Don't expect Miracle May this month on the Colorado River

Five years after a “Miracle May” of record rainfall staved off what had appeared to be the river’s first imminent shortage in water deliveries, the hope for another in 2021 “is fading quickly,” says the Colorado Basin River Forecast Center’s latest report, released Thursday.

That’s one more piece of bad news for the Central Arizona Project. A first-time shortage is now likely to slash deliveries of river water to Central Arizona farmers starting in 2022 but won’t affect drinking water supplies for Tucson, Phoenix and other cities, or for tribes and industries that get CAP water.

Federal forecasts for spring-summer runoff into Lake Powell plunged this week, predicting the third-lowest such flows on record into the big reservoir at the Arizona-Utah border.

The river forecast center predicts that runoff into Powell from April through July will be 28% of average, down from a 45% projection in its April forecast.

The difference in these forecasts amounts to 1.2 million acre feet of lost water. That’s about nine months worth of CAP supplies in a normal year.

“I expect that the forecast for Lake Powell will stay in the 1.6 million to 2.0 million acre-foot range through the end of the runoff season,” said Eric Kuhn, an author of a book on Colorado River management and a former general manager of the Colorado River District in Glenwood Springs, Colorado. “We all need a decent monsoon.”

Short- and long-term forecasts for the entire river basin are calling for above-normal temperatures and below-normal rainfall through the next 90 days.

As of now, key indicators of available future runoff such as snowpack and soil moisture are below normal and, in the case of soil moisture, well below normal, the forecast center said. Snowpack levels are less than 10% of normal in the Virgin and Dolores river basins in Utah and Colorado.

The Upper Green and Upper Colorado River river basins have about 75% snowpack of normal, while the Yampa Basin has 65%, the Gunnison Basin has 55% and the Duchesne Basin has 50%.

In various studies, scientists such as Brad Udall at Colorado State University, Jonathan Overpeck at the University of Michigan and Christopher Milly of the U.S. Geological Survey have linked such chronic, low-water and low-snowpack conditions on the river since 2000 to long-term climate change that has boosted temperatures and increased evaporation. That causes snow in the mountains to melt faster and reduces spring and summer runoff.

Overall, there’s always uncertainty in the long-range weather forecasts, and things can change. But given current conditions, a Miracle May like that of 2015 seems “highly unlikely,” said Ashley Nielson, a senior hydrologist for the river forecast center, in an email to the Star on Friday.

“Unfortunately, no prolonged period of … wet conditions is seen over at least the next 10 days, a pattern that we desperately need, given the very poor water supply,” the center’s report said.

The National Weather Service’s forecast is for below-normal rainfall from May 15 through May 21 across the entire river basin, except for southwest Arizona and Southern California. There, rainfall is likely to be at normal levels.

By contrast, in May 2015, the lower 48 states had not just their wettest May but their wettest month since records started being kept in 1895. Parts of Colorado — home of the river’s headwaters — got 200% to 400% of normal rainfall that month.

Grim outlook for Lake Mead

The U.S. Bureau of Reclamation, in its latest forecast that looks 24 months ahead, predicted on April 15 that the “most probable” scenario for Lake Mead is for it to stand at 1,067 feet at the end of December.

If that forecast holds through the rest of the year, the federal government will declare its first round of significant, mandatory cutbacks of water deliveries for the CAP. That’s what it’s supposed to do when Lake Mead falls below 1,075 feet at the end of a year.

Such a shortage, cutting deliveries to Arizona by 520,000 acre-feet, or nearly one-third of total CAP supplies, would particularly strike at Pinal County farmers. They would lose most of their CAP supplies next year and would lose their entire supply in 2022 should Lake Mead fall far enough over the next year. Until now, the state has only been sustaining cuts of 192,000 acre-feet per year in CAP deliveries.

A new, longer-term forecast for Lake Mead by the bureau looks a bit more grim than earlier such forecasts.

Released Tuesday, that forecast predicted a 20% chance that Mead will drop below 1,025 feet by 2025 and a 26% chance of it falling that low by 2026. Forecasts in September 2020 gave the lake a 19% chance of dropping that low by 2025 and a 23% chance of being that low by 2026.

Those forecasts are based on what the bureau calls “stress test hydrology,” in which the agency uses only precipitation and runoff totals since 1988 to calculate future conditions. Forecasts by the bureau that rely on hydrology covering the river’s historical record from 1906 through 2018 were more optimistic: an 8% chance of Mead dropping below 1,025 by 2025 and a 10% chance by 2026.

The 1,025 figure is considered a major threshold for the river, the point at which the federal government could consider taking over management of how the river water is divided up.

Currently, each state’s share of river flows are split according to the 1922 Colorado River Compact, a pact that actually allocated more water to the seven basin states than what long-term records show are likely to flow downriver.

If Lake Mead fell below 1,025 feet, that would also require the first cuts in CAP deliveries to cities and tribes, under a 2019, seven-state drought contingency plan that aimed to cut water use in early years of shortages to stave off more crippling shortages later.