Impacts of the 20-year Colorado Plateau drought

Western states are alarmed by the two-decade drought on the Colorado Plateau that is steadily diminishing the flow of the Colorado River.

We have assembled one short radio broadcast and two in depth pieces from Yale University on the Colorado River crisis. Although they don't focus attention on it, the agreement reached by the federal government under George W. Bush and Colorado River users, focusing on the Imperial Irrigation District in California, caused a huge increase in pumping out of the Delta to make up the difference in metropolitan Southern California.

That decision put the Delta Smelt on the map of highly endangered species and undoubtedly has had something to do with the recent decision by the state water board to require more water from the San Joaquin River tributaries. -- blj

 

1-29-19

KQED

Just Days Left to Avert Colorado River Water Crisis. Can States Make a Deal?

https://www.kqed.org/science/1937377/with-days-left-colorado-river-state...

Avoiding a long-expected crisis on the Colorado River, a water source for 40 million people, is coming down to a final few days of frenzied negotiations. A 19-year drought and decades of overuse have put a water shortfall on the horizon.

 

'These are the lowest reservoir levels in my lifetime. We are quickly running out of time.'Brenda Burman, Bureau of Reclamation

 

If California and six other states, all with deeply entrenched interests, can’t agree on a plan to cut their water consumption by Jan. 31, the federal government says it will step in and decide the river's future.

The Colorado River has been the object of some of the West's most bruising water battles. If farmers, cities and water districts can agree to conserve water now, it would mark a new era of cooperation.

In California, the Colorado supplies millions of people in San Diego and the Los Angeles area. Its water also shows up on dinner plates across the country. In winter, most leafy greens and lettuce in grocery stores are grown with it, in California’s Imperial Valley or across the border in Arizona.

Still, despite the high stakes, California and Arizona have yet to sign off on the plan. For California, it will take convincing one farming community to share water, even though their supply of the Colorado River is considered the most untouchable.

Climate Change is ‘Water Change’

The long-running drought in the Colorado River basin has brought everyone to the table, but it’s not the entire story.

“Most droughts, it’s pretty obvious,” says Brad Udall, climate and water scientist at Colorado State University. “You get less snow or less rain, you see less water in the river. In this case, there was actually still pretty good precipitation and yet a horrible drought. So, it begs the question: All right, what’s going on here?”

Udall and others found one answer came from the warming climate.

“Climate change is water change,” he says. “You heat up the climate, you are going to get fundamental impacts to the water cycle. We’ve known this for almost 50 years now.”

A warmer atmosphere sucks up water, drawing it out of plants and soils. Udall says that means less runoff going into the river, potentially cutting its flow 20 percent by mid-century.

It Doesn't Add Up

Add to that, the Colorado River has a fundamental accounting problem. Seven states and Mexico rely on the river, but by longstanding agreement are entitled to more water than it delivers in most years.

For decades, many states didn’t use their entire share, something that largely benefited California, which was able to tap into the surplus. But now, demand on the river has grown.

That’s led to falling water levels in Lake Mead, the biggest reservoir on the river (and largest man-made reservoir in the nation). Its level is close to the specified low point that would trigger the federal government to declare an official shortage of water on the river for the first time.

“These are the lowest reservoir levels in my lifetime,” said Brenda Burman, Commissioner of the U.S. Bureau of Reclamation, as she addressed water users in December. “We are quickly running out of time.”

So, states on the river have been negotiating a water-sharing deal known as the “drought contingency plan.” Water users would conserve and cut back, so that Lake Mead's surface wouldn’t sink to its trigger point of 1,075 feet above sea level.

Negotiations have been slow-going. Some hoped a few wet winters would solve things. But with a shortfall in sight, the federal government turned up the heat.

 

“I am here today to tell you all that we will act if needed to protect this basin,” Burman said last month. If there’s no deal by Jan. 31, the Bureau of Reclamation will start a process to make new rules to govern water on the river.

States like Arizona and Nevada have the biggest incentive to make a deal. That’s because of the pecking order of who gets water first, known as the water rights system.

Under the current system, if a shortage is declared in Lake Mead, Arizona and Nevada would be the first states required to cut back.

