LAS VEGAS – The patroller stopped his water district truck and grabbed his camcorde “Here we go,” he said, sliding from the cab and pointing his lens at the fine spray of water and rainbow rising from pop-up sprinklers on the lawn of a low-slung ranch home.

“Thursday,” he spoke, recording the day as evidence. No watering allowed on Thursdays.

Welcome to the future, where every drop of Colorado River water is guarded and squeezed. Only here, in the city that gets 90 percent of its water from the fickle and fading river, the future is now.

The vast and highly urbanized Southwest, built on the promise of a bountiful river propped up by monumental dams, is up against its limits. Already tapped beyond its supply, the river is now threatened by a warming climate that shrinks its alpine source.

To support fast-growing urban populations in a time of dwindling supply, the Southwest is due for rapid and revolutionary changes.

A region that uses two-thirds of its water outdoors, and mostly for agriculture, will have to find ways of sharing and boosting efficiency — a shift that many experts believe will mean city dwellers paying to upgrade rural irrigation systems.

Cities such as Phoenix and Las Vegas, which have reduced their per-person water usage through better landscaping and appliances, will have to do better. They lag behind Los Angeles, whose growing population, by necessity, uses no more water than it did 40 years ago.

Water suppliers from Denver to San Diego will spend billions of dollars to squeeze more out of each drop, and to clean and use wastewater and salt water. It means a future of higher water bills, further promoting conservation.

Problem can’t be deferred

“We’re in a drought,” water patroller Robert Kern said after hanging a warning notice on the home’s doorknob. Two more violations and the water district will fine the owner $80.

“Everyone has to do their part.”

Residents in this part of town — known as Zone C to the Las Vegas Valley Water District — may only water on Monday, Wednesday and Friday from fall through spring. They’re freer to soak their grass at will in summer, when the withering heat demands it.

The cooler months are for austerity, to give the plummeting water levels behind Hoover Dam a break. The river’s massive storage tub, Lake Mead, is draining.

The Colorado isn’t all that we thought it would be when we divvied up the rights in the Roaring ’20s. Most years, it gives less than it once did, and there are more users taking from it.

A 2012 government study of supply and demand predicted a 2060 annual shortfall of nearly a trillion gallons — enough to cover the sprawling city of Phoenix 9 feet deep or to supply 6 million Southwestern households for a year.

How the Southwest’s leaders, farmers and lawn waterers respond will help decide how many millions of people this drying corner of the continent can sustain in the next century.

Throughout this year, The Arizona Republic will examine the twin stresses of climate change and population growth, and ways to ensure reliable water for the next generation of Southwesterners.

“This is not one of the problems you can defer and let your grandkids deal with,” said Doug Kenney, a University of Colorado law professor.

Last year, the Arizona Department of Water Resources published a “strategic vision” for the coming century. The department stopped short of calling the state’s current situation a “crisis,” but said Arizona is at a “crossroads” and needs to decide on actions to secure new water.

Many potentially costly steps for metro Phoenix were included: conservation, treated water recycling, watershed forest thinning, cloud seeding and seawater desalination among them.

Kenney chairs the newly formed Colorado River Research Group, an independent group of 10 river and climate experts from regional universities. This winter, they made a simple recommendation that would have sounded outlandish in the past century.

Use no more water.

Cities will have to grow within their means, through conservation and by paying farmers to save and transfer water, he said. When the river already falls short of supplying everyone who has a legal right to it, there’s no sensible way of taking more from it.

“If everyone takes what they’re legally entitled to,” Kenney said, “the system crashes.”

That’s true even if the wetter 20th century hydrology repeats. But that’s not what the big water suppliers are expecting.

Actual flow of Colorado River versus water promised for Southwest

Agreements have promised 16.5 million acre-feet of water annually to come out of the Colorado River for use by Western states and Mexico. But in many years, the actual flow of the Colorado has been lower than what’s promised, which is marked by the solid line. The 110 year average is shown by the dotted line.

“In my opinion, the future of the Colorado Basin is a future where we have less water than we have right now,” said John Entsminger, general manager of the Southern Nevada Water Authority.

“The future of the Colorado Basin also has less grass.”

But it won’t be just the urban lawns that attract scrutiny. Farmers from Wyoming to Mexico — by far the biggest users of the river — will have to back off on hay production

They’ll also have to embrace expensive but efficient drip irrigation, Entsminger said. Urban water users will help pay for that through higher rates.

