Report: On the causes of declining #ColoradoRiver streamflows — Mu Xiao, @BradUdall, Dennis P. Lettenmaier #COriver #aridification

Click here to read the paper. Here’s the abstract:

The Colorado River is the primary surface water resource in the rapidly growing U.S. Southwest. Over the period 1916‐2014, the Upper Colorado River Basin naturalized streamflow declined by 16.5%, despite the fact that annual precipitation in the UCRB over that period increased slightly (+1.4%). In order to examine the causes of the runoff declines, we performed a set of experiments with the Variable Infiltration Capacity (VIC) hydrology model. Our results show that the pervasive warming has reduced snowpacks and enhanced evapotranspiration (ET) over the last 100 years; over half (53%) of the long‐term decreasing runoff trend is associated with the general warming. Negative winter precipitation trends have occurred in the handful of highly productive sub‐basins that account for over half of the streamflow at Lee’s Ferry. We also compared a mid‐century drought with the (ongoing) post‐Millennium Drought, and find that whereas the earlier drought was caused primarily by pervasive low precipitation anomalies across UCRB, higher temperatures have played a large role in the post‐Millennium Drought. The post‐Millennium Drought has also been exacerbated by negative precipitation anomalies in several of the most productive headwater basins. Finally, we evaluate the UCRB April‐July runoff forecast for 2017, which decreased dramatically as the runoff season progressed. We find that while late winter and spring 2017 was anomalously warm, the proximate cause of most of the forecast reduction was anomalous late winter and early spring dryness in UCRB, which followed exceptionally large (positive) early winter precipitation anomalies.

From Arizona Central (Ian James):

Since 2000, the amount of water flowing in the Colorado River has dropped 19 percent below the average of the past century, a decline that has left the Southwest on the brink of a water shortage.

Now, new research indicates that a large portion of that decline isn’t due to less rain and snow falling from the sky, but to warmer temperatures brought on by climate change.

Scientists from the University of California-Los Angeles and Colorado State University found that about half the trend of decreasing runoff from 2000-2014 in the Upper Colorado River Basin was the result of unprecedented warming across the region.

“A good chunk of the decline we’re seeing right now is temperature-related. And as the Earth continues to warm, we’re going to see less flow in the river,” said Brad Udall, a water and climate scientist at Colorado State University who co-authored the research. “We need to prepare for a river that has significantly less water in it.”

Udall, together with UCLA researchers Mu Xiao and Dennis Lettenmaier, used a hydrologic model to examine the streamflow in the Upper Colorado River Basin from 1916 through 2014. They found the flow declined by 16.5 percent over the past century.

They calculated that 53 percent of the trend was linked to warming, which has shrunk the average snowpack in the mountains, boosted the uptake of water by plants and increased the amount of water that evaporates off the landscape.

“We separated the effect of different factors and determined the influences,” said Xiao, the study’s lead author and a doctoral student in UCLA’s Geography Department. “Aside from warming temperature, the precipitation also contributed to the drought. This was not obvious to us as the precipitation in the Upper Basin doesn’t show a significantly decreasing trend.”

The researchers attributed the remaining 47 percent of the decrease in the river’s flow to shifts in precipitation patterns, with less rain and snow falling in four areas of Colorado that tend to be especially productive in feeding tributaries in the Rocky Mountains.

“The precipitation is falling in different places, places that aren’t nearly as effective in generating runoff,” Udall said. “It’s the heat and it’s also this changing pattern in precipitation from areas that are really productive in generating runoff, like the state of Colorado, to areas that are much less effective, like the deserts of Utah.”

Variable Infiltration Capacity (VIC)
Macroscale Hydrologic Model

For you geeks: Here’s the link to the Variable Infiltration Capacity (VIC) Macroscale Hydrologic Model download and docs.

From KUNC (Luke Runyon):

Colorado State University researcher Brad Udall co-authored the study with UCLA scientists Mu Xiao and Dennis Lettenmaier…

Warming temperatures throughout the Colorado River watershed accounted for more than half the decline in flows, Udall says. Other factors include changes to precipitation patterns and loss of snowpack in high altitudes.

“The impacts of temperature are very large on this river and if you believe temperatures are going to increase — as every reputable scientist now does — you then have to conclude that the future of the river is going to be a future with much less water in it,” Udall says.

The amount of precipitation increased by about one percentage point during the last 100 years, but it didn’t end up in areas that would boost the river’s flow in significant ways.

Udall says most water managers agree that climate change is fundamentally altering how the river functions. Officials in Arizona, California and Nevada are currently negotiating a new plan to voluntarily cut back how much water they take from the river to avoid mandatory reductions. Federal officials have given the states until the end of the year to craft a plan.

The latest study builds on a 2017 study Udall published with University of Michigan researcher Jonathan Overpeck that compared flows in the Colorado River during a period of hot and dry conditions in the basin starting in 2000 to long-term average flows. From 2000 to 2014 flows in the river averaged 19 percent below those recorded the previous 93 years.

He notes too that the current drought conditions in the basin differ from droughts in the recent past. An extended drought from 1953 to 1968 was mostly driven by lack of precipitation, whereas the 21st century drought is being driven by increasing temperatures, a product of climate change.

“It’s disturbing,” Udall says. “I think we’re going to see a very different world as the 21st century unfolds with all these climate change impacts.”

