Colorado River basins could face tipping point, drought study warns

The study, published in the journal Earth's Future, is the largest and most comprehensive exploratory modeling analysis of drought vulnerability in the Colorado West Slope basins -- six watersheds along the Colorado River that feed the Lake Powell reservoir and support a $5 billion annual agriculture economy. The finding comes at a critical time as state and federal policymakers negotiate water-sharing agreements set to expire in the coming years.

Patrick Reed, senior author of the study and professor at Cornell's School of Civil and Environmental Engineering, said the research was designed to better understand the limits of current management policies and clarify where new strategies for sustainable management could be beneficial.

By pairing Colorado's water planning model, StateMod, with a new hidden Markov modeling framework, the study created hundreds of thousands of streamflow scenarios under both historical and climate-change conditions. The results imply that drought vulnerability analyses relying only on the historical streamflow record may severely underestimate the magnitude of potential drought events and their impacts on water storage, agriculture and municipal water supply.

"Our work shows that even relatively middle-of-the-road climate change and streamflow declines in these basins flows can threaten to put the system at risk of breaching a tipping point where the basins are no longer able to maintain the levels of deliveries to Lake Powell that we're accustomed to over recent history," Reed said.

In 2021, drought led Lake Powell to unprecedented lows, prompting the U.S. Bureau of Reclamation to declare the first-ever water shortage in the Upper Colorado River Basin. The study shows that such shortages can emerge from the basins' internal variability even in the absence of climate change, and that drought doesn't just affect each basin individually -- their combined effects create an even greater strain on the entire region.

"This finding highlights the need for advanced drought analysis methods that can capture these widespread and interconnected impacts," said the study's lead author, David. F. Gold, an assistant professor of physical geography at Utrecht University who conducted the research as a postdoctoral associate at Cornell.

Gold added that the exploratory modeling methodology also provides a roadmap for other work exploring drought vulnerability in institutionally complex river basins across the globe.

The research was sponsored by the U.S. Department of Energy's Office of Science as part of research in the Multisector Dynamics area within the Earth and Environmental System Modeling program.