It's a picture-perfect day in Southern California. The sun is beating down on this Carlsbad beach, where volleyballs hit the sand and surfers paddle out into the waves. Just steps from here, the salty water lapping the shore is being transformed.

This beach neighbors the largest desalination facility in the Western Hemisphere. The Carlsbad Desalination Plant uses a complex web of pipes, tanks, and specialized filters to pull salt and impurities out of ocean water, turning it into part of the drinking supply for San Diego County.

Water managers are feeling the crunch of a supply-demand imbalance along the Colorado River. Fresh water reserves are shrinking as climate change squeezes the river that supplies 40 million people and fields of crops across seven states. Some have proposed desalination technology as a way to augment that supply, easing the strain on a river that supplies a growing population from Wyoming to Mexico. Experts say it could be part of the solution, but likely won't make much of a dent in the region's water crisis.

At the Carlsbad plant, former seawater was poured into a cup from a freshwater spigot. Michelle Peters, technical and compliance manager for plant operator Poseidon Water, held it and took a drink.

"At 10 a.m, the morning surfers were swimming in it off the coast in the ocean here," she said. "Now it's high-quality drinking water, ready for consumption."

On an early-afternoon tour of the facility, Peters walked through neat rows of white pipes on the way to the "reverse osmosis gallery." In here, towering stacks of pipes and blue tubing fill a warehouse-like room, and water roared through them at a din. "This is where the magic happens," Peters said. "This is really what makes desal, desal. It's the heart of the site."

Water laden with salt, bacteria, and other impurities enters these vessels. It's forced through membranes about the width of a human hair, passing through tens of thousands of times before it emerges on the other side just about ready to drink.

Where else could it work?

It's a striking and seductive proposition for those managing the Colorado River Basin's ongoing water scarcity crisis. The workhorse river is overallocated and still shrinking. This process would add new fresh water, seemingly created out of thin air.

Policymakers wonder whether similar plants could offer some relief for parts of the basin that are most strapped for drinking supply. Desalinated ocean water already enters the basin through a handful of plants in California, and some have proposed increasing the scale of desalination around the region.

Perhaps the most prominent of those is Arizona Governor Doug Ducey's ambitious proposal to strike a deal with Mexico in which the state would fund an ocean desalination plant on the Gulf of California, allowing Mexico to use the newly-desalted water in exchange for some of Mexico's share from the Colorado River.

The plan is expensive and largely unprecedented but has added a degree of legitimacy to the idea that ocean desalination could help parts of the Colorado River basin that are far from the coast. Policy analysts say it's not the best way to add to Arizona's supply but merits further research.

"People should understand that it's probably the most expensive water supply that would be available as one of the many solutions to Colorado River shortage issues," said Sarah Porter, director of the Kyl Center for Water Policy at Arizona State University.

Porter said desalinated water is likely too expensive for the agriculture sector, the river's largest water user.

"It probably is — and will be someday — a solution for municipal water users," she said. "And that's when it becomes a solution — when the less expensive water supplies and less expensive opportunities to stretch current supplies have already been taken advantage of," Porter said. Arizona's project would carry a hefty price tag. A 2020 binational study of potential desalination deals between Arizona and Mexico showed construction costs in the billions and annual operating costs in the tens of millions.

A big chunk of operating costs would come from the steep expense of moving water over a long distance. Arizona's proposed project would pump water nearly 300 miles north to the Morelos Dam near the U.S. border, driving up total costs for the project that would ultimately make the freshwater itself more expensive for users.

That study said desalinated water could cost around $2,000 per acre-foot, roughly ten times more expensive than the current cost of Colorado River water delivered by the Central Arizona Project — a canal that carries water across more than 300 miles of desert. An acre-foot is the amount of water needed to fill one acre of land to a height of one foot. One acre-foot generally provides enough water for one to two households for a year.

Because of the high costs associated with water transport, most experts say adding desalinated water to the drinking supply really only makes sense when it's implemented near the coastline, like in Carlsbad.

Even then, the expensive process only yields relatively little drinking water. The desalination plant in Carlsbad, which is the largest on the continent, only provides about 10% of the water supply to San Diego County.

"If they get desperate enough, desalination could work"

Experts overwhelmingly agree that most water-starved cities in the Southwest should try to find fresh water elsewhere before turning to desalination, and list a myriad of cheaper and less complicated ways to augment the drinking supply.

Jay Lund, a professor of civil and environmental engineering at the University of California Davis, studies the economics of water supply. He said cities should start by paying farmers and ranchers for their water.

"If they get desperate enough, desalination could work," he said. "It'll be far cheaper for Las Vegas or Phoenix or other cities to buy the farmland and fallow that lower-value economic activity in order to keep their higher-value economic activity."

That practice, often called “buy and dry,” is already employed in some parts of the basin, but Lund sees room for expansion. Agriculture uses about 70-80% of the water in the Colorado River basin. For a long time, policymakers have resisted cutting into that number.