With the oldest water rights, California wouldn’t have to make any cuts, at least until the reservoir dropped so much that the whole system would have to be renegotiated.

Under the proposed drought plan, California would cut back voluntarily to help keep water in Lake Mead. On a river where water rights have ruled, it marks a major moment of collaboration.

Some water users, like the Metropolitan Water District, a major supplier in Southern California, have largely supported the plan, seeing their fate tied to the future of the river.

But in the subsidiary pecking order within California, Metropolitan is at the bottom. It would have to cut back before others in the state with more senior rights -- like the Imperial Irrigation District. That’s where the drought plan has been a tough sell.

Water's For Fightin' Over

“It’s extremely important that we have a reliable water source,” says Erik Ortega, president of the board of directors of the Imperial Irrigation District. “It’s the Colorado River, our only source.”

The Imperial Valley uses more Colorado River water than Arizona and Nevada combined, mostly for agriculture. It’s the dominant industry there, thanks to its year-round growing season.

Public meetings about the drought plan have turned contentious. Some raised another water-sharing deal from 2003, where tensions ran high.

 

Back then, California had been using more than its allocation from the river and was being forced to stick to the limit. San Diego's proposed solution was to buy some of Imperial Valley’s water. Farmers would either fallow land or install more efficient irrigation equipment to free up that water.

 

The irrigation district approved it, but by a very tight margin.

“I was voted out of office over that vote,” recalls Bruce Kuhn, who was a board member of Imperial Irrigation District at the time. “I was the swing vote. I lost friends and I lost business associates over that.”

Kuhn is now back on the board, with another water-sharing vote in front of him.

“If we have to do some things," says Kuhn, "as long as it’s not to our detriment, we stand ready to be good neighbors and do our part."

He hopes his friends will stay with him this time.

“One thing I will not lose this time: I will not lose customers,” Kuhn says of his former agricultural services company. “I sold my business a year ago.”

The board said it would support the Colorado River drought plan under a few conditions, including rights to store more water in Lake Mead.

The largest sticking point is the Salton Sea, a vast inland lake that was created in 1905 by a breached levee. The lake bed has been drying up, creating dust and air quality problems for the region.

“This is an ecological disaster at this point, and it needs to be addressed,” says Ortega.

In order to approve the drought plan, the irrigation district wants to ensure that millions of dollars in Salton Sea restoration funding is approved. The federal Farm Bill, passed in December, had language supporting that, but the district says it’s still looking into how to get that funding.

The Imperial board said it won’t vote to implement the drought plan until that happens. Arizona is also debating how to cut water there, with some farm groups opposing the cutbacks.

 “We’re very hopeful it will be resolved by the 31st," says Ortega, "but I don’t know if that’s realistic at this point."

 

 

1-14-19

Yale.edu

Crisis on the Colorado: Part I

The West’s Great River Hits Its Limits: Will the Colorado Run Dry?

As the Southwest faces rapid growth and unrelenting drought, the Colorado River is in crisis, with too many demands on its diminishing flow. Now those who depend on the river must confront the hard reality that their supply of Colorado water may be cut off. First in a series.

Jim Robbins

https://e360.yale.edu/features/the-wests-great-river-hits-its-limits-wil...

        

The beginnings of the mighty Colorado River on the west slope of Rocky Mountain National Park are humble. A large marsh creates a small trickle of a stream at La Poudre Pass, and thus begins the long, labyrinthine 1,450-mile journey of one of America’s great waterways.

Several miles later, in Rocky Mountain National Park’s Kawuneeche Valley, the Colorado River Trail allows hikers to walk along its course and, during low water, even jump across it. This valley is where the nascent river falls prey to its first diversion — 30 percent of its water is taken before it reaches the stream to irrigate distant fields.

The Never Summer Mountains tower over the the valley to the west. Cut across the face of these glacier-etched peaks is the Grand Ditch, an incision visible just above the timber line. The ditch collects water as the snow melts and, because it is higher in elevation than La Poudre Pass, funnels it 14 miles back across the Continental Divide, where it empties it into the headwaters of the Cache La Poudre River, which flows on to alfalfa and row crop farmers in eastern Colorado. Hand dug in the late 19th century with shovels and picks by Japanese crews, it was the first trans-basin diversion of the Colorado.

