The Colorado River basin, a source of water for 40 million people and several million acres of agricultural land in the western United States, has been experiencing drought conditions for more than two decades. In mid-September, the U.S. Bureau of Reclamation warned that such conditions will continue during the next several years, increasing the likelihood of water shortages by 2025.
On Sept. 15, Reclamation released the latest results of its Colorado River Simulation System — the modeling program used by the agency to predict long-term conditions on the river and its critical reservoirs, lakes Powell and Mead. Formed by the Glen Canyon and Hoover dams, respectively, the two reservoirs are the largest in the country in terms of capacity. The modeling results extend through 2026.
The most recent CRSS projections “indicate an increase by as much as 12 percent in the chance of Lake Powell and Lake Mead falling to critically low reservoir levels by 2025 as compared with the projections released this spring,” according to a Sept. 15 news release from Reclamation. The worsening outlook stems from the below-average runoff within the Colorado River basin this year.
The outlook is worse for the Lower Basin, meaning that the water levels on Lake Mead could reach critical levels if drought conditions persist.
For regulatory purposes, the Colorado River is divided into an Upper Basin and Lower Basin, with the demarcation point being Lee Ferry in northern Arizona, just downstream of Lake Powell. The Upper Basin comprises Colorado, New Mexico, Utah, and Wyoming, while the Lower Basin comprises Arizona, California, Nevada, and Mexico.
The outlook is worse for the Lower Basin, meaning that water levels on Lake Mead could reach critical levels if drought conditions persist. “The chance of a Lower Basin shortage determination increased by as much as 20 percent through 2025, assuming a dry hydrologic future similar to what the Basin has experienced over the past 2 decades,” according to Reclamation’s release. “These increases put the chances of reaching critically lower levels near 20 percent and a Lower Basin shortage near 80 percent by 2025.”
“Because there’s so much uncertainty with respect to future hydrology (within the Colorado River basin), we employ two different methods to give us a probabilistic outcome for future conditions,” says Carly Jerla, an operations research analyst for Reclamation, who spoke to the media as part of a Sept. 15 virtual press conference. The two methods are known as the “full” hydrology and the “stress test” hydrology. The full hydrology method includes the river’s entire historical natural flow record from 1906 to 2018, while the stress test hydrology includes only the flow records from 1988 to 2018, which are more representative of the recent drought conditions. Modeling results based on the stress test hydrology indicate a “much drier future,” Jerla says.
Reclamation’s findings reinforce the well-known fact that the historical allocations of water from the Colorado River exceed what can be sustained in the long run.
Under a 2007 agreement between Reclamation and the seven basin states, the first of three designated shortage conditions occurs when the elevation of Lake Mead at Hoover Dam falls to 1,075 ft, which equates to the reservoir being approximately one-third full. Additional shortage conditions are triggered if the reservoir elevation dips to 1,050 ft as well as 1,025 ft, which equates to about 22 percent capacity and is considered critically low. In the event of a shortage declaration, cuts will be made to water deliveries, beginning with Arizona and Nevada. At the end of September, the elevation of Lake Mead was approximately 1,083 ft, according to Reclamation’s website.
Under the full hydrology scenario, Lake Mead has a 23 percent chance of falling below 1,075 ft but remaining above 1,025 ft in 2022, according to information provided by the agency on its website. Between 2023 and 2026, the chances of this same outcome increase, ranging from 44 to 49 percent. As for falling below the critical level of 1,025 ft, Lake Mead faces only a 1 percent chance of doing so in 2024, increasing to a 5 percent chance in 2025 and a 6 percent chance in 2026.
As would be expected, the projected conditions for Lake Mead are more dire under the stress test scenario modeling results. This approach assigns a 32 percent chance of the reservoir reaching a level between 1,075 and 1,025 ft in 2022. Subsequent years have even higher chances of this outcome, ranging from 55 percent in 2023 and 2026, 58 percent in 2025, and 65 percent in 2024. More concerning, Lake Mead is given a 19 percent chance of dipping below the 1,025 ft level in 2025 and a 23 percent chance in 2026.
“What the projections are showing us is we have greater uncertainty than we did last year,” says Brenda Burman, the Reclamation commissioner, during the Sept. 15 press conference. However, Burman cited the drought contingency plans that Reclamation finalized in May 2019 with several states and water providers as evidence that past efforts to address future risks have paid off. Under the plans, states have agreed to leave certain amounts of water in Lake Mead in the event that its elevation falls to certain levels. “We know the drought contingency plans are working,” she says.
Reclamation’s findings reinforce the well-known fact that the historical allocations of water from the Colorado River exceed what can be sustained in the long run, says Bill Hasencamp, P.E., the manager of Colorado River resources for the Metropolitan Water District of Southern California, a major user of water from the river. “We know shortages are coming,” Hasencamp says. “It’s a matter of when, not if.”
Although not surprising in themselves, the modeling results from Reclamation highlight the deleterious effects that the long-term drought continues to have on the Colorado River basin, says Tina Shields, P.E., the water department manager for the Imperial Irrigation District. Located in California’s Imperial County, the IID is one of the largest recipients of Colorado River water. The results also show the “need for water users to continue to look for opportunities for conservation, reuse, and recycling within their service areas as well as the need to reduce future demands,” Shields says.
This article first appeared in the November 2020 issue of Civil Engineering.