Groundwater benefits indicate that Arizona’s policy is working, but climate risks still threaten water supply

Research led by Austin schools in Arizona and Colorado and collaborators shows that despite areas dealing with long-term water stress, it has contributed to a prominent increase in groundwater in central Arizona.

Since 1980, central Arizona has been working to recharge depleted aquifers. Much of that charge depends on water from the Colorado River in the southwestern United States. Water, which accounts for 36% of Arizona’s water supply, is brought to the states more than 300 miles away via the Central Arizona Project’s Aqueduct System.

Some state policies encourage farmers to use surface water from rivers rather than groundwater. Other policies could lead river water directly into aquifer charging zones, where it could penetrate groundwater.

These policies saved a total of 10.5 cubic kilometres of groundwater from 1989 to 2019 in the Phoenix, Tucson and Pinal Active Management areas where these policies are in place. According to a study published in Communications Earth & Environment, these policies saved a total of 10.5 cubic kilometres of groundwater from 1989 to 2019. Furthermore, the researchers found that an additional 14.2 cubic kilometres of the aquifer could be attributed to penetration from surface water irrigation that is not part of an active management program.

Together, the 25-kilometer water body is roughly five times the annual demand for water in the three active-controlled areas of the state, the state’s seven largest. These are substantial agricultural districts with a total size larger than Maryland.

“This study demonstrates the value of conjunctival management in the surface and groundwater,” says Bridget Scanlon, a research professor at the UT Jackson School of Geosciences Bureau of Economic Geology.

Researchers show that these benefits are working for Arizona’s groundwater charging policies, indicating that these policies can provide a roadmap for other water-bound regions. However, the water provided by the Colorado River faces an uncertain future. The extreme droughts of 20 years have led to a massive decline in water. The country’s two largest reservoirs, Lake Powell and Lake Mead, are expected to drop even further due to climate change, from 90% capacity in 2000 to about 30% capacity in 2025 due to a decline in river flow.

That means that even with these groundwater benefits, sound aquifer management in central Arizona will face new challenges, said co-author Kathryn Sorensen, director of research at Arizona State University’s Kill Water Policy Center and former director of Phoenix Water Services.

“It’s a lot of water and that’s great. We should celebrate it. But the shortage in the Colorado River means there’s less water to boost our aquifer and more pressure to draw groundwater as a replacement supply,” Sorensen said.

The findings of the Arizona Aquifer are just one aspect of a study that has tracked the increase and loss of surface and groundwater across the Colorado River basin for decades. Using Grace Satellite Data, regional models and groundwater level monitoring data, the researchers showed how water levels changed in response to wet and dry periods and human intervention.

In addition to showing the importance of intentional groundwater banking policies, this study showed how accidental penetration from irrigation promoted charging of aquifers first depleted by intensive groundwater pumps in the 1940s and 1970s, and how moisture from the 1980s to 1990s provided an additional boost.

“I don’t know if we’ll see wet periods again like the ’70s and early ’90s, but we need to use these wet periods to put water into the ground wherever we can.”

Water allocations from the Colorado River are governed by the 1922 Compact and Reservoir Operating Rules, which are set to expire in 2026. Various states, indigenous communities and Mexican policymakers have been convened to renegotiate water pullouts. Scanlon said the wide range and span of the research would help plan sustainable water use from the river.

“You can see how things change over time,” Scanlon said. “And that’s important when you’re trying to understand climate change, irrigation pump variation, managing coupled management, putting that into context.”

Additional co-authors of this study are Ashraf Rateb and Robert Reedy from the Bureau of Economic and Geology, Branch Conway from the Arizona Department of Water Resources, and Bradley Wudal from Colorado State University.