 Many more trans-basin diversions of water from the west side of the divide to the east would follow. That’s because 80 percent of the water that falls as snow in the Rockies here drains to the west, while 80 percent of the population resides on the east side of the divide.

The Colorado River gathers momentum in western Colorado, sea-green and picking up a good deal of steam in its confluence with the Fraser, Eagle, and Gunnison rivers. As it leaves Colorado and flows through Utah, it joins forces with the Green River, a major tributary, which has its origins in the dwindling glaciers atop Wyoming’s Wind River Mountains, the second largest glacier field in the lower 48 states.

The now sediment-laden Colorado (“too thick to drink, too thin to plow” was the adage about such rivers) gets reddish here, and earns its name – Colorado means “reddish.” It heads in a southwestern direction through the slick rock of Utah and northern Arizona, including its spectacular run through the nearly 280-mile-long Grand Canyon, and then on to Las Vegas where it makes a sharp turn south, first forming the border of Nevada and Arizona and then the border of California and Arizona until it reaches the Mexican border. There the Morelos Dam — half of it in Mexico and half in the United States — captures the last drops of the Colorado’s flow, and sends it off to Mexican farmers to irrigate alfalfa, cotton, and asparagus, and to supply Mexicali, Tecate, and other cities and towns with water.

While there are verdant farm fields south of the border here, it comes at a cost. The expansive Colorado River Delta — once a bird- and wildlife-rich oasis nourished by the river that Aldo Leopold described as a land of “a hundred green lagoons” — goes begging for water. And there is not a drop left to flow to the historic finish line at the Gulf of California, into which, long ago, the Colorado used to empty.

Nature, in fact, has been given short shrift all along the 1,450-mile-long Colorado. In order to support human life in the desert and near-desert through which it runs, the river is one of the most heavily engineered waterways in the world. Along its route, water is stored and siphoned, routed and piped, with a multi-billion dollar plumbing system — a “Cadillac Desert,” as Marc Reisner put it in the title of his landmark 1986 book. There are 15 large dams on the main stem of the river, and hundreds more on the tributaries.

The era of tapping the Colorado River, though, is coming to a close. This muddy river is one of the most contentious in the country — and growing more so by the day. It serves some 40 million people, and far more of its water is promised to users than flows between its banks — even in the best water years. And millions more people are projected to be added to the population served by the Colorado by 2050.

The hard lesson being learned is that even with the Colorado’s elaborate plumbing system, nature cannot be defied. If the over-allocation of the river weren’t problem enough, its best flow years appear to be behind it. The Colorado River Basin has been locked in the grip of a nearly unrelenting drought since 2000, and the two great water savings accounts on the river — Lake Mead and Lake Powell — are at all-time lows. An officially announced crisis could be at hand in the coming months.

Some scientists believe a long-term aridification driven by climate change may be taking place, a permanent drying of the West.

Meanwhile the Lower Basin states — Arizona, California, and Nevada — have, despite much debate, been unable to come up with a Drought Contingency Plan to keep water in Lake Mead below levels that would trig ger a crisis and lead to mandatory cuts in water. And if the states do not agree on a plan by the end of this month, U.S. Bureau of Reclamation Commissioner Brenda Burmansays she will step in and force hard decisions.

There are large, existential questions facing the 40 million people who depend on the river — there simply is not enough water for all who depend on it, and there will likely soon be even less.

Most of the water in the Colorado comes from snow that falls in the Rockies and is slowly released, a natural reservoir that disperses its bounty gradually, over months. But since 2000, the Colorado River Basin has been locked in what experts say is a long-term drought exacerbated by climate change, the most severe drought in the last 1,250 years, tree ring data shows. Snowfall since 2000 has been sketchy — last year it was just two-thirds of normal, tied for its record low. With warmer temperatures, more of the precipitation arrives as rain, which quickly runs off rather than being stored as mountain snow. Many water experts are deeply worried about the growing shortage of water from this combination of over-allocation and diminishing supply.

There is tree ring data to show that multi-decadal mega-droughts have occurred before, one that lasted, during Roman Empire times, for more than half a century. The term drought, though, implies that someday the water shortage will be over. Some scientists believe a long-term, climate change-driven aridification may be taking place, a permanent drying of the West. That renders the uncertainty of water flow in the Colorado off the charts. While not ruling out all hope, experts have abandoned terms like “concerned” and “worrisome” and routinely use words like “dire” and “scary.”

