Arizona water officials expect the state’s mandated reductions in use of Colorado River water to hold steady next year but potentially worsen after that unless a wet winter in the Rocky Mountains refills some of the Southwest’s lost reservoir storage.
Federal guidelines for doling out water based on Lake Mead’s diminished current storage this year placed the river in what is known as a Tier 1 shortage, meaning Arizona is giving up about 30% of the Central Arizona Project’s normal supply, which equals 18% of the state’s share of the Colorado.
The U.S. Bureau of Reclamation is expected to release its projection on Aug. 15 for Lake Mead’s 2026 water levels, and unless anything changes, the current tier should persist through next year, according to Doug MacEachern, an Arizona Department of Water Resources spokesperson.
Lake Mead is at elevation 1,054.15 feet above sea level, about 175 feet below what’s considered full. Based on the water stored behind Hoover Dam, the reservoir is at 31% of capacity.
It’s unclear what might happen after the 2026 operating plan is put in place, as the federal guidelines for administering shortage expire next year. The seven states that use the river have struggled to reach a new agreement restricting their consumption as the river continues shrinking. The Trump administration has said it needs them to strike such a deal by this winter or Interior Secretary Doug Burgum will impose his own plan.
The last projections for Lake Mead’s water line in 2027 indicated a most-probable elevation dipping near what would be the trigger for a deeper Tier 2 cut if the current guidelines weren’t expiring. The outlook is worsening because the 2025 runoff season in the Rocky Mountains, the Colorado’s headwaters, is shaping up to be the seventh-driest since 1963, when the government built Glen Canyon Dam to help store the river’s flow.
Central Arizona Project officials bemoaned the rough water year during an Aug. 7 board meeting.
“This year has really kicked us,” said Vineetha Kartha, CAP’s Colorado River programs manager.
While it’s anything but clear what water-use reductions Arizona will face in 2027, MacEachern said, “What can be said about the ‘most probable’ projection is that elevations in Mead will continue to decline.”
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Bad hydrology is ‘crushing’ conservation measures
As with other recent years, factors including warming air, parched soil and lengthening growing seasons have kept some of the melting snowpack from reaching the river or its reservoirs.
“Fundamentally, bad hydrology is crushing valiant conservation and management efforts,” said Kathryn Sorensen, research director at Arizona State University’s Kyl Center for Water Policy. Precipitation upstream of Lake Powell registered 83% of the 30-year average, she said, but the flow into that reservoir during the crucial April-July runoff season was significantly lower — 42% percent of average through June and projected at 19% for the month of July.
Enrique Vivoni, a professor of hydrosystems engineering at Arizona State University, told CAP board members on Aug. 7 that dry soils are soaking up precipitation before it can reach the river, meaning good snow doesn’t always mean good flow.
“Under drought, the relationship of hydrology with precipitation is changing in Colorado River,” Vivoni said.
Lakes Mead and Powell, the largest constructed reservoirs in both the Colorado River Basin and the nation, were roughly full when drought began at the beginning of this century. In late July they held 31% and 32% of capacity, respectively.
“The only good news is that municipal water providers in central Arizona have long been preparing for Colorado River cuts,” Sorensen said. “Tap water deliveries are unlikely to be impacted, though water providers may ask for or even demand additional conservation efforts from their customers.”
To compensate for the loss of some river water, central Arizona is likely to pump more groundwater, depleting a resource not easily replenished during long-term drought.
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It is possible that Arizona cities would need to tap new supplies to make up for river losses in coming years, depending on how low Lake Mead sinks and what rules the feds impose next year. One option, which also would deplete non-renewable groundwater, would be to drill wells to pump water that the state banked in the ground during years when it had excess Colorado River water.
Phoenix relies on the Colorado for about a third of its tap water, running about 120,000 acre-feet of it through its surface water treatment plants, according to a 2023 report from the Kyl Center. That kind of water remains available through Tier 1 and Tier 2 but some of it becomes questionable in Tier 3, according to the report.
Under the expiring rules, that would happen if Lake Mead’s surface sank about 25 more feet. That’s about how far the reservoir has dropped since 2020, though the decline can quicken the lower the water goes because the reservoir is essentially V-shaped. It remains unclear how much water CAP and Phoenix may get under new rules.
The Arizona Department of Water Resources and the CAP have begun new talks among Arizona water users to craft a new plan to share cuts in the future. Some cities have avoided the worst effects of existing cuts because other farmers, tribes and cities have given up extra water in exchange for compensation, but those compensation arrangements are set to end in 2025 and 2026, meaning new burden-sharing agreements may be necessary to avoid harsh consequences.
The states have spent part of this year debating a potential deal that would roughly split up the river between the Upper Colorado and Lower Colorado basins based on the quantity actually flowing in a given year, rather than the old system of following a 1922 compact’s allocations that don’t match reality in a warming climate. So far that idea has not yielded an agreement as the regions continue to dispute how much the Rocky Mountain states must yield to the Southwest.
Brandon Loomis and Austin Corona cover environmental and climate issues for The Arizona Republic and azcentral.com. Reach Loomis at brandon.loomis@arizonarepublic.com and Corona at austin.corona@arizonarepublic.com.
Environmental coverage on azcentral.com and in The Arizona Republic is supported by a grant from the Nina Mason Pulliam Charitable Trust.
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This article originally appeared on Arizona Republic: Arizona’s declining share of Colorado River to hold steady for a year