ASU-led study sharpens Colorado River forecasts as Arizona faces deepening water cuts
Lake Mead is formed by Colorado River water held back by Hoover Dam in Boulder, Nevada. Photo by Carlin Harris
For Arizona, where much of the state’s water supply depends on the Colorado River, even small improvements in hydrologic forecasting can have significant consequences.
More accurate estimates of water availability could help state agencies anticipate shortages earlier and make more informed decisions about conservation, which is crucial as Arizona continues to face mandatory cutbacks tied to declining river and reservoir levels.
A new study led by Arizona State University researchers aims to improve those forecasts by changing how water is tracked across the Colorado River basin.
The research was conducted through the Center for Hydrologic Innovations, a pillar of the Arizona Water Innovation Initiative, which is housed in the Julie Ann Wrigley Global Futures Laboratory in collaboration with the Ira A. Fulton Schools of Engineering, with support from NASA’s Earth Science Division and Central Arizona Project.
The work centers on improving a widely used hydrologic tool known as the Variable Infiltration Capacity, or VIC, model that is used by researchers and water managers to simulate how water moves through the Colorado River basin.
Looking beyond the river
For decades, scientists and policymakers have relied largely on streamflow, or the amount of water moving through rivers, to measure water availability. But new research published in Scientific Reports shows that approach captures only part of the system.
By incorporating satellite data that tracks water stored in soils and underground, researchers significantly improved how well computer models represent total water availability across the Colorado River basin.
“The Colorado River flow at Lees Ferry is often treated as the main indicator of water availability, but it’s really the result of many processes happening across the landscape,” said lead author and ASU research scientist Zhaocheng Wang. “By bringing in satellite observations, we’re able to better account for water stored in snowpack and soils, which improves how we understand and predict the system as a whole.”
The Colorado River supplies water to about 40 million people across seven U.S. states, including Arizona, and Mexico. But prolonged drought and rising temperatures have reduced flows and pushed major reservoirs to historically low levels.
Satellites reveal what models have been missing
Hydrologic models like VIC are widely used to estimate how water moves through the basin to better inform allocation decisions and drought planning. But many of these models have been calibrated primarily using streamflow data, meaning they can match river flows while misrepresenting what’s happening inside the system.
Water managers rely on these models to guide decisions about reservoir operations and water allocations, but they can overestimate available water or delay recognition of shortages. These risks are especially critical in a system already under strain.
In the new study, researchers integrated data from NASA satellite missions that measure near-surface soil moisture and changes in total water storages, including groundwater.
The results show that the VIC model not only reproduces river flows with high accuracy, but also closely matches independent satellite observations. It captures both surface and deeper soil moisture, as well as basin-wide changes in water storages over time, and successfully tracks how water is stored and depleted during drought.
Those improvements matter for Arizona, where changes in soil moisture and groundwater directly influence how much water ultimately reaches the river at key locations such as Lake Powell. During drought, dry soils can absorb more water before it becomes runoff, further reducing inflows. In a basin where supply is already stretched, even small miscalculations can ripple through interstate agreements and water deliveries.
“If we want accurate forecasts, we need to understand the entire system, not just the visible parts,” said Swastik Ghimire, co-author on the study and recipient of Central Arizona Project’s first-place award for water research. “Modeling results showed that even near-average snowpack can fail to produce expected streamflow volumes if soil moisture is depleted.”
Improving decisions for a drier future
The study underscores the growing role of satellite technology in managing water resources in arid regions. What once relied on sparse ground measurements can now be tracked across large areas with consistent, high-resolution data, which allows scientists to monitor changes in water storages that would otherwise remain hidden.
“As drought conditions continue in the Colorado River basin, every drop of water is becoming more and more important,” said Nolie Templeton, a senior policy analyst in the Colorado River Programs Department at Central Arizona Project. “By capturing a more holistic representation of hydrological processes across the basin, this study provides additional insights to what CAP ultimately needs to know: how much water is in the river, and how that can vary depending on wet or dry conditions.”
The researchers note that some uncertainty remains, particularly in representing subsurface processes like deep groundwater and in the satellite data used to evaluate the model, pointing to the need for continued integration of observations and modeling.
“The Southwest faces increasing water stress, and improving our ability to monitor and predict water availability is essential,” said Enrique Vivoni, senior author on the study and Fulton Professor of Hydrosystems Engineering in the School of Sustainable Engineering and the Built Environment. “These advances give us a more complete understanding of the system, which is critical for making informed decisions about water management in Arizona and across the Colorado River basin.”
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