From The Arizona Daily Star (Tony Davis):

Lake Powell cannot rescue Lake Mead forever, said Karl Flessa, a University of Arizona geosciences professor who worked on the report [From the Colorado River Research Group, It’s hard to fill a bathtub when the drain is wide open: the case of LakePowell].

“Those of us here in the Lower Basin are so focused on Lake Mead, but what’s propping up Mead is Lake Powell, and Lake Powell is going down, too,” Flessa said.

A total of about 11 million acre-feet of the extra water has been sent from Powell to Mead since 2000, the report says. That’s more than seven years’ worth of CAP water. Powell has dropped 94 feet since 2000. Had all that water stayed in Powell, that lake wouldn’t have dropped at all since 2000, the report says.

“The math suggests that, without these extra releases, we could today have a full Lake Powell and an empty Lake Mead. We are certainly not saying that would be a ‘better’ outcome; that would be chaos,” said another researcher involved in the report, Douglas Kenney, director of the University of Colorado’s Western Water Policy Program.

“What we are saying is that it’s important to understand that the actions taken to keep Lake Mead out of shortage have had real impacts upstream at Lake Powell, and all users dependent upon Lake Powell now face risks associated with looming Lake Powell shortages.”

The scientists say “the status quo (of reservoir management) is untenable,” and that a crisis on Lake Powell may already be at hand.

“It is impossible to keep a bathtub full while the drain is left open,” the report says.

From KJZZ (Holliday Moore):

It’s hard to fill a bathtub when the drain is open; it’s even harder when the tub is shaped like a bowl said Doug Kenney, who led the Colorado River Research Group study.

He said visitors to Lake Mead and Powell who measure the water levels by the bath-tub-like rings on the canyon walls see only part of the problem.

“As the reservoir gets lower and lower, it gets narrower at the bottom,” he explained. “So, what looked like a 10-foot decline one year, might look like a 30-foot decline the year after.”

Lake Mead is currently only 38-percent full, but draws its water from Lake Powell, which was last measured as 48-percent full.

If Lake Mead relied solely on its own winter runoff, Kenney said, “It would be a dry river bed now.”

From Mashable (Mark Kaufman):

[Hoover] dam is a proud place, built by thousands of hands and with 5 million barrels of concrete. Its golden elevator doors, Gotham-esque pillars, and stoic guardian angel statues line the lofty walkways atop the structure. A U.S. flag beating patriotically over the desert gets swapped out every few days, and then put out for sale in the visitor center.

Yet, in the 80 years since the great dam’s completion, the 1,450-mile Colorado River – which sustains some 40 million Americans in places like Phoenix, Las Vegas, and Los Angeles — has been gradually growing weaker, and the water level beyond the noble dam has fallen considerably over the last two decades. The writing is easily spotted on the steep rocky walls of the Lake Mead reservoir, where a bathtub-like ring shows where the water once sat during more fruitful times.

Today, however, the water sits 150-feet below that line, and human-caused climate change is a major reason why.

Over the last century, the river’s flow has declined by around 16 percent, even as annual precipitation slightly increased in the Upper Colorado River Basin — a vast region stretching from Wyoming to New Mexico.

New research published in the journal Water Resources Research argues that over half of this decline is due to sustained and rising temperatures in the region, which ultimately means more water is evaporated from the river, diminishing the flow.

But it’s really been in the last twenty years that matters have deteriorated into a major drought, edging the region toward a potential water-rationing crisis.

It’s the worst drought in Colorado River history.

“The river since 2000 has been in an unprecedented decline,” Brad Udall, coauthor of the new study and senior water and climate research scientist at Colorado State University, said in an interview.

“There’s no analog, from when humans started gauging the river, for this drought,” said Udall.

From The New Mexico Political Report (Laura Paskus):

a new study shows that even though annual precipitation increased slightly between 1916 and 2014, Colorado River flows declined by 16.5 percent during that same time period. That’s thanks, in large part, to “unprecedented basin-wide warming.” Warming reduces snowpack and increases the amount of water plants demand.

Using experiments and a hydrology model, the trio of authors from the University of California-Los Angeles and Colorado State University, found that 53 percent of the decrease in runoff is attributable to warming; the rest to reduced snowfall within regions that feed into the system.

One of the study’s authors, Bradley Udall, co-authored an earlier paper with Jonathan Overpeck showing a drop in river flows. What’s striking about the new study, Udall explained, is how much of the decline is due to warming relative to precipitation.

“Climate change isn’t in the future: it’s here now, it’s affecting all of us, and it will become increasingly worse as time goes on,” said Udall, senior water and climate research scientist at the Colorado Water Institute at Colorado State University. “Climate change is in our face right now: It’s western fires, it’s drought, it’s river flows.”

He cautioned that the study’s results are based on one model and one data set.

“That said, I think the model is telling us something that’s really valuable,” he said. “The feedback loop—self-reinforcing cycles, where dryness begets heat, which further begets dryness—is probably at the root cause of what’s causing these 50 percent declines.”

In short, he said, the model points toward the further aridification of the region.

The model also shows how sensitive the Colorado River Basin is to shifts in precipitation patterns. Not only does it matter whether precipitation falls as snow or rain, it matters where it falls. Snowfall in Colorado, he explained, contributes more to the river’s flows than if it falls in Utah. Unfortunately, the researchers noted a decline in snowfall within four important sub-basins of the river, all within the state of Colorado.

The paper, he said, is also a reminder that we need respond to climate change right now.

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