“Everybody’s going to have to figure out how to do the same or more with less water.”

Robert Kern, a Waster Water Investigator for the Las Vegas Valley Water District, documents a watering restriction violation in a west-central Las Vegas neighborhood. Las Vegas residents are only allowed to water on assigned days, Kern issued a warning to the homeowner.

At Lake Mead, America’s most voluminous water impoundment when it was full and a lifeline to everyone from Phoenix to San Diego, the crisis has already arrived.

Desiccated palm trees flap over the cracked and peeling shell of a resort hotel at Echo Bay Marina at the northern end of the lake, the tattered banners of a man-made oasis now drained and vacant. Dormant boat docks lie stacked against each other.

To nearby innkeeper Chris Wiggins, it’s a sign of government mismanagement.

“Climate change?” he scoffed. “That’s the biggest joke.”

You don’t have to believe in a climate connection to recognize the risks in doling out on paper more water than a river can give.

“In the lower basin, we use more water than in a normal year we receive,” said Chuck Cullom, Colorado River program manager for the Central Arizona Project, whose canal pumps water to Phoenix and Tucson.

“Even absent the drought we would still be facing a declining Lake Mead.”

A sustained regional drought that started in the late 20th century shrank the reservoir to its record low by last summer. Federal officials say there’s a 1-in-4 chance it will sink low enough — to 1,075 feet above sea level — by next year that Arizona will have to cut back substantially on what it takes from the river.

After that, the government projects, the odds are better than even — about 60 percent — for a declared shortage and restrictions in 2017.

The reservoir has fallen by more than 100 feet since 2000. Its stored water, paired with upriver sister reservoir Lake Powell, is at about half-capacity.

The water’s retreat is a slow-blooming crisis that many have seen coming for years. Some communities have used the time to curb their thirst.

Los Angeles residents use 129 gallons a day each. That’s stingier than the 160-gallon average in Phoenix, whose use rate has nonetheless plummeted in recent years.

Now, though, even conservation-minded Los Angeles is following the unlikely lead of a gaudy, electrified billboard for sustainability. Still ridiculed in some corners as a wasteful and whimsical boomtown in the desert, metro Las Vegas has nonetheless turned its precarious relationship with the river into a powerful incentive to cut back.

Southern Nevadans use 212 gallons a day, which is more than their counterparts in either Los Angeles or Phoenix. But they also return almost 40 percent of that to the river as treated and reusable wastewater, making their net usage rate 124 gallons.

They have slashed usage steeply and deeply, by more than 100 gallons in about a decade.

Las Vegas has cut use of the river by nearly a third in a 12-year period that saw its metro population grow by 25 percent.

Vegas did it by regulating outdoor watering, and by paying $205 million — up to $2 a square foot — to entice people to remove lawns and “embrace living in the Mojave Desert,” Entsminger said.

That was crucial, because in 2002, Nevada was using more than its legal entitlement to the river.

Now Los Angeles is following, paying homeowners even more money to strip lawns.

For decades, the Colorado River hasn’t typically flowed as high as it did about a century ago, when Congress authorized impounding it at what would become Hoover Dam.

Climate scientists say there’s a strong chance it never — or rarely — will again. Yet unlike in those pioneering days of last century, more than 30 million people and several billion dollars in farm production are now counting on a river that is so tapped that in most years it no longer reaches the sea.

What’s left after the U.S. uses most of the water is diverted to farmers in Mexico.

“The Colorado River Compact appears to have been negotiated during an unusually wet period,” said Connie Woodhouse, a University of Arizona geosciences professor who has studied historic flows on the river. “I don’t think anyone would argue with that.”

The 1922 agreement split the river’s flow between upper- and lower-basin states, with the divide just upstream of Grand Canyon, at Lees Ferry. In the first few decades of the 20th century, an average approaching 17 million acre-feet — each acre-foot gushing 326,000 gallons, 51/2 trillion gallons in all — flowed past Lees Ferry every year.

For most of the past 90 years, though, the average flows have sagged below even the 15 million acre-feet that the states legally share, let alone the 1.5 million owed to Mexico by treaty.

The enormous but shrinking reservoirs at Lake Mead and Lake Powell, capturing spikes in runoff during occasional wet years, have forestalled shortages. The flow was 20 million acre-feet in 2011, and just half that in 2013.