“These conditions could mean a hell of a lot less water in the river,” said Jonathan Overpeck, an interdisciplinary climate scientist at the University of Michigan who has extensively studied the impacts of climate on the flow of the Colorado. “We’ve seen declines in flow of 20 percent, but it could get up to 50 percent or worse later in this century.”

Even in rock-ribbed conservative areas, those who use the water of the Colorado say they are already seeing things they have never seen before — this year state officials in Colorado cut off lower-priority irrigators on the Yampa River, a tributary of the Green, and recreation had to be halted, for example — and have grudgingly come to believe “there is something going on with the climate.”

If water cuts are mandated, some states will be required to send others their allotted water, whether they have it to spare or not.

As the authors of a 2015 study on the region’s climate future put it: “Our results point to a remarkably drier future that falls far outside the contemporary experience of natural and human systems in Western North America, conditions that may present a substantial challenge to adaptation.”

So there are conventions and meetings and papers being written throughout the Colorado Basin, seeking an agreeable adaptive future for a river in crisis. One of the big rubs in an incredibly complex debate is this: In 1922, California was booming and helping itself to an increasing share of the water, while other states were growing far more slowly. The other basin states wanted to assure their share before California could suck it up and, with guidance from then-Secretary of Commerce Herbert Hoover, created a Colorado River Compactthat divvied up 15 million acre-feet of water — 7.5 million to the Upper Basin States and the same amount to Lower Basin States.

It’s well known now that this Law of the River was a product of irrational Manifest Destiny exuberance, a false premise based on unrealistic projections, because it was signed in one of the wettest periods on the river in centuries. Yet the actual amount apportioned among the states is more than 16 million acre-feet, 1.2 million acre-feet over the too-optimistic apportionment. These extravagant numbers are baked into the system, something known officially as a “structural deficit.”

This winter is make or break for the short term. A crisis is underway — record low flows were seen throughout last summer and fall. Another low- snow winter could light the fuse of a major crisis — an escalating crescendo of emergencies ending in a “compact call” when the lower basin states call on the upper basin states to send them their legally mandated allotment of water — whether they have it to spare or not. All of the players along the river are jockeying for “water security,” an oft-heard term in the region these days.

Cities from Tucson, Arizona, to St. George, Utah, to Denver are booming and need more water to keep growing. Municipal officials across the basin are apprehensive about the future of their growth economy in a time of an increasing likelihood of limits.

Another friction point is the fact that the Upper Basin States — Wyoming, Colorado, New Mexico, and Utah — still have “paper water,” meaning water owed to them by the 1922 compact, which they have not yet taken out of the river. And while many experts say no new straws should be dipped into the river to suck more water out, even if a state is entitled, there are plans afoot to do just that, setting up even more contention and wranglings.

Speculators are quietly buying up farms with water rights and holding them for the day that the price of water soars.

The future of farms and ranches that depend on Colorado River water is most uncertain. Agriculture uses about 80 percent of the Colorado’s flow to irrigate 6 million acres of crops, the largest share of which is alfalfa grown to feed cattle; cities use just 10 percent. While agriculture’s rights are senior — it staked the first claims and so, by law, is the last to lose its water in a crunch — if the going gets tough and cities start running out of water, political and economic clout would favor the millions of people who live there. In that case, agriculture would start losing some of its allotment, either willingly or unwillingly.

“You don’t have to be a rocket scientist to see that there is going to be pressure on your water,” says Mark Harris, general manager of the Grand Valley Water Users Association, a group of irrigators in western Colorado trying to adapt to a new era. “We have a target on our back.” Dewatering agriculture could lead not only to the buying and drying of farms, but the collapse of many small towns whose raison d’être is growing food.

And then there is the recreation industry, a $26 billion part of the Colorado River economy. Last year, raft companies had to reduce their season and cut back the number of trips on the river because of diminished flows.

Meanwhile, speculators and investors have waded into this complex “Chinatown”-like scenario, and are playing a quiet, though growing, role. Hedge funds and other interests, a breed of vulture capitalists, are quietly buying up farms with water rights, and holding them for the day things become more dire and the price of an acre-foot of water soars.

