Editor's note: This article originally appeared in the Utah Water Research Laboratory's newsletter. It has been edited lightly for style.
As drought conditions persist in the Colorado River Basin, managers, researchers and the public are all left wondering: Just how bad can the drought get?
At the Utah Water Research Laboratory, graduate student Homa Salehabadi and Director David Tarboton sought to answer this question by using historical drought data to project flowrates of the Colorado River as future drought scenarios. Guided by the idea that if it has happened in the past, it might happen again in the future, they found that future drought conditions could produce even lower flow rates than the present drought. This suggests a need for alternative management strategies in the Colorado River Basin.
“The hope is that this research provides easy-to-use, openly available, documented scenarios for water managers to evaluate different release and management alternatives,” Tarboton said.
Digging into the Data
To establish the significance of this drought in the history of the river, as well as project potential scenarios for the future, Salehabadi and Tarboton evaluated Colorado River flow rates at Lees Ferry in northern Arizona. For periods that predate modern measurements of the river, they relied on data from dendrochrolologists who have, through the correlation of tree ring widths with streamflow, been able to reconstruct streamflow back from 1416.
More recent flow rates originate from Lees Ferry, just below Glen Canyon Dam, where flow rate is measured and the natural flow is calculated by the Bureau of Reclamation. Natural flow is what the flow in the river would be, were it not for upstream diversions, storage and consumptive use. The mean natural flowrate at Lees Ferry calculated using data from 1906 to 2021 was 14.67-million-acre feet per year (maf/yr). Using more recent observations from 2000 to 2021, however, that amount dropped to 12.3 maf/yr.
For water managers and policy makers, this decrease in flowrates is cause for concern.
“We seem to be in a period of aridification, one that is perhaps a new normal,” Tarboton said. “We should expect and plan for lower uses to match reduced supply.”
Typically, the total demand for Colorado River water hovers around 15 maf/yr, while recent annual flows have been closer to 12 maf/yr. This deficit has exhausted the supply offered by Lake Powell and Lake Mead, which provide about 50 maf of combined storage.
Past and Future Droughts
Looking at tree-ring data, as well as natural flowrates at Lees Ferry, Salehabadi and Tarboton identified three major drought periods affecting the Colorado River Basin:
- A paleo tree-ring derived drought from 1576 to 1600, in which flowrates dipped more than any other period in the historical record.
- The mid-20th century drought from 1953 to 1977.
- The current drought from 2000 to the present day.
Salehabadi and Tarboton selected random flows from the records of each of these droughts and created 100 sequences of low-flow scenarios (traces) for each of the three. Each trace comprised 42 years of monthly streamflow for the Colorado River basin. The three sets of traces produced drought scenarios extending from 2018 to 2060. Salehabadi and Tarboton also compared these simulations to climate models that project future Colorado River flowrates and found that, while the climate-projected streamflows from the different climate models show considerable variability, simulations from the randomized drought scenarios are not inconsistent with the climate models. This indicates that their simulations can serve as plausible extreme scenarios for the Colorado River Basin.
The findings conclude that future droughts may be even worse than the current one and suggest that alternatives to the current paradigm for managing the Colorado River may need to be considered. This has implications for both water supply and hydroelectric power generation as reservoirs along the river may fall below minimum pool levels necessary for power generation. While it is tempting to suppose that the current drought is an extraordinarily rare occurrence that should not evoke any permanent change, this research pushes back against that notion. Results suggest that the measures we now may think of as temporary may need to become permanent.
Drought research is not new to UWRL and USU researchers. In 1995, Tarboton was part of a large multi-state Colorado River Severe Sustained Drought study led by then-UWRL Director L. Douglas James that, based on tree-ring reconstructions, contemplated a much more dire water supply scenario than had occurred in the past century. The present situation is quite similar to the severe sustained drought foreseen then. The study explored institutional arrangements regarding how the river is operated, noting that institutional inflexibility presents a significant challenge.
More recently, others at USU have added to the research informing drought management. In a series of white papers, USU’s Center for Colorado River Studies led by Jack Schmidt has evaluated the accuracy of the Bureau of Reclamation’s models, researched alternative management paradigms and streamflow losses, among other topics.
To serve as a basis for discussion and to encourage the flexibility needed to address the Colorado’s dwindling supplies, David Rosenberg at the UWRL has developed a Colorado River Basin participatory modeling activity that allows participants to explore flexible water accounting to achieve sustainable water allocation solutions. Thus, while the current drought situation in the Colorado is alarming, considerable research-based information is available from these prior and ongoing studies to help water managers work through these difficulties.
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