In 1911, a group of farmers on the lower Salt River, struggling to cope with floods and drought, built Roosevelt Dam and developed a reliable water supply that has met the needs of a growing region for 100 years.

In 2011, the challenge is no longer how to ensure enough water to develop farming in the arid Valley, but where Phoenix and other Arizona cities will find water for the next 100 years.

Demand has spread beyond the capacities of Roosevelt and the other dams on the Salt and Verde rivers. Drought has underscored the vulnerability of Colorado River water, delivered to the Valley in the last big water project built here, the Central Arizona Project Canal. Climate change has added uncertainty.

What is certain is that there will be no Roosevelt Dam for the next generation, no single piece of infrastructure that can create another 100-year supply of water.

"The big dams are all built," said Craig Kirkwood, an Arizona State University researcher who has studied water-resource issues.

"You can reallocate water. You could augment water supplies, but that would probably be very expensive. Or you could say, 'We're living in the desert,' and just accept that."

Still, most water experts agree that if the region continues to grow, its cities will need to find more water - or use less of what is now available - to avoid drawing too deeply on non-renewable groundwater supplies.

"We've probably got another 30 years to run without doing anything too dramatic," said Grady Gammage Jr., a senior fellow at ASU's Morrison Institute for Public Policy.

"Beyond the 30-year point, if you assume we will continue to grow even close to the pace we have - and I'm not at all convinced we will - beyond 30 years, something dramatic has to happen."

Storing the supply

In 1968, Congress approved a plan to build a third dam on the Verde River northeast of Phoenix. Orme Dam would expand water-storage capacity and help capture runoff during unusually wet years.

But Orme Dam was never built, and it has become a case study for why dam-building is not without limits.

Indian tribes and conservation advocates objected to the project's location and argued that it would destroy valuable wildlife habitat. Water agencies across the West recognized the growing influence of environmental groups, who would willingly fight for their cause in court.

Water experts now say that if Orme had been built, there's no assurance the Verde would have filled it. During the recent drought, Horseshoe Lake, the smaller of the two existing reservoirs on the river, sat empty for months at a time.

Meanwhile, Arizona pioneered large-scale water banking, storing unused Colorado River water by pouring it into recharge basins to percolate into the ground for later use. So far, the state has banked about 1.6 trillion gallons.

What worries some water managers is that the banked water could end up being used as a long-term source instead of an emergency supply.

Cities could start drilling wells just as they did before groundwater-management laws were enacted to protect aquifers.

"The philosophy behind those laws was that we would use surface water and save our groundwater for drought or other unexpected events," said Kathryn Sorensen, water-resources manager for Mesa. Using banked water as a long-term source, she said, "is not sustainable."

"It's cheaper and easier," she added, "but it's not sustainable."

Conserving resources

Conservation is the cheapest and most direct way to stretch water resources, but it would require fundamental changes in the way people use water.

"When you talk about conservation, people think about shorter showers or maybe reclaiming effluent," Gammage said.

To make a real dent in water use, Phoenix would need to change the entire landscape - the actual landscape, Gammage said. "If you quit watering landscape at the rate we do, just drop the per capita water use in Phoenix to what they use in Tucson. That's about a 40 percent increase in water supply right there."

The problem is most people don't want to give up lawns and trees, Gammage said, adding that they're not wrong.

Grass and trees improve quality of life and reduce urban-heating effects. Phoenix doesn't get enough rain to support the greener native landscape evident in Tucson.

Conservation programs have succeeded in reducing indoor water use, and water experts said most Phoenix homeowners could use less water outdoors without sacrificing greenery. The question is how far water providers and users are willing to go to conserve.

"It's an infrastructure issue in a sense," said Patricia Gober, co-director of ASU's Decision Center for a Desert City.

"You build a city that's not set up for water reuse, you build an oasis city with lots of lawns and trees instead of a desert city, and 50 years later, with climate change upon us, we have no quick adaptation strategy."

Importing what's needed

Arizona and other Western states grew because they were able to move water from where it was to where it wasn't, importing water through mountains, across deserts and up hills toward the money of developing cities.

Dams made that movement possible. Without new dams, importing water will become more expensive and more difficult.

There are some quick fixes: Engineers have already studied the possibility of enlarging the CAP Canal, which moves water 336 miles from the Colorado River to Tucson and Phoenix.

But the idea gaining the most attention right now is desalinating ocean water. Officials from SRP, the CAP and the state have met with private investors and officials from Rocky Point, Mexico, about building a desalination plant on that resort city's coast.

The plant, built with money from both sides of the border, would produce water for Rocky Point and export the treated water to Arizona and California.

With the right mix of pipelines and trades within the Colorado River system, such a project could benefit areas of rural Arizona that lack a renewable source.

But the cost would be enormous, as would the energy requirements. Even if those obstacles were overcome, the plant might not produce water for decades.

"It might fill a small short-term immediate need, and maybe it's part of the solution mix, but these plants are incredibly expensive," said Brad Udall, director of the University of Colorado's Western Water Assessment. "And if you build them and the drought ends, you have a very expensive plant sitting there waiting for the next drought. It's not a silver bullet."

Reallocating uses

Farmers built Roosevelt Dam and have developed water supplies across the West. But now, many urban water managers say agriculture needs to give back some of what it has used.

San Diego was one of the first major cities to strike a deal with farmers to lease water made available when the farmers took land out of production. Now, other cities see that as an option.

ASU researcher John Sabo said agriculture transfers could help stretch water supplies but only as part of a broader program that includes aggressive conservation efforts in cities.

"You can't just get rid of farms," he said. "We need to find a way for everyone to benefit."

Cities are increasingly willing to pay more for water, especially in new developments, Sabo said.

Maybe the cities could pay farmers not to grow low-value, high-water crops like alfalfa and use the water that would have irrigated those crops.

The farmers could take the income from those deals and install more efficient irrigation systems, which could reduce the amount of water diverted from rivers, creating an environmental benefit. Higher water costs for municipal customers would have the same effect.

"You can't hammer everybody, or no one's going to buy into it," Sabo said. "This idea that there are three parties that are mutual enemies, that's the part that needs to be solved so each one is helping the others."