Groundwater pumping has caused stream flow in U.S. rivers to decline by as much as half over the last century, according to new research by a University of Arizona hydrologist that strengthens the connection between groundwater and surface water. 

The research confirms that groundwater losses, primarily due to pumping water from below the surface for agricultural and municipal uses, decrease the overall surface water supply and have caused some smaller streams to dry up. This has a downstream effect that influences water levels far beyond the groundwater pumping location.   

Laura Condon, assistant professor of hydrology and atmospheric sciences at University of Arizona, worked with hydrology professor Reed Maxwell from the Colorado School of Mines on an article published in Science Advances. Working with funding from the U.S. Department of Energy, Condon and Maxwell created a high-resolution nationwide modeling system to visualize groundwater and surface water interactions on watersheds over the last 100 years. 

The central finding has been known anecdotally and through prior studies, the researchers said. New is the large-scale, high-resolution model that explores the groundwater and stream water relationship across the country and over 100 years. The researchers’ goal was to understand the cumulative impacts nationally of groundwater pumping over time.    

Groundwater pumping supplies more than 40% of global irrigation demand, according to the study. Agriculture in arid areas is often only possible due to pumping water from aquifers underground. When combined with use of surface water, it becomes an essential way to balance out the total water supply. 

Depletion of groundwater is significant because it decreases the amount of water available in aquifers for human use. But also important is its impact on other water in the ecosystem.

Although they seem like separate sources, groundwater and surface water are closely linked, and changes to groundwater levels greatly impact water above the surface in rivers and streams. Conservation advocates would like to see that connection recognized in water laws.

“Everything is connected in the hydrologic cycle,” Maxwell said. “Whether it’s a strong or weak connection, you’re going to see some impacts downstream; you’re going to see some impacts if you overuse.” 

Important findings for Arizona 

The study focused on the continental United States, but it has important implications for Arizona as the state continues to grapple with groundwater pumping issues and drying waterbeds. 

In 2003, the U.S. Geological Survey reported that increased groundwater pumping in south-central Arizona due to population growth had resulted in widespread water level declines in aquifers of 300 to 500 feet. Significant groundwater pumping has continued in Arizona, exacerbating those water level issues.  

Condon said their modeling showed Arizona has experienced significant declines in stream flows — between 10 and 50% — due purely to groundwater pumping.  

Two river systems in particular, the San Pedro River in southern Arizona and the Verde River in northern Arizona, are flashpoints for water debates in the state today, both due to their size and their broader ecosystem impacts. Robin Silver, co-founder of the Center for Biological Diversity, said these rivers are in “extreme peril.”  

“We’re losing what’s left of our desert rivers,” Silver said. “If you take two of the most perilous examples and two of the most valuable areas that we really don’t want to lose, you’re looking at the San Pedro and the Verde.”   

Silver said these two rivers matter the most now because they are some of Arizona’s last, as many others have been depleted due to a combination of groundwater pumping, dams and drought.  

“Say we’re driving from Texas and just go through these rivers: Rio Grande is dead, and then you start moving into Arizona. The Gila, dead. Santa Cruz, dead. Salt, dead. You cross the San Pedro at Benson, it’s pretty dead, but the river flows from Mexico as you go south and it’s still alive. Same thing with the Verde,” Silver said.  

The Santa Cruz River through Tucson has been Arizona’s worst example of the perils of groundwater pumping. The river has been without base flow for decades due to pumped water in the vicinity.  

Sandy Bahr, Arizona chapter director of the Sierra Club, said the state has seen clearly the impacts of groundwater pumping, combined with other threats, on a number of river bodies.  

“We have these long stretches of river that have no water in them, and that’s really not their natural condition,” she said. “We have groundwater pumping, we have diversions, we have climate change, long term droughts, so there are a number of factors that go into killing a river, but groundwater pumping is a big one.” 

The Verde and San Pedro matter for their water supply for human use, for their ecosystem value of providing a home for myriad species, and for their economic impacts in allowing bird watching, wildlife viewing and tourism.  

“In a land like Arizona that is already a pretty dry place and where we’ve already dried up huge portions of our rivers, really trying to sustain what’s left is critical,” Bahr said. 

Case study: San Pedro River  

The San Pedro River, flowing north toward Tucson from across the Mexican border, is likely Arizona’s most visible example of how groundwater pumping reduces river water levels.  

The Center for Biological Diversity has blamed unsustainable pumping of groundwater for decreasing the river’s base flow by nearly 70% since the 1940s. In recent years, population growth in the upper basin of the river has increased groundwater pumping by billions of gallons annually, levels that outpace river recharge by rainfall.  

