YUMA - Along its final miles, the Colorado River snakes through a dizzying series of dams, canals, siphons and ditches, diverted to hundreds of users in Arizona and California until barely a trickle remains.

What flows through this watery Grand Central Station could fill the needs of all the homes and offices in Phoenix, Tucson, Las Vegas and much of Southern California.

But it doesn't.

The water, more than a billion gallons a day, irrigates vast fields of wheat, alfalfa, cotton, lettuce, cauliflower, broccoli, melons and a produce aisle of other fruits and vegetables, feeding an industry tilled from the desert more than a century ago.

In Arizona, the crops yield about 1 percent of the state's annual economic output, yet the fields soak up 70 percent of the water supply. That outsize allotment has painted a target on the farms as urban water managers search for the next bucket of water to meet future demands.

Because so much of what farmers use flows from renewable surface supplies, such as the Colorado River, agricultural water seems like a good pool to tap. Using that water would reduce the pressure on the state's vanishing groundwater resources and reduce the need to pay for expensive alternatives, such as desalinated seawater.

But the issues have grown more complicated than transferring water from one user to another. The conflicts have evolved from the clearly defined us vs. them - cities vs. farmers - to more nuanced sustainability issues, such as trading future urban water supplies for locally grown food.

Some of the biggest questions ahead for Arizona's water managers will have less to do with whether we have enough water than how we use the water we have. They are questions that will force Arizonans and their elected leaders to decide what they value.

"We don't want to get into a situation of saying 'My use is better than yours,' " said Tom Davis, general manager of the Yuma County Water Users Association. "But there needs to be a better way than just whoever has the most money gets the most water."

The farmers' water

There's no getting around the facts: Farmers use a lot of water in Arizona. With so little rain, they must irrigate every field of hay, every row of vegetables. In a year, agriculture consumes almost 5 million acre-feet of water, or 1.6 trillion gallons.

The annual value of the crops alone is about $2 billion. Industry groups say that, including suppliers and related businesses, agriculture overall contributes more than $9 billion a year to the state's economy. At either estimate, though, agriculture accounts for just a fraction of Arizona's gross domestic product, which in 2007 was an estimated $245 billion.

Rick Sellers farms about 4,000 acres between Yuma and the U.S.-Mexico border. He makes his best money during the winter, when he grows lettuce, cauliflower, broccoli and fennel - crops not native to the desert.

"The vegetables take a lot of water," he admitted. "But we manage the heck out of it. We try to stretch it as far as we can."

Sellers strings sprinkler pipes across many of his fields instead of running water down ditches, reducing losses to seepage. He spent $48,000 this year to line a section of dirt ditch with concrete. He employs a small army of irrigators to make sure crops get only the water needed.

"Frankly, I think we're about as efficient as we're going to be," he said.

With a reliable water supply, farmers can manage their resources down to the drips in a tangle of irrigation hoses, a precision that helped Yuma grow into the nation's winter vegetable capital. Sellers said rain is an unwanted intruder during the growing season because it can't be managed.

One reason farmers can take so much water is that they staked their claim first, which is what matters in Western water law. Most of the irrigation districts in Yuma hold rights to the Colorado River that predate Hoover Dam, which means if the river starts to run dry, the farmers get their share before anyone else. They can lease water to other users but keep the long-term rights.

Together, the largest water districts in the Yuma area can divert more than 750,000 acre-feet of water from the Colorado each year. Metropolitan Las Vegas, with a population of more than 2 million, can draw just 300,000 acre-feet a year.

An acre-foot is 325,851 gallons, enough to supply two typical households for a year.

Critics complain that farmers grow too many water-needy crops, such as cotton and alfalfa, and pay too little for the water they use. Environmental groups say the water amounts to a federal giveaway because it flows through government-built reservoirs and canals.

But the farmers say they manage their resources carefully, more so, they argue, than the cities that covet agriculture's water. The payoff is on display in the produce aisles of supermarkets in those same cities.

"The real end user of that water isn't the farmer, it's the guy who buys the lettuce or the cauliflower," said Davis, the water district manager. Farms fill tables with locally grown food, which reduces the demand for imported fruits and vegetables.

"If this basic industry is reduced because cities want their golf courses and their lawns, how long before we can't feed our own country?" Davis said.

