As a hotter, drier #climate grips the #ColoradoRiver, #water risks grow across the Southwest — #AZ Central #COriver #aridification

Hoover Dam, straddling the border between Nevada and Arizona, holds back the waters of the Colorado River in Lake Mead. In 2016, Lake Mead declined to its lowest level since the reservoir was filled in the 1930s. Source: Bureau of Reclamation

From Arizona Central (Ian James):

The water level of Lake Mead, the country’s largest reservoir, has dropped more than 130 feet since the beginning of 2000, when the lake’s surface lapped at the spillway gates on Hoover Dam.

Twenty-one years later, with the Colorado River consistently yielding less water as the climate has grown warmer and drier, the reservoir near Las Vegas sits at just 39% of capacity. And it’s approaching the threshold of a shortage for the first time since it was filled in the 1930s.

The latest projections from the federal government show the reservoir will soon fall 7 more feet to cross the trigger point for a shortage in 2022, forcing the largest mandatory water cutbacks yet in Arizona, Nevada and Mexico.

The river’s reservoirs are shrinking as the Southwest endures an especially severe bout of dryness within a two-decade drought intensified by climate change, one of the driest periods in centuries that shows no sign of letting up.

With a meager snowpack in the Rocky Mountains and the watershed extremely parched, this month’s estimates from the federal Bureau of Reclamation show Lake Mead could continue to decline through next year and into 2023, putting the Southwest on the brink of more severe shortages and larger water cuts.

“What really is starting to emerge is this really long pattern, that we’re in a megadrought in a lot of the western U.S.,” said Laura Condon, an assistant professor of hydrology and atmospheric sciences at the University of Arizona. “It’s kind of like a cumulative impact, that we’ve just been getting hotter and drier and hotter and drier.”

One of Lake Mead’s spillways the last time water lapped at the top of the spillway was 1999.

Many scientists describe the past two decades in the Colorado River Basin as a megadrought that’s being worsened by higher temperatures with climate change. While the Southwest has always cycled through wet and dry periods, some scientists suggest the word “drought” is no longer entirely adequate and that the Colorado River watershed is undergoing “aridification” driven by human-caused warming — a long-term trend of more intense dry spells that’s here for good and will complicate water management for generations to come.

Both Lake Mead and the upstream reservoir Lake Powell are dropping. Taken together, the country’s two largest reservoirs now hold the smallest quantity of water since 1965, when Powell was still filling behind the newly built Glen Canyon Dam.

The Colorado River has long been overallocated to supply farmlands and growing cities from Denver to Phoenix to Los Angeles. And the growing strains on the river suggest that Lake Mead, its sides coated with a whitish “bathtub ring” of minerals along its retreating shorelines, will continue to present challenges as the Southwest adapts to a shrinking source of water.

“There will still be ups and downs and we will have wetter and drier years going forward but overall warmer temperatures mean we should expect a drier basin with less water,” Condon said. “Warmer temperatures increase the amount of water plants use and decrease snowpack. Even if we get exactly the same quantity of precipitation, a warmer basin will produce less streamflow from that precipitation.”

Lake Mead was about 40% full in December 2019, but will almost certainly fall further this year, as will its companion reservoir of the desert southwest, Lake Powell. Photo/Allen Best

[…]

Representatives of the seven states that depend on the river met at Hoover Dam in 2019 and signed a set of agreements, called the Drought Contingency Plan, laying out steps to reduce the risks of a damaging crash. Arizona and Nevada agreed to take the first cuts to help prop up Lake Mead, while California agreed to participate at lower shortage levels if the reservoir continues to drop.

The states’ water officials described the deal as a “bridge” agreement to temporarily lessen the risks and buy some time through 2026, by which time new rules for sharing shortages must be negotiated and adopted.

Under the deal, Arizona and Nevada have left some water in Lake Mead in 2020 and 2021. Those reductions are set to increase next year under the “Tier 1” shortage, which the federal government is expected to declare in August.

Looking downstream from the base of Hoover Dam. Concrete structure in the center of the photo is the outlet for the Nevada side emergency spillway.

Arizona is in line for the largest cuts, which will reduce the Central Arizona Project’s water supply by nearly a third and shrink the amount flowing through the CAP Canal to farmlands in Pinal County. Nevada is also taking less water, and Mexico is contributing under a separate deal by leaving some of its supplies in Lake Mead.

“We have a plan to deal with these shortages,” said Tom Buschatzke, director of the Arizona Department of Water Resources. “We’ve known this was possible for a long time and have planned for it.”

He and other officials say the Drought Contingency Plan never guaranteed the region would escape a shortage, but that it has reduced the odds of Mead falling to critical lows and has pushed back the possibility of more severe shortages and larger cuts. Buschatzke said voluntary conservation measures by the states and Mexico since 2014, plus the initial mandatory cuts over the past two years, have left about 40 feet of conserved water in Lake Mead.

“We would already be in a Tier 2 shortage had that water not stayed in the lake,” Buschatzke said during a panel discussion hosted by the Arizona Capitol Times. “It’s what we can do to slow the reduction in Lake Mead and minimize the depth and length of the shortages.”

