A line along a cliff in this 2018 photo illustrates where the surface of Lake Mead once stood near South Cove.

 David Wallace / The Arizona Republic 2018

For the second time in two years, federal officials are warning that Lake Mead could drop in five to six years to levels low enough to possibly warrant major Central Arizona Project water cutbacks to Tucson and Phoenix.

These warnings were ratcheted up significantly compared to forecasts made earlier this year. That’s due to the severely hot and dry weather that struck most of the West during the summer, including the hottest two months on record in Tucson and Phoenix in August and July, respectively.

The negative forecasts come despite 13 years of major government-run water conservation efforts in the Colorado River Basin. These have raised Lake Mead 40 feet higher by the end of 2020 than it otherwise would have been, the U.S. Bureau of Reclamation says.

These forecasts are nothing new. In fact, predictions somewhat gloomier than this one were released by the bureau about two years ago, following another hot, dry summer.

Since then, the Colorado River has had one far-above-average year, in 2019, and one way-below-average year, in 2020, although it wasn’t as bad as in 2018.

The bureau was making the gloomy 2018 forecasts as it was trying to persuade Arizona to adopt a drought contingency plan to keep Lake Mead from falling to catastrophically low levels. Now, it finds a lower but still real chance of the lake dropping that far again, even with the drought plan in place, having been approved in 2019.

At a press briefing Tuesday, bureau officials forecast a 19% chance of a river shortage severe enough by 2025 to lower Lake Mead to below 1,025 feet. A separate forecast, using what it called alternative hydrology assumptions, predicted the lake had a 23 percent chance of falling below 1,025 in 2026.

Below 1,025 feet is the level that federal officials could theoretically step in to manage the river; it’s today managed under a series of guidelines approved by Arizona and the other six Colorado River Basin states.

Under that scenario, CAP cutbacks to Arizona cities have been seen as possible or likely.

β€œThe 1,025 numbers frankly are the end of the world,” said Brad Udall, a Colorado State University water researcher who has warned for years of the potential for major river shortages. β€œIt’s sort of a map that has uncharted territories.

β€œBelow 1,025, our ability to reliably deliver water to everyone who depends on the Colorado River in the Lower Basin is seriously called into question. No sane person wants to be below 1,025 or near 1,025. Yet your numbers show an almost 1-in-4 chance.”

During Tuesday’s briefing, Reclamation Commissioner Brenda Burman said officials have tried very hard to plan for such outcomes by adopting the drought plan last year.

β€œIn plain terms what we’re trying to say is that we are gathering building blocks. We are gathering the best information out there,” Burman said. β€œThere is uncertainty and risk on the horizon but policy decisions have been made to address that risk. We knew we had risk in the next five to six years,” she said.

Asked if she thought climate change contributed to lower river flows, Burman replied, β€œI will say that we know that warmer temperatures have contributed to the drought in the last 21 years. We know they exacerbated it.”

Val Little, director of a water conservation group in Tucson, said she’s surprised that the shortage risks the bureau is forecasting are as low as they are.

β€œThey’ve just about squeezed all they can out of the river,” said Little, of the Water Conservation Alliance of Southern Arizona. β€œGiven the summer we had, all bets are off.”

The bureau also predicted:

  • A 77% chance Lake Mead could drop low enough between 2022 and 2026 to require some level of CAP water cutbacks, first to farmers in Pinal and Maricopa counties. Such cuts would occur if Mead drops below 1,075 feet, compared to 1,083 feet today.
  • Mead has a 32% chance of dropping below 1,075 feet in 2022 and a 55% to 65% chance of doing that over the following four years.
  • Lake Powell, which generates electricity for 6 million residents, has a 16% to 23% chance of dropping below 3,525 feet from 2024 through 2025. At that level, some outside experts have warned that the reservoir’s power plant could suffer damages, although the plant is capable of producing power at as low as 3,490 feet. There's also a 10 percent chance in 2025 of Powell falling below 3,490 feet.

Speaking of the drought contingency plan, bureau spokeswoman Patti Aaron noted that its purpose was to reduce the risk of the two reservoirs falling to critical levels such as 1,025 at Lake Mead.

"The DCP was not designed to eliminate all risk and there will always be some risk facing this system," Aaron said.

These projections were based on what bureau officials call a β€œstress test.” It assumes the hot, dry weather that has persisted in the river basin since 1988 will continue.

When the bureau instead used as a basis for its forecast the entire period of records for the river basin, from 1906 through 2018, the forecasts become much less pessimistic. For 2025, for instance, the bureau sees only a 5% chance of Mead falling below 1,025 using those assumptions.

The odds of the lake even falling below 1,075 feet would drop to 53 percent over the next five years.

Overall, the latest forecasts are 12% to 20% more pessimistic than similar forecasts the bureau made in the spring, before the worst of the hot, dry weather struck the river basin, the bureau said in a news release.

"These numbers show the importance of the Drought Contingency Plan contributions and the need to continue to conserve additional volumes of water in Lake Mead," Arizona Department of Water Resources Director Tom Buschatzke said in a statement.

The forecast also show that the drought plan's goal of reducing the probability of Lake Mead going below 1,025 is being achieved, he added. He cited the 5 percent probability figure using the longer-term hydrology as opposed to the stress test hydrology that shows a 19 percent chance by 2025.

In 2021, Arizona water users will conserve another 107,000 acre feet of water -- an action consistent with the drought plan, Buschatzke said.

Burman said the drought plan led to β€œremarkable contributions” in conservation last year, adding as much as 6 feet additional elevation to Lake Mead in 2019 alone due to the unused water.

Overall, the combination of high river flows in 2019, significant amounts of conservation in 2019 and 2020 and the presence of the drought plan made the new forecast less pessimistic than the one in 2018, spokeswoman Aaron added.

β€œWe had a history of dealing with uncertainty and that uncertainty clearly is growing,” said Terry Fulp, director of the bureau’s Lower Colorado regional office. β€œWe have programs in place to implement conservation and other measures that are good through 2026. I’m very confident we will put together other measures to deal with whatever Mother Nature throws us.”

Photos: Oro Valley Water Utility to replace 500,000-gallon tank

Become a #ThisIsTucson member! Your contribution helps our team bring you stories that keep you connected to the community. Become a member today.

Contact reporter Tony Davis at tdavis@tucson.com or 806-7746. On Twitter@tonydavis987.