount of water the desalting plant can treat is small, every drop that can be sent to Mexico from treated irrigation runoff is a drop that can be kept upstream in Lakes Mead and Powell to supply Las Vegas, Phoenix and other cities. Currently, those reservoirs release the water needed to meet the U.S. obligation to Mexico.

The amount of treated water released into the river during the yearlong trial run was enough to supply the needs of about 116,000 people for one year.

Should the plant be resurrected permanently? Almost everyone involved with the plant's pilot run says it is too early to make that call. For one thing, running the desalting plant is expensive -- the pilot cost $23 million -- and it would likely cost $55 million to run the plant at full capacity. It is unclear who would pick up the tab.

"That's a big number for us," said Jennifer McCloskey, manager of Reclamation's Yuma office. "It would be tough to absorb in Reclamation's budget."

Southwestern cities and other water users paid most of the cost of the trial run. But they may not be willing to pay for its extended operation, which would require a hefty long-term investment.

Technologically, the plant -- the largest desalting facility in the United States and the second-largest in the world -- appears to be in good enough shape to operate on a long-term basis, Norris said.

"When the Yuma Desalting Plant was built, it was ahead of its time," he said, looking out over the warehouse-like desalting chamber, stacked with 12-inch membrane cylinders.

The plant puts water through an extensive pretreatment process to remove debris, silt and contaminants before it enters the desalting chamber and flows through the membranes.

"It's like a toilet-paper roll," he said of the cylinders. "We spin it, and it comes out the center."

'Environmental water'

A big question for environmentalists is how full-time operation of the plant would affect a 40,000-acre wetland known as the Cienega de Santa Clara in Mexico that was revived by irrigation return flows that bypass the plant.

The cienega -- originally part of a larger complex of wetlands in the Colorado River Delta -- all but dried up after Hoover and Glen Canyon dams were built in 1936 and 1963, respectively. Irrigation farther south diverted water to California's Imperial Valley and southern Arizona.

But the influx of runoff after the Wellton-Mohawk Irrigation and Drainage District canals were built near Yuma to bypass the lower Colorado River expanded the wetland. It has become an important stop for migratory birds. In all, about 95 species of birds, including the endangered Yuma clapper rail, the Southwest willow flycatcher and a type of desert pupfish, now rely on the cienega for habitat.

"Before all the upstream diversions, that whole area was marshland," said Karl Flessa, a professor of geosciences at the University of Arizona in Tucson who has been studying the cienega. "This is an accidental re-creation. When dumping the water, no one set out to create this terrific habitat. It just happened."

The cienega, which is just a few miles south of the border, has spurred a growing ecotourism industry.

About 25 percent of the flow that feeds the wetland was diverted to the plant during the pilot project, but much of that water was replaced under a binational agreement. Under the accord -- unprecedented in its collaboration across two countries and among both public and private entities -- the United States, Mexico and environmental groups, through water rights purchases from Mexican farmers, provided about 10,000 acre-feet of water each to make up for what the plant used. The U.S. portion came from overflows during storms, McCloskey said.

"We were depending on conditions in the river, because it was opportunistic," she said, sitting at the kitchen table of her home on the outskirts of Yuma. "I wanted to take advantage of these opportunities as they happened. I got with the staff and pressed the issue."

The wetland typically receives about 107,000 acre-feet of agricultural runoff water per year, but while the plant was operating during the test run, it lost about 25 percent of those flows. All the replacement water was successfully delivered, although it came in pulses rather than in consistent flows, according to a delivery chart McCloskey shared from a forthcoming report on the test run to be released soon by Reclamation.

That delivery blueprint could provide an example for water managers and environmental groups who are trying to ensure some water is allocated to environmental uses, she added.

"The buzz word these days is 'environmental water,'" she said. "The lesson for anyone that's been managing water is, 'Be flexible. Take advantage of the supply as it becomes available. Mother Nature is unpredictable.'"