Lastly are the natural attributes of the Colorado River. The needs of fish, wildlife, and native flora have always been at the bottom of the priority list, lost to the needs of booming cities and thirsty crops. That’s changing, as a growing number of people and organizations are working to carve out a future for a more natural river — from the sandy beaches of the Grand Canyon, to the endangered fish along the river’s length, to the birds and jaguars of the Colorado River Delta in Mexico.

The bill for a century of over-optimism about what the river can provide is coming due. How the states will live within their shrinking water budget will depend on how severe the drought and drying of the West gets, of course. But however the climate scenario plays out, there is a good deal of pain and radical adaptation in store, from conservation, to large-scale water re-use, to the retirement of farms and ranches, and perhaps an end to some ways of life. Worst case, if the reservoirs ever hit “dead pool” — when levels drop too low for water to be piped out — many people in the region could become climate refugees.

“I hate to use the word dire, because it doesn’t do justice to the good-thinking people and problem solvers that exist in the basin, but I would say it is very serious,” said Brad Udall, a senior scientist at the Colorado Water Institute. “Climate change is unquantifiable and puts life- and economy-threatening risks on the table that need to be dealt with. It’s a really thorny problem.”

 

1-22-19

Crisis on the Colorado: Part II

On the Water-Starved Colorado River, Drought Is the New Normal

With the Southwest locked in a 19-year drought and climate change making the region increasingly drier, water managers and users along the Colorado River are facing a troubling question: Are we in a new, more arid era when there will never be enough water? Second in a series.

Jim Robbins

https://e360.yale.edu/features/on-the-water-starved-colorado-river-droug...

In the basement of the University of Arizona’s Laboratory of Tree-Ring Research, the fragrant smell of pine hangs in the air as researchers comb through the stacks of tree slabs to find a round, 2-inch-thick piece of Douglas fir.

They point out an anomaly in the slab — an unusually wide set of rings that represent the years 1905 to 1922. Those rings mean it was a pluvial period — precipitation was well above average — and so the trees grew far more than other years.

“In 1905, the gates opened and it was very wet and stayed very wet until the 1920s,” said David Meko, a hydrologist at the lab who studies past climate and stream flow based on tree rings. “It guided their planning and how much water they thought was available.”

The planning was that of the states that share the water of the Colorado River. Worried that a burgeoning California would take most of the water before it was fairly divvied up, representatives from the other Colorado River Basin states, presided over by U.S. Secretary of Commerce Herbert Hoover, came together in 1922 to develop an equitable apportionment. They looked at flow measurements and figured that the river contained an average of 15 million acre-feet. They divided the Colorado River states into two divisions – the upper basin and the lower basin, with the dividing line in northern Arizona near the Utah border. The upper basin states — Utah, Wyoming, Colorado, and New Mexico — agreed not to take more than a total of 7.5 million acre-feet and to allow the other half to flow south to the lower basin. The agreement they signed was called the 1922 Colorado River Compact, also known as the Law of the River.

The 1922 compact, though, is based on a premise that the tree rings in the University of Arizona lab now show is false. The river’s long-term average flow is about 12 to 15 million acre-feet, in a good year. Meanwhile, the lower basin states — Arizona, California, and Nevada — use 7.5 million acre-feet, and in 1922 no one factored in evaporative losses from the desert sun at the yet-unbuilt Lake Mead reservoir, which amount to another 1.2 million acre-feet, or the water taken up by plants. Nor did anyone factor in a subsequent 1944 treaty that requires the United States to provide 1.5 million acre-feet to Mexico. A conservative estimate on how much Colorado River water is actually used is 20 million acre-feet.

This over-appropriation is problem enough, but in recent years the river’s flow has been dwindling. The region is locked in a 19-year-long drought, the most severe in 1,250 years. And it may continue much longer. The tree ring data shows that there have been numerous multi-decadal or mega-droughts in the basin in the last 1,000 years. The prospect that drought could be the new normal for the region is creating a good deal of anxiety along the Colorado.