The cities' water

Urban areas have long viewed agricultural water as a backup supply during a drought or other shortage, such as the failure of a pipeline or a canal, but rural farmers fear some cities want the water as a permanent supply for future growth. It's happened before.

Phoenix exists because farmers settled along the Salt River and built a series of dams and reservoirs to manage the water supply. Citrus groves and cotton fields once blanketed the valley, just as melons and lettuce do in Yuma.

As Phoenix grew, the land available for farms shrank and water use shifted. As recently as 1965, 80 percent of the water delivered by Salt River Project was used by agriculture. Today, that number is just 15 percent.

Converting retired agricultural water supplies to urban uses helped Phoenix grow without the water issues facing Las Vegas, where there were never many irrigated fields. The canals that move water - particularly the Central Arizona Project canal, which taps the Colorado River - could help fuel more population growth if farmers were to give up some of their water.

But if farmers go out of business, the crops they sold in Arizona would be imported from another state or country. As their water is claimed by new development, cities would lose the backup water source, which would become more important if droughts worsen as climate scientists predict.

In 2007 and 2008, a group of water managers, farm-industry representatives and university researchers traveled Arizona asking farmers about the future of agriculture. The farmers were clearly worried.

"They were concerned about having enough water for the future," said Chris Udall, executive director of the Agribusiness Council of Arizona, a group that works to protect farmers' water rights. "There are a lot of farmers who want to stay in business."

One of the messages from those sessions was clear: Cities shouldn't assume they can just shift water from farms to cities. Growers with senior water rights can refuse to give up their supply, even temporarily, and groundwater rights are usually considered a property right, which carries its own protections.

Some experts say cities should consider paying farmers to modernize irrigation systems, in hopes of using the water that would be saved. Nevada, Arizona and California have already agreed to spend $172 million to build a reservoir west of Yuma to capture water that had been lost to inefficient procedures.

Pat O'Toole, president of the Farm Family Alliance, a national trade group, warned lawmakers about the risks of focusing on the agricultural allotment of water as a future source for growth.

Testifying before a U.S. Senate committee in 2007, O'Toole said relying on farmers' water "is not planning. It's a choice to put our heads in the sand."

But is it sustainable?

The bigger question is what Arizonans want to do with their water. Most planners start with the idea that water shouldn't limit growth.

But farmers and others will argue that growing food and fiber is a more sustainable use of water. Locally grown produce reduces the need to import fruit and vegetables, which shrinks fuel consumption and adds certainty to the food supply.

"We can start out asking, 'What food do we need to nourish Arizona's population?' and 'What do we do to meet Arizona's food security?' " said Gary Nabhan, a research social scientist at the University of Arizona's Southwest Center. "I think the most water- and energy-efficient way to do that is dedicating our farmland to low-water food crops."

Those would be vegetables and some grains that can grow on drip irrigation systems and require less water than alfalfa, cotton or some kinds of corn.

Nabhan, the author of several books about the connection between food and the environment, said Arizona has grown under the false impression that retiring farmland in favor of cities will conserve water.

"The farmers have already reduced water use for reasons of economics and water restrictions," he said. "We've already seen greater efficiency without a very straightforward program. And yet we've put tons of money into urban water programs and we haven't made that great a gain."

Most water experts say it's likely that cities and states will look for ways of moving at least some water from farms to urban uses.

"The last bucket is transfers," said Robert Glennon, a University of Arizona law professor and the author of the recent book "Unquenchable: America's Water Crisis and What To Do About It."

"Right now, it's the farmers' water and they're doing fine," Glennon said. "Farmers are very nimble and adept at changing crops. If there's no reason to be nimble, it'll be business as usual."

Strong water laws will give growers a negotiating position as outsize as their water supply. Any shifts will almost certainly hinge on lucrative short-term leases as long as the farms can stay in business. As a result, cities will struggle if they only search for new sources of water instead of examining more closely how they use existing supplies.

"Having so much land in agriculture does give us flexibility," said Herb Guenther, director of the Arizona Department of Water Resources. "Our downfall is that we've done a pretty good job for a lot of years. We haven't had any major crises. We do have the water. We just need to make sure we manage it right."