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.”

Arizona implications

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. These rivers are in “extreme peril,” said Robin Silver, co-founder of the Center for Biological Diversity.

“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.”

“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.”

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.

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.

Complicated water laws

Arizona’s current water system is riddled with uncertainty, said Sarah Porter, director of Arizona State University’s Kyl Center for Water Policy. 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.”

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.”

More stress coming

Meanwhile, as legislative change crawls along, Friends of the Verde River has piloted a community-led 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.