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Salt River Project touts research indicating reservoirs are well-prepared for rising temperatures. Other scientists caution that climate risks remain.

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Roosevelt Lake sits 97% full on March 31, 2020. The wet winter filled reservoirs managed by Salt River Project. (Photo: Mark Henle/The Republic)

Scientists have found that climate change is playing a big role in shrinking the flow of the Colorado River, but recent research suggests Arizona's reservoirs on the Salt and Verde rivers could fare better as temperatures continue to rise. 

The findings back the assurances of water managers at Salt River Project that their system of reservoirs appears to be relatively resilient in the face of climate change.

Some climate scientists still caution that uncertainties remain and that the Salt and Verde rivers could be hit hard as the burning of fossil fuels continues to heat up the planet. 

Across the American West, rising temperatures have begun to intensify droughts and add to the strains on water supplies. To assess the potential effects and worst-case scenarios in central Arizona, Salt River Project’s hydrologists and meteorologists teamed up with the federal Bureau of Reclamation to study how higher temperatures in the coming decades will likely affect reservoirs that supply cities in the Phoenix area.

Their conclusion, in a report published earlier this year, was that SRP’s strategies are “sufficient to maintain a secure and reliable water supply.”

SRP manages six reservoirs on the Salt and Verde rivers and provides water for more than 2 million people across the Phoenix metropolitan area. 

SRP officials who were involved in the research said the differences between the Colorado River and the Salt and Verde rivers relate to geography and the timing of precipitation.

The upper Colorado River is fed largely by runoff from melting snow in the Rocky Mountains during the spring and early summer, when warmer days intensify the evaporation process and leave less water flowing into the river. Scientists have found that the upper Colorado basin is acutely sensitive to warming and that higher temperatures have sapped the river's flow during two decades of mostly dry years. 

On the Salt and Verde watersheds, in contrast, the rivers get much of their flow from rain and snow during the winter months, when cool temperatures tend to not drive as much evaporation off the landscape.

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“This research as a whole suggests that with warming, the reductions in streamflow on the Salt and the Verde are expected to be less than the upper Colorado River basin,” said Bo Svoma, an SRP meteorologist who worked on the study. “SRP plans for drought, and you would expect droughts to get deeper with warming. But the overall decrease in water supply from warming expected in the future appears to be something that SRP can manage.”

The research team studied the reservoir system under different scenarios and examined the rivers’ flows over the past 650 years using records of wet and dry periods captured in tree rings. They also studied future climate projections and considered how the reservoirs could be affected with a temperature increase of 3.1 degrees Celsius, or 5.6 degrees Fahrenheit.

One key takeaway, Svoma said, is that “intensification of drought is not as severe for the Salt and the Verde as it is for the upper Colorado and other parts of the West.”

FUELED BY GLOBAL WARMING: Supercharged by climate change, ‘megadrought’ points to drier future in the West

Hoover Dam and the Mike O Callaghan–Pat Tillman Memorial Bridge, March 17, 2019, in the Lake Mead National Recreation Area near the Arizona/Nevada border. (Photo: Mark Henle/The Republic)

The Colorado River has long been overallocated, and the levels of its reservoirs have fallen dramatically since 2000. SRP’s researchers and water managers said the Arizona reservoirs are able to weather dry spells better in part because the area has been using less water in recent years due to progress in conservation.

The amount of water the agency delivers each year has declined dramatically during the past two decades. Severe drought and low reservoir levels in 2002-2003 forced SRP to reduce water deliveries by a third for almost a year. But the area was using significantly more water at that time.

During the past five years, records show SRP has delivered about 19% less water than it did from 2000 to 2004.

“Because of that decreasing demand, our inflows are in a long-term balance with that demand. So it's a balanced system, unlike the upper Colorado,” Svoma said.

Some climate scientists who weren’t involved in the bureau's study said that while the findings focus on the effects of higher temperatures, decreases in precipitation could still take a major toll on water supplies. They pointed to a growing body of research showing a heat-driven drying trend in the Southwest, and they warned that unchecked climate change threatens all the region’s rivers, including the Salt and Verde.

