The Bureau of Reclamation’s influential monthly forecasting report for water levels at Lake Powell and other reservoirs are consistently “too rosy,” said one of the five experts who co-published a new Colorado River basin research paper.
Each month, the Bureau of Reclamation, which manages Lake Powell, releases a widely read report known as the 24-Month Study. The document projects three scenarios of monthly conditions in the Colorado River basin two years out — a most probable outcome as well as a best- and worst-case scenario. The August version of the 24-Month Study is particularly important; the bureau uses that report to determine how it will operate the basin’s reservoirs for the upcoming calendar year. The bureau also uses the April forecast to make decisions about Lake Powell operations.
“We’re all living and breathing these 24-month studies and these studies include a range of uncertainty,” said Jack Schmidt, director of the Center for Colorado River Studies at Utah State University, which published the paper. “We decided to go under the hood on this for no particular reason other than this is an important piece of the puzzle that is guiding a lot of public conversation.”
The paper, titled “Evaluating the Accuracy of Reclamation’s 24-Month Study Lake Powell Projections,” found that the bureau’s most-probable calculation in the August study “tended to overestimate the end-of-calendar-year Lake Powell elevation by as much as ~10 feet.” Similarly, the study, released Feb. 18, found that the April forecast for end-of-year elevation numbers were off by as much as 20 feet in either direction.
Ten or 20 feet can make a big difference at Lake Powell these days. Water managers are closely watching whether the water level will drop below 3,525 feet above sea level. If it does, it threatens the ability of Glen Canyon Dam to generate power. More than 3 million customers use Glen Canyon Dam electricity and the federal government generates roughly $150 million in average annual revenue from selling that hydropower. Brad Udall, a senior water and climate research scientist at Colorado State University’s Colorado Water Institute, said the water level at Lake Powell could dip below that point at the end of this year or early next year.
Last summer, U.S. water engineers made emergency releases from other reservoirs, including Blue Mesa west of Gunnison, to protect the water level at Lake Powell. The releases dropped the water level at Blue Mesa 8 feet, which forced an early end to the boating season and significantly impacted the local economy.
Bureau of Reclamation public affairs officer Patti Aaron said the bureau appreciates the kind of academic analysis contained in the Colorado River Studies paper and is reviewing the research.
The short-term projections in the 24-Month Study are based on widely accepted hydrologic models compiled by NOAA’s Colorado Basin River Forecast Center; the more long-term projections, however, are made based on an analysis of average inflows during the 30 years from 1991 through 2020. That’s a problem, according to the new report.
“Reclamation uses the last 30 years of flow data to run these projections. And because the last 30 years includes the end of the 20th century, which was quite wet, it makes these projections think the future is rosier than it is,” Udall, a co-author of the research paper, said.
Until last year, the Bureau’s 24-Month Study used a 30-year period that included the 1980s. The latest update drops the 1980s and adds in the 2010s, which helps, Udall said. But Udall advocates going further by dropping the 1990s from the projections and just relying on data from the past 20 years. This approach, he said, would provide a more accurate picture.
Climate scientists who studied tree ring data have recently identified the current 22-year period as the driest on record stretching back 1,200 years. The report, published in the journal Nature Climate Change, estimated that climate impacts driven by human causes increased the severity of the drought by 42%.
“As water supplies fundamentally change we’re going to have serious problems if we use these older statistics to consider our risk,” Udall said. “The climate is not stable and if we utilize this old data, it’s too rosy.”
In 2000, for example, the reservoir system in the basin was 95% full; as of fall 2021, the reservoirs were at 39% capacity, the lowest levels on record, according to the Department of the Interior.
“What we expect to see based on the most probable scenario doesn’t reflect the changes we’ve seen in the climate,” Eric Kuhn, who was the general manager of the Colorado River District for 37 years, and a co-author on the paper, said.
Kuhn, now an author who writes about Colorado River issues, said what surprised him most upon reflecting on the paper’s findings is why this topic doesn’t come up more often. “Why hasn’t the water management community had discussion about this before?” Kuhn said. “I’m hopeful this will help to have more discussion of general management.”
“We need to plan for something different.” Udall said. “That’s why we’re trying to prompt a discussion around this.”