The Shark is Upon Us

Lake Powell’s water level continues to fall behind Glen Canyon Dam due to overuse of the Colorado River. The Bureau of Reclamation projects that it will lose another 21 feet of water by the end of this year, leaving it at 41% of its storage capacity. Photo by Brian Richter

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“Climate change is the shark in our future and water is its teeth.” — Director General of Mexico’s Water Ministry at the 2012 water and climate conference in Mexico City.

Three years ago I read a remarkable paper written by Brad Udall and Jonathan Overpeck that documented the influence of climate warming on the Colorado River’s flow during recent decades. These scientists found that the river’s flow from 2000-2014 had decreased by ~10% due to a warmer climate that is causing more of the Rocky Mountain snowpack to evaporate before melting and causing soils to dehydrate faster. They extrapolated what they had observed from the past decades and projected that with continued climate warming the river would dry by another 10-25% by mid-century, a forecast that has subsequently been confirmed by other climate scientists.

This is a simple pictorial representation of what climate warming is expected to do to river flows (based on 10-year running averages).

The historical record of natural flows in the Colorado River shown here is based on 10-year running averages at Lees Ferry. The climate change projections illustrate the average declines in river flows across the range of potential climate-warming effects presented in Udall and Overpeck (2017).

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Reading the Udall & Overpeck paper fundamentally changed the way I think about water management challenges in the Colorado River basin. Until that time I was focused on the overdraft of the river due to human over-consumption, and the resulting depletion of Lakes Mead and Powell. Reading their paper made me realize that our challenge is not just to rebalance the water budget in its present state; true long-term sustainability is going to challenge us to adaptively reduce our use to an increasing degree to stay ahead of climate change.

Climate change is not something out on the horizon. The shark is upon us.

Ever since reading that paper I’ve been wondering if these recent climate projections are being adequately factored into planning and negotiations over how to manage the river in the future. When water managers talk about the need (or lack thereof) for water-saving “demand management” programs, are they explicitly accounting for the fact that we’re not going to have as much water in the future as we do now?

I recently decided to do my own estimation of the implications of this river-drying trend for the balance of supply and demand in the river system, to help me better understand the potential magnitude of water management challenges in coming decades.

Possible Futures for Lake Mead and Lake Powell

Any future projections rely upon a set of key assumptions. Here are mine:

My analysis is based on long-term averages for supply and demand out to the year 2050. We’ll continue to have a lot of variability in water availability and use from year to year, but my analysis focuses on longer-term averaged trends. An important recent paper by the Center for Colorado River Studies at Utah State University provides insights into the potential future vulnerability of the reservoirs during severe drought periods that could be much worse than my long-term averages.
Future precipitation will not change substantially from the recent past.
Water use in both the Upper and Lower Basins will remain the same as the recent 5-year averages.
I used the Bureau of Reclamation’s most recent 24-Month Study to estimate reservoir levels for the next two years.
The mandatory reductions in water use associated with different water levels in Lake Mead (known as the Lower Basin Drought Contingency Plan) will stay in place; these DCP rules mandate cutbacks to each of the Lower Basin states and Mexico as the water level in Lake Mead falls.
Mindful of the huge effect of variable Lake Powell releases to the Lower Basin and Lake Mead’s water level, I looked at two levels of annual releases: an average of 8.23 MAF/yr or 9.0 MAF/yr.
I looked at the range of climate-warming effects that Udall and Overpeck projected between now and 2050: a 20% reduction in the long-term average (1906-1999) river flow, and a 35% reduction.
I assumed that once Lake Powell drops to elevation 3490 (4.0 MAF in storage), very little water can be released from Glen Canyon Dam to the Lower Basin because the water level will be below the dam’s hydropower intakes.

The Key Take-Aways

You can see the results in the four graphs below. My conclusions:

Under the best-case scenario for Lake Powell (loss of 20% to climate warming, dam releases to the Lower Basin average 8.23 MAF/year), the Lake recovers for a bit and then starts to decline as human uses and reservoir evaporation exceed the climate-impacted flows in the Upper Basin.
IF the climate-induced river flow reductions are greater than 20%, OR IF the releases from Lake Powell to the Lower Basin are greater than 8.23 MAF/year, Lake Powell tanks within the next couple of decades. It’s important to note here that the Lake lost another 16% (2.0 MAF) of its remaining volume this year; with three more years like this one the Lake will be functionally dry.
The Lower Basin DCP does an excellent job of stabilizing Lake Mead’s levels under all scenarios, EXCEPT when Lake Powell drops to elevation 3490 (only 15% of storage capacity remaining), at which time Lake Mead tanks as well because not enough water can be released from Lake Powell to serve the Lower Basin’s needs
Given the massive economic, environmental, social and cultural risks associated with reservoir drying, I cannot fathom why an aggressive demand-reduction program has not yet been implemented in the Upper Basin as a strategy to prevent Lake Powell from dropping to a critically low (e.g., 3490) elevation. Lake Powell has been losing an average of 600,000 AF/year since 2000 (and lost 2 million AF this year), and this loss rate will accelerate with climate warming if water use is not reduced accordingly.
The Lower Basin states need a definitive plan for implementing the large reductions called for in the DCP. The DCP was a giant leap forward, but now it’s time to examine and communicate which specific uses of water will be curtailed at each level mandated in the DCP. A thoughtful, coordinated plan of response will help to minimize economic and environmental disruption and the associated public alarm over these impacts.

As always, please let me know if I’ve gotten anything wrong or you’ve got a different perspective to share. (please use the comment box below)