“Many water managers like me are struggling at not panicking,” said Mark Harris, general manager of the Grand Valley Water User’s Association in Grand Junction, Colorado. In his farm cap and jeans, Harris is a no-nonsense type, not given to hyperbole. This year, though, some “junior” water users on the Yampa River, a tributary to the Colorado, were told they would not get their water because others had priority, the first time that has ever happened, and late-season water flows near Grand Junction were near crisis levels. “The crunch is here,” Harris said. “It’s here, and it will stay here. We will never be out of the woods, we are in the woods forever.”

Another low-snow winter would trigger the first emergency declaration in the basin, forcing states to deal with water cutbacks.

Never has the question of “what will the winter be like?” loomed larger than it does this year in the Colorado River Basin. If it is anything like last year (when about two-thirds of the usual snow fell) and many other low snow years since 2000, it will trigger the first emergency declaration in the basin, which could force states to deal with cutbacks in the water they are appropriated. And even if it is a big snow year, it will likely only delay what now seems inevitable.

The last time Lake Mead was full was 1983. Since then it has slowly declined. It is now 40 percent full: 1,082 feet above sea level. It may never be full again, experts say. If it drops 7 feet, to 1,075 feet, it will trigger the first Tier 1 water cutbacks. A flyover reveals a giant white ring all the way around the lake’s 112-mile-long perimeter, dramatically showing how far water levels have dropped.

There are three levels of cutbacks. When Lake Mead falls to 1,050 feet, a Tier 2 crisis occurs, and Tier 3 at 1,025. At each level, states in the lower basin have to give up more of their water. Lake Mead would have already hit 1,075 feet and a First Tier declaration if it weren’t for the fact that farmers, ranchers, and many others have been working to avoid an emergency by keeping more water in the river through conservation efforts. For example, in 2017, state, federal, municipal, and private entities funded the purchase of 40,000 acre-feet from the Gila River Indian Community to be left in Lake Mead in perpetuity as part of a system conservation agreement.

Last August, the U.S. Bureau of Reclamation issued a report on the water future of the region. The agency’s predictions were sobering. By May of this year, the bureau forecast the level will dip just below 1,075 feet, and at the beginning of 2020, the level is expected to drop to 1,070. By the summer of 2020, the prediction is 1,050 feet, almost Tier 2. If these predictions come true, users will have to begin giving up their water allotments, starting with the most junior.

If water levels continue to drop, sinking below 1,050 feet, Hoover Dam — which impounds Lake Mead and provides power to millions of people in Southern California, Nevada, and Arizona — will stop generating electricity, as water levels will be too low to flow through it. And should Lake Mead keep dropping all the way to 895 feet, it will fall below the level at which water can be piped out — the dreaded “dead pool.” Moreover, because Lake Mead is funnel-shaped, the lower it gets the faster it drops. At some point there is the likelihood that the lower basin will force the upper basin to send water to meet its obligations — a compact call — something that’s never happened before.

A few wet years in a long dry spell would be critical these days to keep the Colorado from completely drying up.

All of this is uncharted crisis terrain. “If the drought is multi-decadal the system will fail,” said Jack Schmidt, a professor of watershed science at Utah State University. “But nobody knows what failure means.”

Arizona officials have a sense of it and are coming to grips with the reality. They are the most junior users in the Lower Basin and a Tier 1 shortage would mean Arizona would have to start cutting allocations to users. “If the current climate trend continues,” said Kathryn Sorensen, director of the Phoenix Water Services Department, “you could have ‘dead pool’ in four years. That’s worst case.” Should that happen, the whole region, she says, would be thrown into crisis.

If these were normal times, past droughts might give us a sense of what might be in store. The climate information stored in tree rings show that the longest drought in this region occurred in medieval times and lasted for 62 years — with no very wet years in between the dry ones. A few very wet years in a long dry spell would be critical on the Colorado these days to keep it from completely drying up.

But it may be even worse than that. This drought is unusually hot. “Temperatures keep going up,” said Meko, of the University of Arizona tree ring lab. “We keep breaking records year after year. It’s additional stress on the water system.” Meanwhile, the two driest years all the way back to the 1200s occurred in 1996 and 2002. “It’s a little worrisome to see the most extreme years right near the present,” he said.