The flexible time frame in the agreement for the cienega allowed all of the parties involved to move the water when there was enough in the system, and that is why the delivery plan was successful, she said.

Researchers from the University of Arizona monitored the wetland before, during and after the pilot run. Their report, which is completed but still awaits approval by Mexico, is due out within the next two weeks.

While the authors of the binational monitoring report and those familiar with it declined to discuss its contents in detail, since it hasn't been officially released, Francisco Zamora Arroyo, director of the Sonoran Institute's Colorado River Delta program, said the wetland appears to be in good shape.

"The results of the monitoring effort will be published soon, but what I can say after the [desalting plant] pilot run and after recently flying over the ciénega is that the ciénega is in good condition," he said in an email.

During the trial run, the cienaga was hit by an earthquake, which changed the elevation in parts of the wetland, and a fire, which scorched vegetation. At the same time, the wetland saw changes in the amount of water coming in and when it arrived, due to the intermittent delivery of replacement water.

But "overall, the ciénega showed to be a resilient ecosystem," Zamora said.

The wetland's vegetation recovered fairly rapidly after the fire, added Jennifer Pitt of the nonprofit Environmental Defense, a key player in negotiating the environmental agreement. "It's amazing at how quickly it began to green up afterwards," she said. "It does seem like it came back very well after the fire."

All things considered, "the replacement water worked well," Pitt said. "The cienega appears to still be a healthy habitat in the delta, and probably the most significant habitat area in the delta at this point in time."

Cooperation counts

The success of the agreement is also a political triumph, Pitt said.

"It's the first time that any government made a dedicated delivery of water to an environmental resource in the delta, and they did it in cooperation with the water trust [which provided the water secured by environmental groups]," she said. "It's a really fantastic example of how these different players working together could come together to do good and ensure the viability of this important resource."

The University of Arizona's Flessa called the effort "a good example of where ... cooperation can get you."

"This is much better than tying things up in courts," Flessa said. "We have enough problems along the border. We don't need more problems."

With so many diversions upstream, the cienega is one of only a few small wetlands left in a delta that has largely been dewatered for decades. Protecting the cienega is part of a larger delta restoration effort by the Sonoran Institute, Environmental Defense, Mexico's Pronatura Noroeste and other organizations.

It remains to be seen whether the cienega could withstand a longer-term and likely higher-volume run of the plant -- and whether all the parties involved could or would ensure replacement deliveries for the cienega. The plant can run at a third to full capacity.

"If they were to run the plant at full capacity, it would not only cut back on the volume of delivery; it would also increase the salinity" of water going into the cienega, Pitt said. "It's hard to predict what would happen. It's somewhat intuitive to say there would be major impacts to the cienega, but we just don't know."

Cattails, which constitute most of the vegetation in the cienega, stop growing when the salinity reaches more than 6 parts per thousand, Flessa said. Salinity levels of water entering with the plant in operation, without replacement water, would likely result in a smaller vegetative footprint.

Complicating matters is the newly completed Brock Dam, the latest diversion structure on the lower Colorado River system, which funnels water on a tributary of the mainstem river to the Imperial Irrigation District in Southern California. The dam, which did not exist during the pilot run, could make it more difficult to send replacement water to Mexico during plant operations, McCloskey said.

Whatever the future holds for the desalting plant, McCloskey, for one, is optimistic that the cienega will continue to receive water, despite the challenges that inevitably would arise in figuring out where to find it were the plant to come back online full-time.

"I think we have a good example that we've been able to build relationships and demonstrate that the deliveries can occur," she said.

Flessa agrees. "We've built up goodwill along the border on water issues," he said.

It is possible to have both a fully operating plant and a healthy cienega -- particularly if brackish groundwater from the Yuma area could be used to supply the plant instead of irrigation water, he said.

"That that would be a great solution," Flessa said.

Reese writes from Santa Fe, N.M.