[…]

A warmer watershed, a shrinking river

Scientists have found that the Colorado River is sensitive to rising temperatures as the planet heats up with the burning of fossil fuels. In one study, scientists determined that about half the trend of decreasing runoff in the river’s Upper Basin since 2000 was the result of unprecedented warming.

In other research, scientists estimated the river could lose roughly one-fourth of its flow by 2050 as temperatures continue to rise. They projected that for each additional 1 degree C (1.8 degrees F) of warming, the river’s average flow is likely to drop by about 9%.

The past year has been especially harsh. Ultradry conditions intensified across much of the West, with extreme heat adding to the dryness throughout the Colorado River watershed. According to the National Weather Service, the past 12 months were the driest on record in Utah, Nevada, Arizona and New Mexico, and the fourth-driest in Colorado, where much of the river’s flow originates.

Lake Powell now stands just 36% full.

Lake Powell is seen in a November 2019 aerial photo from the nonprofit EcoFlight. Keeping enough water in the reservoir to support downstream users in Arizona, Nevada and California is complicated by climate change, as well as projections that the upper basin states of Colorado, Utah, Wyoming and New Mexico will use as much as 40% more water than current demand. A recent white paper from a lineup of river experts calls those use projections into question.
CREDIT: ECOFLIGHT via Aspen Journalism

The reservoir typically gets a boost in the spring and summer as the river swells with runoff from melting snow. But this winter, the snowpack peaked at 88% of the long-term median and has since dropped to 71% of the median. The dry soils in the watershed are soaking up some of the melting snow like a sponge, leaving less water running into the Colorado and its tributaries.

The amount of water that will flow into Powell from April through July is now estimated at just 38% of average.

Water researchers Eric Kuhn and John Fleck said their analysis of the latest federal numbers points to some alarming possibilities. The two — who coauthored the book “Science Be Dammed: How Ignoring Inconvenient Science Drained the Colorado River” — wrote in separate blog posts that a careful reading of the data in the 24-month study, which only goes out to March 2023, shows the projections point to bigger troubles at Mead and Powell later that year.

Fleck wrote that the “most likely” scenario would put the level of Mead at an elevation around 1,035 feet at the end of September 2023, which would trigger larger cuts for Arizona, Nevada and Mexico, as well as California’s participation in reductions.

“I’m talking about the midpoint in a range of possible outcomes,” Fleck wrote. “A run of wet weather could make things substantially better. But a run of dry weather could make them worse.”

Kuhn wrote that the assumptions in the government study “do not fully capture the climate-change driven aridification of the Colorado River Basin.” He said the projections suggest Lake Powell could drop in 2023 to “a level that is troublingly close to the elevation at which Glen Canyon Dam could no longer generate hydropower.”

[…]

Across the West, snow has traditionally stored a vital portion of the water, gradually melting and releasing runoff in the spring and summer. But that’s changing with higher temperatures. Researchers from the University of California, Irvine, found in a study last year that the western U.S. has experienced longer and more intense “snow droughts” in the second half of the period from 1980 to 2018.

“The main issue is the snow drought everywhere in the entire West, including Arizona, Utah, California, Colorado,” said Amir AghaKouchak, a professor in UC Irvine’s Department of Earth System Science. “When the snow is below average, it means low-flow situations in summer, drier soil moisture. And drier soil moisture increases the chance of heat waves.”

The upshot, he said, is that “we have to prepare for a different hydrologic cycle, basically.”

Warm and dry in the headwaters

With higher temperatures, more snow has been melting earlier in the year. Scientists recently examined 40 years of data from snow monitoring sites across the western U.S. and Canada and found increasing winter snowmelt at a third of the sites…

With higher temperatures, more snow has been melting earlier in the year. Scientists recently examined 40 years of data from snow monitoring sites across the western U.S. and Canada and found increasing winter snowmelt at a third of the sites.

Other researchers have discovered that the dry periods between rainstorms have grown longer on average across the western United States during the past 45 years. Scientists with the U.S. Department of Agriculture and the University of Arizona found this trend throughout the West in their study, but they saw the most extreme changes in the desert Southwest, where rainstorms have been happening much less frequently.

The average dry period between storms in the desert Southwest has gone from 31 days to 48 days, an increase of about 50 percent since the 1970s, the scientists found. Annual precipitation declined by about 3.2 inches in the region over that period, a much larger decline that the West as a whole.

“In the desert Southwest, we were averaging around 10 inches and now we’re averaging around 7 inches,” said Joel Biederman, a hydrologist at USDA’s Southwest Watershed Research Center in Tucson. “That’s much more impactful when you consider that the amount in our region is smaller to begin with.”

Biederman and his colleagues focused on changes that have been measured and didn’t attempt to parse the influences of natural variations and climate change.

A separate analysis of climate data over the past 30 years by the National Oceanic and Atmospheric Administration shows the nation’s “normals,” or averages, have shifted dramatically in a decade, growing wetter in the central and eastern U.S. and drier in the Southwest while climate change has pushed temperatures higher.

Another group of scientists at Los Alamos National Laboratory recently looked at how interconnected extremes influenced by climate change — from floods to droughts and heatwaves — are expected to intensify in the future in the Colorado River Basin. They found these sorts of concurrent extreme climatic events “are projected to increase in the future and intensify” in key regions of the watershed.

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