The snowpack in the Sierra Nevada is the deepest in decades, but these storms that have been a boon to Northern California won’t do much for the river basin’s long-term water shortage. Colorado – an essential source of supply for Southern California.
In fact, the recent storms haven’t changed a view shared by many Southern California water managers: don’t expect Lakes Mead and Powell, the nation’s largest reservoirs, to are filling up again so soon.
“Thinking that these things would ever fill up takes some kind of leap of faith that I, for one, don’t have,” said Brad Udall, a water and climate scientist at Colorado State University.
Lake Mead, located on the Arizona-Nevada border and held back by the Hoover Dam, filled in the 1980s and 1990s. By 2000, it was nearly full and lapping at the spillway gates. But the mega-drought of the past 23 years – the worst in centuries – has worsened the water deficit and left Lake Mead around 70% empty.
Upstream, Lake Powell has shrunk to just 23% of its full capacity and is approaching a point where the Glen Canyon Dam would no longer generate electricity.
Even with this winter’s above-average snowpack in the Rocky Mountains, water officials and scientists say everyone in the Colorado River Basin will need to plan for low reservoir levels for years to come. And some say they think the main reservoirs in the river are unlikely to fill in our lifetimes.
“They’re not going to fill up. The only reason they filled up the first time was because there was no demand for water. In the 1950s, 1960s, 1970s and 1980s there was no Central Arizona project, there was no Southern Nevada Water Authority, there was not as much use in the Upper [Colorado River] Basin,” said Bill Hasencamp, Colorado River resource manager for the Metropolitan Water District of Southern California. “The water consumption was therefore low. So that filled up the storage.
Demand for Colorado River water increased in the late 1990s and early 2000s. The Central Arizona Project, a 336-mile-long water distribution system, delivers water from the Colorado River to the most populous counties in Arizona and was not completed until the 1990s. The Southern Nevada Water Authority was established in 1991.
Arizona began taking its full allocation of water from the river in the late 1990s and Nevada in the early 2000s. California continues to use the largest share of the river.
“Now the water consumption is at its maximum. Every state is taking too much, and we need to cut back. And so there just aren’t enough of them. You would need wet year after wet year, after wet year after wet year, after wet year. Even then, because the demand is so high, it still wouldn’t be filled,” Hasencamp said in an interview.
Climate change has significantly altered the river. Over the past 23 years, as rising temperatures have intensified the drought, the river’s flow has decreased by around 20%.
Scientists have found that about half of the decline in river flow has been caused by higher temperatures, and that climate change is driving the aridification of the southwest. With global warming, average temperatures in the upper watershed – where most of the river’s flow comes from – have risen by about 3 degrees since 1970.
Research has shown that for every additional degree Celsius (1.8 degrees Fahrenheit) the average river flow is likely to decrease by around 9%.
In multiple studies, scientists have estimated that by the middle of this century, the average river flow could drop 30% or 40% below the average of the last century.
“The last 23 years are the best lessons we have right now, and they should scare people away,” said Udall, who co-authored research showing how warming is undermining river flows.
Based on low levels in Lake Mead and Lake Powell, Udall said, he would estimate it would take about six consecutive extremely wet years to fill the reservoirs, with water flows similar to 2011.
“We would need six years like this to fill this system, in a row, based on the current operating rules,” Udall said. “And I just don’t see that even remotely possible.”
The Colorado River Basin could very well have a few wet years, he said.
“We might even have a wet decade. But, boy, the long-term trend of warming and drying seems very clear to me,” Udall said. “And a bet on anything other than water management malpractice, that we have to plan something that looks like a worst-case scenario.”
The Colorado River supplies water to seven states, tribal nations and Mexico. States are under pressure from the federal government to accept reductions to prevent reservoirs from falling to dangerously low levels.
California and the other six states are at odds over how to make the cuts and have submitted separate proposals to the federal government, with some disagreements over the legal system that governs the river’s management.
Scientists have been warning of a coming crisis for many years.
In a 2008 study, scientists Tim Barnett and David Pierce looked at likely declines in flow with climate change and estimated that there was a 50% chance that the usable water supply in Lake Mead and Lake Powell is gone by 2021. They titled their study “When Will Lake Mead Dry Up?” In research published in 2009, they wrote that based on projections of climate change or even long-term average flows, “currently planned future water deliveries from the Colorado River are unsustainable”.
“Climate change is reducing flow in the Colorado River system, so agreements are dividing more water than exists,” said Pierce, a climatologist at the Scripps Institution of Oceanography. “This drop in reservoir levels is happening because we’re sticking to agreements that don’t take into account changes in the influx of water into the system due to climate change.”
There’s always the possibility of a few extremely wet years with the potential to fill reservoirs, Pierce said.
“It’s just that in the decades to come, that probability decreases. Our work has estimated that the probability of reservoir filling drops from about 75% today to about 10% by 2060 if no change in [water] delivery schedules are set,” Pierce said. “We should plan for the situation where warmer temperatures will decrease river flow in the future.”
The capacity of Lake Mead and Lake Powell is gargantuan compared to the capacity of California’s two largest reservoirs, Shasta Lake and Lake Oroville. Lake Mead can store over 27 million acre-feet of water and Lake Powell 25 million acre-feet. In contrast, Lake Shasta can hold about 4.6 million acre-feet and Lake Oroville 3.5 million acre-feet.
The Colorado River provides, on average, about 25% of coastal Southern California’s water supply, while the region also receives water from Northern California through the State Water Project and other sources. .
The Sierra Nevada snowpack in California is now about 200% of average at this point in the season, while the snowpack in the upper Colorado River basin so far is about 140% of average. median over the past 30 years.
The higher snowpack could help Colorado Reservoir levels somewhat this year. How much will not be clear for a few months.
“Absolutely this snow is welcome. The cold is welcome. The real question will be in the spring,” Hasencamp said.
In recent years, hot and dry conditions have resulted in reduced river flows. “That’s what has killed the last few years is that a hot, dry spring has washed away the snow that was there and it’s not reaching the reservoirs,” Hasencamp said.
Daniel Swain, a UCLA climatologist, said an exceptionally wet decade could one day turn things around.
“But the problem is that it not only has to be wetter than average, it should be considerably wetter than long-term average,” Swain said. And for many years.
Scientists say higher temperatures make the atmosphere “thirstier,” causing more moisture from the landscape to evaporate. Vegetation also absorbs more water as temperatures rise, allowing less runoff to flow into streams.
“There is no doubt that there will be a continued downward trend in inflows, but extremely high events are also more likely to occur in the context of climate change, according to the United States National Climate Assessment. United,” said University Principal Kathy Jacobs. from the Arizona Center for Climate Adaptation Science and Solutions.
Jacobs noted that researchers predict atmospheric rivers will become more intense with rising temperatures, and scientists expect more intense extreme storms and periodic flooding.
“I strongly suspect that dams on the Colorado will be needed in the future for flood control as well as water supply,” Jacobs said.
As for the future, Jacobs said a lot hinged on reducing greenhouse gas emissions “to net zero in the near term.”
There are practical ways to deal with the reduced water supply from the river, she said. “The longer we wait to build more flexible future management schemes, the more difficult it will be.”