But the severity of the drought and the need for additional supply to keep taps flowing in cities has forced hard conversations that suggest municipal users are increasingly likely to invest in the temporary or permanent purchase of agricultural water. Farm groups worry that water leaving the land will harm rural economies and ecosystems.

Lund listed other alternatives to desalination that could be employed before cities turn to pulling water from the sea — xeriscaping or fallowing lawns, more stormwater capture and more wastewater reuse. Cutting back on lawn watering has already helped many cities, which have conserved effectively enough to stretch a finite supply of water over decades of population growth. Wastewater recycling programs, which essentially purify sewage back into clean drinking water, are gaining steam in some parts of the basin, heralded as expensive but necessary means of keeping fresh water from leaving the system in the first place.

Lund said all of those programs should be used before desalinating ocean water.

"It's very unlikely that the water would get so scarce that the cities would pay for desalination," he said. "Unless there were some policies — which they would likely resist — that forbid the fallowing of agricultural land to provide substitute water."

Lessons from abroad

Other countries with limited water supply have turned to desalination and found varying degrees of success. Israel is often heralded as a global leader in turning ocean water into fresh water. About 80% of the country's domestic water comes from desalination.

Sharon Megdal, director of the University of Arizona Water Resources Research Center, cautions against the idea that success in Israel would translate to the Western U.S.

"Israel is about the size of the state of New Jersey in terms of square miles," she said. "The population is about 10 million people. So just think about scale and then their water use on a per capita basis is much lower than our water use."

Megdal said differences in water law and governance also give Israel an easier path to success with desalination, and its proximity to the ocean brings down the cost.

Australia has also experimented with desalination, and Lund said that country's expensive mistakes should be seen as a cautionary tale for the Colorado River basin. In the early 2000s, Australia was gripped by the "Millennium drought," which strained both agricultural and municipal water supplies — straining agriculture in some areas and cutting sharply into cities' water reserves.

"They figured that maybe this was a new permanent change in climate," Lund said, "So they built about six coastal desalination plants. And only two of those are now currently in operation. The others, several billion dollars worth of plants, were just mothballed. You still have to pay for that capital expense, but you don't get any water at all."

While water returned to many parts of Australia that suffered through the drought, climate scientists say the Colorado River basin is unlikely to see any substantial recovery from its drought.

Warming temperatures accelerated by climate change, mean dry soils and early snowmelt that will keep the Colorado River basin dry in the future.

Wetter weather patterns may not be the reason that desalination plants would be constructed but unused, but Lund says they may not be needed if large amounts of water are reallocated from agriculture, which was not an option in Australia.

"Probably the worst time to build a desal plant is in a drought," Sarah Porter said. "It's kind of like grocery shopping when you're hungry. 'I don't ever eat this stuff. Why did I buy this?'"

Still worth exploring

Despite the costs and challenges of expanding desalination in the U.S., experts say the technology is at least worth a closer look.

"Yes, desalinated seawater is expensive," said Sharon Megdal. "But we have to recognize that water is a scarce commodity. Water is a valuable commodity. And seawater desalination tends to be a drought-proof technology."

Megdal said further studies make sense because it often takes decades to get big infrastructure off the ground. If the Colorado River basin might decide that it wants desalination years from now, the wait will be shorter if policymakers get the ball rolling today.

"When we're looking at the full mix and the full portfolio, I think there's a role for desalinated seawater," she said. "The fact is, it will take time to get projects permitted and built. And so you have to think ahead."

Even a desalination plant that has made it through decades of planning and permitting can find political opposition at the end of the road. A controversial proposed facility at Huntington Beach, California, was rejected by the state's coastal commission after two decades of debate.

The $1.4-billion proposal had support from California Governor Gavin Newsom but drew ire from environmental groups. A study of the facility's impacts found that it would kill marine life through its water intakes and the discharge of a high-salinity brine byproduct. The study also cited risks to the plant's structural integrity and said that rising sea levels or earthquakes posed a threat of damage. Poseidon Water, which also runs the Carlsbad facility, would have operated the plant.

In Carlsbad, the users of that water say it's a valuable tool in their toolbox. San Diego County gets about two-thirds of its water from the Colorado River. Jeremy Crutchfield, a water resources manager for the San Diego County Water Authority, says desalinated water gives the area valuable flexibility, and that flexibility is worth a five-dollar monthly bump to each household's water bill.

"It really goes back to reliability," he said. "That's what this brings to our water supply portfolio. When you look at an investment portfolio, you don't want to put all your eggs in one basket. You want to diversify and have different areas where you can bring in revenue, or in this case, bring in water."

This story is part of ongoing coverage of water in the West, produced by KUNC in Colorado and supported by the Walton Family Foundation. KUNC is solely responsible for its editorial coverage.

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