Some parts of the San Pedro no longer flow perennially due to increased water demands for both home use and irrigation. Some of that water is pumped from aquifers near the river.

“It is such an important place, and to see it robbed of its lifeblood, which is the water, through groundwater pumping is very disturbing,” Bahr said. “We know it’s happening, and it’s well documented in the research, so we know part of the answer for saving that river is to limit groundwater pumping.” 

Groundwater pumping draws water away from underground flows, some of which otherwise would have found its way into a river system. That decreases the river’s levels, which in turn impacts the diverse ecosystem of birds, mammals, reptiles and plants a healthy river supports. 

According to the USGS, the San Pedro’s ecosystem supports more than 400 species of birds and the second-highest known number of mammal species of any river basin globally, making it an important site biologically.  

Silver said the only reason the San Pedro still has water flowing is because environmental advocates have litigated to protect the threatened and endangered species that call the river area home.

Because there are no laws that protect surface water, he said, advocates argue to protect species. Silver said they first filed a lawsuit in 1994 and have continued since, winning each time. They faced a recent setback, however, in an Arizona Supreme Court ruling against them that allowed for large-scale development and significant groundwater pumping near the San Pedro.

“We live in a desert, we live in the arid west, and no one’s really been paying attention for generations here,” Silver said, urging legislative action paired with more sustainable individual and municipal water use to save Arizona’s rivers.  

COURTS: Lawsuit targets development near San Pedro River

Case study: Verde River  

The Verde River, which originates near Chino Valley and flows toward Phoenix, has also faced issues surrounding groundwater pumping and decreased water levels, serving as another concrete example of the study’s findings.  

Salt River Project holds most of the water rights in the Verde based on the legal principle that it first used the water, dating back to 1869. The Verde contributes about 40% of the total surface water SRP delivers to the Phoenix area, although this percentage varies year to year due in part to impacts from groundwater pumping.  

Greg Kornrumph, SRP’s water rights manager, said the utility has seen reduced flows in the Verde River over the past several decades due to increased pumping and a drought since 1996.  

Max Wilson, sustaining flows project manager at the group Friends of the Verde River, said one stretch of the river has even dried up. The only logical reason, he said, is groundwater pumping.  

The USGS found in 2013 that base flow levels at the Camp Verde stretch of the river had decreased by 10,000 acre-feet per year between 1910 and 2005. They are predicted to further drop by 5,400 to 8,600 acre-feet per year by 2110, depending on continued groundwater pumping.

One acre-foot is the volume of water needed to cover one acre to a depth of one foot, about 326,000 gallons.  

The Verde is a perennial steam, meaning it has river flow year-round, largely because groundwater reservoirs add water to the river, allowing it to flow even when there’s no rain, Kornrumph said. An increased number of groundwater wells reduces the amount of water available to flow into the stream, impacting both users of the water like SRP and the watershed environment.  

According to SRP, wells along the Verde have proliferated from 250 in 1950, to 1,600 in 1974, and to 8,900 in 2017. The nearly 9,000 groundwater production wells today combine to have a negative impact on river water levels, something Kornrumph said was certain “beyond reasonable dispute.”


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Nancy Steele, executive director of Friends of the Verde River, said the river is essential for numerous reasons: for a water ecosystem in a largely arid state, for sustaining plant and animal species, for providing a transitional zone between the Sonoran Desert to the south and forests to the north, and for providing a space for recreation and outdoor learning. Reduced river flows impact all these benefits. 

Reduced surface water levels due to increased groundwater pumping has led SRP to supplement some of its Verde River supply with locally pumped groundwater, thus increasing cost to customers, Kornrumph said. Pumping groundwater is more costly, so in dry years or years where river flow is insufficient, the cost to supply groundwater goes up. 

“As the communities upstream continue to grow and they continue to sink more wells and they utilize that groundwater without regard to the impacts to the river, that creates a continuing impact to our users going forward,” Kornrumph said.  

He warned that with the combined impacts of expanding groundwater pumping upstream and a continued drought and lower levels of runoff, SRP could face challenges meeting demand for water. And the Verde could face challenges surviving.  

“A hundred years from now, we don’t want the Verde River to end up the way of the Santa Cruz,” Kornrumph said. “The Santa Cruz River dried up when Tucson drilled all their wells many years ago, and that could potentially be where the Verde’s heading unless we get this situation under control.” 

MORE ON DROUGHTS: Unlikely allies fight drought along the Verde River

Better information, better regulations  

Maxwell said he knows groundwater use won’t stop, so he hopes their study will draw attention to the need to map out pumping and better plan for its effects on broader watersheds so it can be sustainable.  