These different perspectives reflect areas of ongoing and active research. Scientists are continuing to work on downscaling methods to apply global climate models to specific areas, aiming to fine-tune future projections to help in adaptation efforts.  

Researchers who prepared the study said their analysis indicates SRP’s water system is relatively well-positioned to withstand higher temperatures.

Kevin Murphy, a member of the research team, said while warming will undoubtedly affect SRP’s water resources, “I think we've put our finger on what the impact is going to be, and the impact is going to be small.”

“Even if we get to the high end of temperature change expectations in the future, we're still going to come out fine,” said Murphy, an adjunct researcher in hydroclimatology with Arizona State University’s School of Geographical Sciences and Urban Planning.

“We can survive it. It's quite manageable,” Murphy said. “We are operating within what nature gives us.”

The researchers looked at severe “megadroughts” recorded in the tree rings — droughts that have previously lasted between a decade and three decades. In the scenario of another megadrought similar to one in the late 16th century, and with higher temperatures due to human-caused climate change, their analysis indicated that SRP’s system would be “sustainable.”

The reservoir system can be “considered 100% reliable at the present time and for the rest of this century,” the authors said in the report.

SRP officials pointed to similar findings about the Salt River watershed in other research that has yet to be published but has been presented at conferences.

SRP is the largest provider of water in the area, supplying cities including Phoenix, Mesa, Scottsdale, Tempe, Tolleson, Glendale, Peoria, Chandler, Avondale and Gilbert. In addition to water from its reservoirs, SRP uses groundwater from about 270 wells.

Phoenix and other cities also draw Colorado River water from the Central Arizona Project Canal. 

Cities that don’t have access to water from SRP instead rely on Colorado River water and groundwater, or groundwater only. In some areas, groundwater levels have declined even with restrictions on pumping. Researchers at ASU’s Kyl Center for Water Policy have warned that a set of water rules that has allowed rapid suburban growth is riddled with weaknesses and needs to be overhauled to ensure sufficient water supplies for the future. 

SRP touted the locally focused research in a news release after other scientists published a regionwide study showing that since 2000 the West has been in a dry spell so severe that it ranks among the biggest megadroughts of the past 1,200 years, and that higher temperatures have been responsible for about half the drought’s severity.

SRP said in a statement that the latest research indicates there will be sufficient water “and room for growth” even with climate change, and that the agency will continue to plan strategies for “building resilience.”

SRP and the federal Bureau of Reclamation have had a partnership for more than a century. The bureau owns the dams and canals, while Salt River Project operates and maintains the infrastructure. 

The bureau initiated and paid much of the study’s cost. SRP also contributed.

Charlie Ester, SRP’s manager of surface water resources, said he and other officials aim to plan for worst-case scenarios and sought to do that through the study.

One apparent difference between the Colorado and the Salt and Verde rivers has to do with soil moisture, Ester said. In watersheds where runoff flows to the Colorado River, multiple years of drought have dried out the soil and led to a “deficit of soil moisture,” which then soaks up some of the precipitation when wetter weather returns, Ester said.

“Our watershed essentially gets baked, baked completely dry, every single year prior to the monsoon season,” Ester said. “So, we basically are starting from zero moisture every single year. And I think that has a lot to do as well with why we're not as sensitive to the warming.”

Still, the water managers acknowledge uncertainties and say more research on the effects of climate change will be important.

“One of the big uncertainties is, how will winter precipitation change in the future?” Svoma said. “Hopefully there's more clarity and that uncertainty decreases in the next few decades as climate models become better, because it's the winter precipitation component, rather than the warming, that really drives our watersheds.”

Dave White, a water policy expert who leads ASU’s Decision Center for a Desert City, agreed there are significant uncertainties and reasons to be cautious in interpreting the findings, especially given the risks.

“We need additional research to really fully understand the consequences of these climate impacts,” White said. “There's a lot of work that needs to be done to improve the downscaling of those models.”