That Colorado, Wyoming and Utah weren’t using their full shares also postponed a reckoning.

Until now.

The drought that started in 2000 and sent the reservoir holdings plunging is a preview of expected dry spells unprecedented in recent centuries, Woodhouse said. Temperatures are higher than those of the last century’s droughts, compounding the intensity.

“The (rising) temperatures are only going to exacerbate conditions that we would normally expect under natural conditions,” she said.

There are lots of reasons to think the droughts of coming decades will be worse than anything we’ve ever experienced — regardless of whether there’s any change in precipitation.

The first is that as the region warms, the trees and plants using the snowmelt will need and tap more of it before it ever reaches the river or pipes.

The next and arguably bigger threat is that the warmth will melt snow faster or even make it fall instead as rain. Either change will lead to more evaporation and less seepage into the soils that, in turn, release water to streams feeding the river.

Four years ago, the U.S. Bureau of Reclamation — the Southwest’s federal water managers — crunched all of the climate model projections for the Colorado River watershed and determined the average outlook was for a river pumping 9 percent less water through the region by 2050.

There is always a chance that monstrous snowstorms and winter rains will bring enough new winter precipitation to offset the warming’s worst effects, said Jeff Lukas, climate scientist with the University of Colorado’s Western Water Assessment team.

“Increasing flow isn’t precluded,” he added. “It just appears to be less likely.”

Past warm spells, etched as living history in the West’s tree rings and lake beds, indicate that where there’s heat there’s often stinging drought, according to Woodhouse’s work.

She co-authored a 2010 study using regional tree rings from an unusually long and hot medieval drought to project that each increase of a degree Celsius results in a decrease in Colorado River flows of between 2 percent and 8 percent.

Most of the region already has warmed by more than a degree on average in the past quarter-century, according to last year’s U.S. National Climate Assessment. Further warming of at least a couple of degrees in a few decades and up to 5 degrees by 2100 is expected even if global carbon emissions are substantially reduced.

The medieval drought, in its worst decade, baked the river down to about two-thirds of what the U.S. and Mexico draw out of it today.

The drought lasted 60 years, but it was not as hot as today. So it seems the next time there’s a repeat of whatever natural phenomena conspired back then to produce such a long, dry spell, the river will be even drier.

Since Woodhouse’s study, a team of 14 university and government researchers has conducted what Woodhouse calls the “best synthesis” of existing climate and flow models — with jaw-dropping, if imprecise, predictions.

The river’s flow probably will drop between 5 percent and 35 percent in response to warming by midcentury, according to that team, which published a January 2014 report in the Bulletin of the American Meteorological Society.

Lukas’ University of Colorado colleague, snow researcher Jeffrey Deems, said there’s reason to believe the bureau’s predicted 9 percent reduction in flow is optimistic.

Already, the Rocky Mountain snowpack is melting three to six weeks earlier than before American settlement of the region, Deems’ studies have found, because dust drifting up from grazing lands and other disturbances collects solar heat on the snow’s surface. Today’s snowmelt is measured by direct observation and compared with computer models of older trends.

Without emissions curbs, Deems said, his modeling and others project flows slashed by about a fifth on average by midcentury.

“Even if it’s only 9 percent,” he said, in a nod to the Bureau of Reclamation study, “that’s a huge shock to any overallocated system.”

A 9 percent reduction would roughly equal the 1.5 million acre-feet that Arizona is allowed to pump through CAP’s 336-mile canal every year.

But that’s a midcentury outlook with lots of climate variables. What about the near-term effects of the existing drought?

If the government declares a Lake Mead shortage because the water drops below the mandated trigger elevation of 1,075 feet — the 58 percent probability that managers have projected by 2017 — then Arizona would lose 320,000 acre-feet every year that the water is so low.

An acre-foot of water is about the amount two Southwest families use each year. So the loss would be about three times the potable water that Tucson Water pumps to customers each year. But it’s not the cities and their residents who will suffer first or most.

CAP was built largely to fuel growth in metropolitan areas of Arizona. The farmers who have used what until now was excess water have the lowest legal priority. Some of them will voluntarily cut back on watering hay and other crops this year, in an effort to help keep Lake Mead from falling.

In December, CAP signed an agreement with the Bureau of Reclamation and water providers for Southern California and Nevada to save 740,000 acre-feet over the next three years, and to keep it in Lake Mead. Each of those organizations would sacrifice water or improve efficiency.

