Political and Physical Roots for the Colorado River Water Crisis: With a View to the Future

Marty Hoerling

Department of Civil and Environmental Engineering, at the University of Colorado-Boulder

Tuesday, Aug 15, 2023, 2:00 pm
DSRC Room 2A305

Abstract

Two frequent assertions about the origin for the current Colorado River water crisis are examined. One that the dire situation today owes to political decisions by the 1922 Compact Commissioner’s that ignored the best available science on how much water was in the river. The second that the Colorado River water crisis is due to human-induced climate change (with an imminent threat for further river declines). Neither of these two claims of primary causality stand up under closer scrutiny.

The Compact apportions 15 million acre-feet (maf) of water between the Upper and Lower Basin, and the Lee Ferry gage determines how much water is delivered from the Upper to the Lower Basin. At the time, the Commissioner’s viewed the Colorado river and its tributaries to have more than this 15 maf allotment (e.g., Compact Section IIIc allows the Lower Basin to increase its beneficial use by 1 maf). Discussed in this talk are different gage-based (1895-1922) estimates for Lee Ferry flow that were available during Compact ratification. These mostly justify a view of an abundant river at the time – the best available science. Flow inferences from earlier proxy information, less supportive of an abundant river but also less reliable, are briefly discussed. Key to this talk’s argument is that statistics of Lee Ferry flow – especially for decadal periods that dictate Compact performance– based on then available gage data could not have reasonably forseen the low flows that would dominate the post-ratification era (1930-present). Other policies better reflect the political roots of today’s crisis: the Compact’s inflexible governance, and subsequent decisions to fully develop Lower Basin water rights in the 1960s at which time data were adequate to confidently reveal a much lower yielding river.

Likewise, there are strong physical roots for today’s water crisis. These arise from less precipitation and warmer temperatures over the Upper Basin, especially compared to the pre-Compact era meteorology. Evidence from multi-model large ensemble climate simulations reveals precipitation decline almost certainly occurred via natural variability. On the other hand, most of the warming has resulted from human-induced change. The river’s sensitivity to warming is uncertain but likely accounts for much less than ½ of the flow declines since the pre-Compact era. We explore the future using an empirical model of Lee Ferry flow sensitivity to combined effects of precipitation and temperature variations, the latter derived from climate simulations. A range of plausible sensitives are considered. The resulting projections of Colorado River flow for the 2026-2050 period of new Interim Guidelines reveals an appreciable probability for increasing flows, especially when compared to observed low flows of 2000-2022.