Water Resiliency in a Changing Climate

How SRP is working to ensure a reliable water supply for the future

For more than a century, Salt River Project (SRP) has been managing a reliable water supply for a growing population in Greater Phoenix. As more people move to the Valley — increasing the demand for water — our desert home also faces the challenges of a changing climate. At the October 2022 Southwest Water Resiliency Conference, experts from SRP and other organizations discussed the state of our region's water supply and how we're working to secure a resilient water future in the wake of the challenges ahead.

Climate Variability

Looking out to 2099, climate modeling institutions show that warming is certain. In addition to warming, there is also the factor of natural climate variability. This natural variability impacts weather phenomena such as temperature and precipitation. For the Southwest, the impact of drought continues to be top of mind. In fact, Arizona has been in a drought for nearly three decades — the most severe in roughly 600 years. While experts say this megadrought can be attributed to natural variability, climate change is expected to make droughts more severe as temperatures increase. Yet, Arizona experienced one of the wettest consecutive 12-month periods from April 2022 through March 2023. There have only been three years in which we have received more precipitation on the Salt-Verde watershed for a 12-month period: 1984-1985, 1992-1993 and 2004-2005.

Colorado River Streamflow

When it comes to streamflow in the Valley's major water sources — the Colorado River and the Salt and Verde rivers — experts have been analyzing the impacts of rising temperatures and drought. The Colorado River has been most affected by these weather phenomena. According to Dr. Enrique Vivoni, hydrologist and professor of hydrosystems engineering at Arizona State University (ASU), Colorado River streamflow is highly sensitive to warming. In fact, studies show that increasing temperatures have decreased streamflow on the Colorado River since 2000.

Salt and Verde River Streamflow

On the contrary, the Salt and Verde rivers have a low sensitivity to warming. According to Dr. Kevin Murphy, a research professor at ASU, there has been a relatively small decrease in streamflow with increasing temperatures. Furthermore, a pilot study of the Salt and Verde river reservoir system revealed that SRP's current operational strategies are sufficient to maintain a secure and reliable water supply.

But why are the Salt and Verde rivers less sensitive to warming than the Colorado River? For one, more than half of the Colorado River's streamflow originates as groundwater that is recharged by snowmelt while the Salt and Verde rivers receive a substantial amount of water from short-duration flood events generated by rainfall. As SRP Senior Meteorologist Dr. Bo Svoma points out, these differences in sources can have a significant impact on how streamflow will change in the future.

According to Marcos Robles, Lead Scientist at The Nature Conservancy, efficient runoff from large winter storms is buffering the annual streamflow of the Salt and Verde rivers from the negative impacts of warming. Research also suggests that increased greenhouse gas concentration in the Southwest could continue to have a substantial effect on precipitation in Arizona. Matei Georgescu, Director of the Urban Climate Research Center at the University of Arizona, also indicates that extreme winter precipitation events on the Salt and Verde watersheds are expected to increase in the future.

Moreover, Dr. Connie Woodhouse, a geography professor at the University of Arizona, points out that while research suggests no significant decrease in streamflow is evident on the Salt and Verde rivers, continued warming will likely decrease streamflow over time. This is a key reason why SRP continues to plan for a changing climate despite the currently reliable water supply from the Salt and Verde rivers.

It is vital to keep in mind that not all greenhouse gas emission scenarios used in the coordinated global climate modeling experiments are plausible. According to Dr. Steven Rose, an economist at Electric Power Research Institute, it is prudent to plan for greater than 1 degree Celsius of global warming but less than 3 degrees Celsius of global warming.

From Droughts to Floods

As drought remains an issue in the Southwest, it is important to factor in the effect flooding can have on our water supply and the health of the Valley. Charlie Ester, Water Supply Director at SRP, tells us there will eventually be a severe climatological shift from droughts to floods, especially as seasonal precipitation is expected to increase in the Salt and Verde river watersheds.