“I think everybody’s becoming increasingly aware of the importance of regulating, or at least understanding, groundwater pumping,” Maxwell said. He urged legislation, awareness and the use of modeling tools to better utilize groundwater and understand its broad impacts.  

Maxwell analogized groundwater in terms of a bank account: “If you have your checking account fluctuating year to year because of wet years, dry years, you need to dip into the savings. And that’s great, as long as you understand how quickly your savings are being recharged in a wet year.”   

Groundwater pumping is also far less of a local issue than commonly thought, he said. Its impacts instead stretch across state boundaries and extend far downstream from the source, so local and regional decision-making must go hand in hand to balance groundwater pumping, streamflow and water use for people, agriculture and industry.  

After the success of their first study together, Condon and Maxwell have begun working with a team of eight other scientists from seven universities on a second related study, with the goal of combining their national project with more regional and locally focused studies.

With a $4 million four-year grant from the National Science Foundation, the group will work to make groundwater and watershed data and computer modeling available to anyone regionally, Maxwell said.  

READ THE LATEST: Environment news from The Arizona Republic 

Activists, conservationists urge change 

Sarah Porter, director of Arizona State University’s Kyl Center for Water Policy, said Arizona’s current water system is riddled with uncertainty. For the past 45 years, a huge lawsuit has remained unresolved as to how to allocate water rights on Arizona’s rivers and watersheds.

There are currently two bodies of state water laws, one for groundwater that says wells can be sunk as long as the water is being used beneficially and not wasted, and one for surface water that says parties that first used the water have rights over subsequent diverters.  

Complications arise when groundwater wells pump or impact surface flows that a user has prior right to, an inevitability given research findings about the connectivity of all water. Porter said the current uncertainty is not good for anyone involved. Well drillers don’t know whether they’ll have certainty over their water; senior rights owners don’t know whether they’ll have adequate water supply to meet their demand.  

“In the end, we really won’t solve the problem until we start thinking of groundwater and surface water as connected,” Porter said. “Whether we get to it through a big legal battle or we get to it through thoughtful legislation, it depends.” 

Bahr said the last major rewrite of Arizona water law happened in 1980, with the passage of the state Groundwater Management Act, but said it was flawed in overlooking groundwater and surface water connections and in leaving large portions of the state without limits on groundwater pumping.  

“We need to change the laws,” she said. “It’s way overdue.” 

The Sierra Club and a coalition of other groups have been working to advance legislation for the state to consider water more ecologically. The bill did not get a hearing last session, Bahr said, but they are tweaking it to push it forward again next year. 

Silver said Arizona law ignores the holistic science presented, for example, in Condon and Maxwell’s study. Instead, he said Arizona allows parties to pump groundwater regardless of potential impacts on diverting aquifers and decreasing surface water levels.  

“Some of us love rivers, and some of us believe that rivers and the species that are dependent on those rivers have a right to exist,” Silver said. “And legislators in Arizona to date have felt differently.” 

Silver said until Arizonans care enough, meaningful change may not happen. He said the state has lost up to 95% of its historic river habitats but thinks many people don’t know this.  

“They haven’t quite gotten engaged to the point that they’re truly feeling threatened,” he said. “Most people who live in Arizona, they think this is normal to have these washes with no vegetation, and they don’t realize that at one time, Arizona had rivers that looked like the Verde or the San Pedro everywhere.”  

Meanwhile, while legislative change crawls along, Friends of the Verde River has piloted a community-based solution based on voluntary groundwater mitigation. As part of the Verde Water Exchange, large water users such as local businesses can opt into a program where their water use is matched by an equal amount of water saving by another user, decreasing the impact of groundwater extraction on the river.  

“The story of the Verde is a story of solutions,” Wilson said, highlighting the work of his organization and others in improving the river’s flow in the face of groundwater pumping. “It’s not all doom and gloom up here, which I think is a real testament to how much work has already been done.”  

But in the years ahead, climate change is expected to likely only exacerbate these issues, Maxwell, Bahr and Silver said.  

Increased temperatures and increased dryness, in addition to more intense drought events and flood events, are expected to further stress river ecosystems and lower the amount of water stored in aquifers underground and in runoff above ground. 

Condon said their historic findings showed that during hotter or drier times, when surface water is limited, people tend to use the most groundwater, furthering the impact on surface water.  

Condon and Maxwell’s holistic and integrated model could then prove all the more useful in designing a sustainable water management system.

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