For now, SRP’s reservoirs on the Salt and Verde rivers stand nearly full, at 96% of capacity. Three of the past four winters have been wet, helping boost the reservoirs after a series of dry years in the 2000s and the mid-2010s.

A HEAD START: Reservoirs that supply water to Phoenix area nearly full after wet winter

A couple fish from their boat at Roosevelt Lake on March 31. A wet winter has filled reservoirs managed by Salt River Project. They now sit 98% full, their highest level since 2010. (Photo: Mark Henle/The Republic)

Murphy has studied megadroughts of the past using tree rings, and those droughts have lasted up to about 30 years. He said the latest series of dry years began in 1995, and the past two wet years suggest that probably “this could be the end of the megadrought.”

Other climate scientists pointed out that warming is projected to continue to increase the odds of drought and megadrought across the region. They said that likely means trouble for the Salt and Verde rivers, as well as the entire Southwest.

Jonathan Overpeck, a climate scientist at the University of Michigan, said he defers to SRP’s experts on their findings but that the big caveat is what will happen with winter precipitation as temperatures continue to climb.

“It’s exactly that decrease in precipitation during the cool season, which is when you really need it, that is going to endanger water supplies in those watersheds,” Overpeck said.

Climate models have projected that cool-season storm tracks would move north, and that shift has begun to be observed as the planet gets warmer, Overpeck said.

“Climate models indicate it could get a lot worse if greenhouse gas emissions continue. The more fossil fuels we burn, the more carbon dioxide we put in the atmosphere, and the less it will snow and rain in Arizona,” he said, and other parts of the Southwest during the cooler months.

ORGANIZING FOR A BETTER FUTURE: A recent conference focused on solutions for Arizona as the planet heats up

Overpeck also cited other factors. The more warming that occurs, he said, the more demand there will be for water. And it also looks unlikely, he said, that an increase in summer rains could make up for the loss of precipitation in the cool season.

“The details may be different, but the watersheds of the Salt and Verde are going to aridify just as much as the bigger watersheds further north. Moreover, the droughts and megadrought impacts will also be just as hard-hitting when they occur,” Overpeck said.

Overpeck said he respects SRP’s water managers and sees them as some of the best in the country.

“But at the same time, I think their challenge is huge, with climate change likely to decrease the precipitation and therefore the water in our rivers,” Overpeck said. He said climate research indicates the flows of all the region’s rivers, including the Salt and Verde, are threatened by rising temperatures, and that Arizona is “going to get hammered by climate change.”

Other researchers have projected that without action to curb climate change, counties across the Southwest and other southern regions of the U.S. will be hit hardest, facing more economic damage than other parts of the country.

Overpeck has studied how higher temperatures are taking a toll on the Colorado River and leading to long-term “aridification” in the West. In a recent article that Overpeck co-wrote with climate scientist Brad Udall of Colorado State University, the two said there is a “clear longer-term trend toward greater aridification, a trend that only climate action can stop.”

“Greater aridity is redefining the West in many ways, and the costs to human and natural systems will only increase as we let the warming continue,” they wrote.

Udall said in an email that water managers throughout the West “need to be prepared for all kinds of previously unseen challenges, even in places where science tells us the impacts may be less.”

“No one is getting a free pass on preparing for climate change,” Udall said.

And while preparing for the effects is important, he said, it’s equally crucial to address the root cause — greenhouse gas emissions.

“No one is going to solve this for us. It will require everyone — governments, water providers, companies and citizens — acting to reduce emissions,” Udall said. “We’ve got to deal with the root cause.”

Reach reporter Ian James at ian.james@arizonarepublic.com or 602-444-8246. Follow him on Twitter: @ByIanJames

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Environmental coverage on azcentral.com and in The Arizona Republic is supported by a grant from the Nina Mason Pulliam Charitable Trust. Follow The Republic environmental reporting team at environment.azcentral.com and @azcenvironment on Facebook, Twitter and Instagram.