Atmospheric Rivers

Even without climate change, there is a naturally high variability in Arizona winter precipitation. Often, we'll experience very dry years followed by very wet years. According to Dr. John O'Brien, climate change is expected to amplify this natural variability due to more frequent inland penetration of atmospheric rivers.

Atmospheric rivers are weather events that can produce extreme flooding. Dr. Marty Ralph, Director, Center for Western Weather and Water Extremes (CW3E) at the Scripps Institution of Oceanography, UC San Diego, and former Chief of the National Oceanic and Atmospheric Administration's Water Cycle Branch, has examined the impact of warming temperatures on precipitation and the connection to atmospheric rivers. The research shows that intensified precipitation extremes are closely associated with atmospheric rivers. Extreme atmospheric river events are expected to get more frequent in the future due to climate change.

Planning for the Future

With all of this in mind, SRP has been preparing for the future in a variety of ways. At the conference, Ron Klawitter, Water System Projects Manager at SRP, highlighted some of SRP's key focus areas for the short, medium and long term.

Short Term: Roosevelt Dam Capacity

At Roosevelt Dam along the Salt River, there is roughly 1.8 million acre-feet (AF) of available space, or 62 vertical feet, that is designed to manage very large flood events. SRP is seeking permission to extend the flood release period in the first 5 feet for up to 120 days instead of the 20 days currently required to empty the flood control space. This extra time would allow tribal, agricultural, and municipal water users to use flood waters during the hot summer months.

Medium Term: SRP-CAP Interconnect Facility

While SRP's canal system can already take in water from Central Arizona Project (CAP) canals, we're currently working on infrastructure that will allow us to do the reverse as well. The SRP-CAP Interconnect Facility (SCIF) is a proposed project to connect SRP's South Canal with the CAP canal that together with the existing CAP-SRP Interconnect Facility (CSIF) will allow water to move between the SRP and CAP systems in both directions. With 270 wells in our system, we currently have more than 6 million AF of water stored. SCIF will help move floodwater, recovered recharge credits of which there are more than 6 million AF stored, and other existing supplies.

Long Term: Bartlett Dam Modification Project

On the Verde River, SRP is working with the Bureau of Reclamation and 21 tribal, agricultural and municipal water partners to find cost-effective solutions that would restore lost water storage capacity at Horseshoe Reservoir while also adding additional capacity for future use.

Two infrastructure modification options identified in the Verde Reservoirs Sediment Mitigation Study (VRSMS) would restore lost capacity at Horseshoe Reservoir (due to sedimentation) while creating additional water storage capacity. The options include raising Bartlett Dam by about 100 feet to enlarge the reservoir. The existing Horseshoe Dam would remain in place to act as a flood control structure, sediment management structure and fish barrier, as well as provide expanded habitat in the existing Horseshoe Reservoir footprint.

The enlarged Bartlett Reservoir would increase the total water storage capacity on the Verde River by about 350,000 AF — more than doubling the current capacity of the Verde River reservoir system which is 276,467 AF. This volume aids in addressing the vulnerabilities associated with the impacts of climate change on the Verde River reservoir system and provide a renewable surface water supply to reduce groundwater use in central Arizona. The project also looks at how new infrastructure at Bartlett Dam can improve operations for sediment, flood and habitat management at Horseshoe Reservoir.

Strategic Partnerships

In addition to the projects above, SRP is continuing to help build a reliable and resilient water supply through avenues of conservation. For instance, with the SRP Healthy Forest Initiative™, we've teamed up with state, local and federal agencies, businesses and others to support the removal of small trees and thick brush in overgrown forests. This helps to prevent catastrophic wildfires that can negatively impact our watersheds.

SRP is also considering innovative ways to increase water resources to help prepare for the future, including cloud seeding with the White Mountain Apache Tribe, desalination, and research projects with Arizona universities to conserve water.

Planning for the challenges of a changing climate is no easy task, but through strategic partnerships, smart infrastructure and investing in innovation, SRP is helping to ensure a resilient and reliable water supply for the Valley for generations to come.