Arizona, with the most to lose from a shortage, is responsible for the largest share: 345,000 acre-feet.

Of that, the deepest cuts — nearly half — will come out of farm irrigation districts. But CAP will pay those farmers $5 million.

“It could actually protect us (from shortage) for a couple of years, and that would more than repay our efforts.” said Cullom, CAP’s Colorado River program manager.

But in the same agreement, the states predicted that these savings might be only half the job of restoring reliable water by 2019. So they also will join Denver Water in sponsoring $11 million in pilot programs that other customers can use to suppress their needs — some of it perhaps for farm upgrades such as drip irrigation or laser field leveling.

If Lake Mead drops another 25 feet after the first shortage, central Arizona would lose nearly a third of what it draws off the Colorado. Farmers there would get nothing from the river, and cities such as Phoenix, Mesa and Scottsdale could start to lose some of the canal water they’re now leasing from Indian tribes.

Best to act now, Cullom said, and reload Lake Mead.

“It’s like a scene from ‘Jaws,’ when one of the characters says, ‘We need a bigger boat,’ ” he said. “We’re trying to find ways to get a bigger boat.”

Some water managers and politicians have mused about importing the solution, from the Great Lakes or the Mississippi River Basin by pipe, or even from Alaska by ship. But the U.S. Interior Department effectively called those schemes pipe dreams, in a study of options for the Southwest.

For one thing, other states may guard their resources as jealously as Arizona would covet them in a water-strapped future. The Great Lakes states even have a compact prohibiting export, and it is being invoked to prevent a Wisconsin county that touches on the drainage from piping water over the line.

Also, the costs, both environmental and financial, caused the Obama administration to reject the idea. Pumping water from the Missouri River to Denver would cost 21/2 times the predicted price to conserve the same amount within the Southwest.

Conservation probably can provide only a third of the new water needed in 50 years.

Environmentalists generally have recommended starting there, though, and then adding treatment plants to clean salt from used irrigation water and return it to the river. Utility managers are also looking to add costlier, more energy-intensive seawater desalination, which could reduce coastal cities’ reliance on the river.

The biggest sponge out there, though, is agriculture. Its use of two-thirds of the Colorado’s bounty offers future urban residents a tantalizing buffer for growth — or a water grab — if it can be reallocated.

About a third of the Colorado River’s annual flow goes just to alfalfa, pasture and other forage for livestock, according to a 2013 analysis of farming in the 256,000-square-mile watershed, conducted by the Pacific Institute.

Much of that grass is flood-irrigated, putting to work water that farmers earned through settlement claims under a “use it or lose it” system that predates the West’s urban population explosion.

The institute modeled other options for ranchers — modern irrigation equipment and a more judicious schedule for watering — and projected a potential savings of 1 million acre-feet a year.

Farmers won’t give up water if they think it means losing their rights to it, and to the income it can bring them, said Kenney, the University of Colorado law professor. But states are free to change the laws, to ditch “use it or lose it.” They can ensure that farmers and rural areas are compensated.

Kenney expects change to come, and city dwellers to pay up, as the Central Arizona Groundwater Replenishment District is doing in an experimental program that gives 33 farmers $750 per acre per year for three years to cut and fallow some citrus orchards.

“Scarcity drives innovation,” he said.

Back in Las Vegas, water patroller Robert Kern spotted a wet sidewalk near the first violator he nabbed. It wasn’t a sprinkler, though. What grass the lawn had was yellowed and crisp.

“I had to mow her lawn the other day because I was afraid there’d be a fire,” said a neighbor, Danny Hinchcliffe, standing on his own dewy grass.

Kern climbed from the truck, knelt to find moss growing in a slight but steady stream of water flowing from a broken underground pipe. He attached another warning to her doorknob.

Hinchcliffe said his own yard used to be rock, but he switched to grass because it helped cool his home and keep down the electric bill.

Reminded that his grass blades shouldn’t be glistening with water on a day when sprinkling is banned, he said his landscaper likely hadn’t had a chance to adjust his timer for the season.

But he didn’t get a citation.

Kern can’t issue a warning or a ticket unless he actually sees the water spraying.

“Our biggest thing is education,” he said. “Without the water, we’re not going to be here.

“We’re in the middle of a desert.” More