Colorado West Slope water shortages require careful coordination

If the Colorado River Basin is the lifeblood of the American Southwest, keeping the region's agricultural, ecological and economic systems thriving, Colorado's West Slope River basins could be considered the heart. Together they contribute about 70% of the water delivery to Lake Powell, the second-largest reservoir in the nation.

A new Cornell study, the most extensive of its kind, shows how drivers of water shortage, from melting snowpack to growing population demands, may impact this collective of six basins and Lake Powell - a major source of energy and water supply for 40 million people which has been in the grip of severe drought for the past 20 years.

One of the most significant challenges for the region, the study finds, is a potential 50% drop in the West Slope basin's inflows to Lake Powell over the next several decades.

The findings were published July 13 in Earth's Future. The lead author is postdoctoral researcher Sai Veena Sunkara.

The new study builds upon a 2024 modeling analysis of drought vulnerability that determined the Colorado West Slope basins face a potential tipping point, with traditional water delivery levels to Lake Powell and other critical areas becoming unsustainable.

"In this system, we're talking about the potential for decadal or multidecadal droughts," said Patrick Reed, the Joseph C. Ford Professor of Engineering in the Cornell Duffield College of Engineering and the study's senior author. "These are rare, hard-to-model events, and historical streamflow observations don't provide a large statistical sampling. You have to account for the uncertain effects of future precipitation and temperature on streamflows. The basins are also very different in their current and future demands. As a consequence of that, we're asking the question of what factors are controlling water shortage risks in the West Slope basins?"

For the expanded analysis, Sunkara sought to capture a greater range of conditions, from extreme dryness to extreme wetness, and plausible changes in streamflow, snowmelt timing and drought persistence, combined with the future demands of agricultural, municipal and industrial users and their related water rights. This resulted in 20,000 possible midcentury scenarios that represent 2.1 million simulated years.

To understand the uncertainties in future demand, Sunkara mined individual reports of the six basins - the Upper Colorado, Gunnison, Yampa, White, San Juan and Dolores - from the Colorado state planning documents. She then incorporated those projections into the model.

"We explore increases as well as decreases in demands," Sunkara said. "Sometimes agricultural demands change due to multiple factors such as land conversion, improved water efficiency or changing cropping patterns. But all of them were indirectly incorporated as reduction or increase in demands."

By also incorporating water rights, the modeling captures the "human institutional management" side, Reed said, resulting in a complex portrait of "what's going to be important, when, for who, as you're starting to think about, OK, I want to improve the situation, where should I focus?"

Despite the differences across the West Slope basins, the researchers saw a consistent midcentury signal that their major reservoirs are 40% to 55% below their historical medians in terms of storage, depriving them of an effective means of mitigating water shortages. Similarly, the researchers also saw that a shift in the timing of snowmelt is a dominant factor contributing to these systematic shortages. After all, snow is a form of storage, Reed said. If it melts too early, less is available to offset dry periods.

The result is not a prediction of future demands so much as an exploration of the potential ranges for change in different sectors and uses in each basin.

The researchers found that each basin is grappling with a variety of issues and no single management strategy will be able to address all future water shortages.

"If you're starting to distinguish what's driving water shortage, you need a careful map because it's complex and it changes," Reed said. "It changes by basin, changes by sector, changes by user. You have to account for water rights. You have to account for the natural drivers as well as the human drivers. It was more complex than we initially thought. We do give results that highlight where actions such as demand management are going to be more effective."

Former postdoctoral researcher David Gold, Ph.D. '22, now an assistant professor at Utrecht University, is a co-author.

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