 “Droughts impacted by warmer temperatures will be more severe,” says Connie Woodhouse, who also studies climate at the tree ring lab. “A lack of precipitation is one thing. But when a drought happens and temperatures are warmer, the precipitation deficits are exacerbated. You have more evaporation, more ground heating, and it impacts the snowpack.”

From 2000 to 2014, flows in the river were 19 percent below the averages in the previous years, and a third of that loss was caused by high temperatures, according to researchers Jonathan Overpeck of the University of Michigan and and Brad Udall at the Colorado Water Insitute at Colorado State University, in an often-cited paper about the unprecedented nature of this drought and what it means for the future.

The biggest impact of high temperatures is something called runoff efficiency — the amount of stream flow that results from precipitation. Right now about 15 percent of the water in the snow in the watershed makes it into the river. The other 85 percent soaks into the ground, evaporates, or is taken up by plants. As it gets warmer, runoff efficiency is decreasing. Shorter winters mean the ground has less snow cover and is darker, so it warms up more and sooner, which means snow melts faster and more water evaporates and is taken up by plants in a longer growing season. The Colorado River Research Group, 10 veteran academics who study the Colorado, call this most alarming change to the physical environment.

The alarm is palpable among water managers throughout the Southwest. They see the writing on the wall.

Warmer temperatures also mean that of the precipation that does come, more of it will fall as rain instead of snow. The Colorado’s engineering infrastructure was built around the natural long-term storage that snowpack provides, but rain pulses quickly through the system.

Meanwhile, the rapid development of everything from housing developments to solar installations in the Southwest has created more dust particles which go airborne and settle on to the snow fields of the Rockies, five to seven times as much dust as was seen a century ago. The darker snow melts sooner and faster, a phenomenon that costs the river about 5 percent of its flow. And as the drought continues, there’s more dust from more dry ground and that creates more dust.

As the flow of the Colorado diminishes, more water users will be forced to turn to groundwater pumping.The news on that front, though, is also problematic. In a 2014 paper, researchers at the Global Institute for Water Security, which uses a satellite to measure large-scale changes in groundwater by measuring changes in gravitational pull, found that from 2004 to 2013, the loss of groundwater from pumping was 6.5 times greater than the total loss of water from Lake Powell and Lake Mead. “Everybody knows that groundwater will become progressively more important,” said Jay Famiglietti, the institute’s director. “The problem is groundwater is rapidly disappearing so we shouldn’t depend on it being there.”

However the biggest cloud looming over the Colorado River Basin is whether the region is entering a completely new era, a permanent change as opposed to a temporary one, caused by a planet being rapidly warmed by human activity. “Is this a drought or is this aridification of the Southwest and Colorado River Basin?” asked the University of Michigan’s Overpeck, who has long studied the Colorado.

Like Overpeck, many experts believe the drying up of the Colorado is being driven by a changing climate. “It’s going to get drier and drier,” he said. “It could mean a hell of a lot less water in the river. We’ve seen declines of 20 percent, but it could get up to 50 percent or worse later in this century.”

If climate change is locked in, he said, what is going on now is not a new normal, but a stop along the way to a yet-drier new normal somewhere in the distant future. “In that case, every year will be a new reality,” he said. “The aridification of the Southwest will continue as long as we put greenhouse gases into the atmosphere. We need to stop burning fossil fuels and that will help stop the decline in the river flow.

“And even if we did that, there’s warming baked in,” he said. “It would continue for another decade and then stabilize. Then we will get the new normal. And it will be at that level of warmth for centuries.”

That’s why the alarm is palpable among water managers in the Southwest. They see the writing on the wall, and there are few skeptics about climate change among them. The plight of Cape Town, South Africa, which came to the brink of a water system crash last year, is on many people’s minds along the Colorado River.

This era of drying is especially serious because so much — some 40 million people and an economy that includes the world’s fifth largest, in California — is riding on the flow of the Colorado. The specter of a region facing an existential crisis because of a warming climate becomes more real every day. “If you can see it, you should plan for it,” Phoenix’s Sorensen said. “And I can see it.”

Jim Robbins is a veteran journalist based in Helena, Montana. He has written for the New York Times, Conde Nast Traveler, and numerous other publications. His latest book is The Wonder of Birds: What they Tell Us about the World, Ourselves and a Better Future.