By Brad Udall and John Fleck

Earlier this year, we argued in a Science magazine editorial that Colorado River forecasting must take the growing risk of climate change seriously. The latest five-year projections from the U.S. Bureau of Reclamation offer a practical example of the challenge.

Published July 8 (see here and here) with an accompanying news release, the projections suggested that if the trends of the last 30-plus years continues, there is a 79 percent chance that Lake Powell could drop next year below elevation 3,525 – a danger zone for managing power production and releases to the Lower Basin going forward. With the reservoirs behind Hoover and Glen Canyon dams expected to drop below 30% by early 2022, these projections take on a new importance — we no longer have a huge water buffer to protect us from future low flow years.

It is stark news. But perhaps not stark enough.

This forecast takes advantage of an important new tool Reclamation has invested in called the “Stress Test” to give us a sense of the future risks we face.

The Stress test goes beyond the old “the future will be like the past” scenario building we have used in the past on the Colorado River. This new tool takes an important step toward incorporating climate change. But we are concerned that it doesn’t go far enough.

The five-year projections come in two flavors. One, “the future will be like the past,” uses the historical hydrology since 1906 with a mean flow of 14.8 maf. In a stationary climate, this hydrology would be fine. But the climate is not stationary, and the only real use for this hydrology is to see just what we’ve lost as climate change saps the river, not what the future might hold.

The alternative ‘Stress Test’ hydrology uses the period from 1988 to 2019 with an annual flow of 13.3 maf. While more reflective of current conditions than the full hydrology, these flows also do not reflect the past 22 years with its annual runoff of 12.4 maf.  When river managers first began using it, the “Stress Test” marked an important step toward taking climate change seriously. But these flows are no longer what they purport to be.

What we really need, and what we argued for in our Science editorial, is a ‘reasonable worst case future’.  This is the future that a prudent person would plan against, knowing what we currently know.

The last 22 years are the best analog for our 5-year future.  And within those 22 years one period stands out as the worst, the period from 2000-2004.  These years averaged 9.4 maf, and during that time Powell and Mead lost ~25 maf.   This period is what a prudent person would pick as a reasonable worst case — it happened before and it can happen again. In fact, the extreme dry of 2020 and 2021 suggests it may be happening now.

It would be interesting to see how Reclamation’s model performs against 2000-2004 and also against 2000-2021.

Within the Colorado River management community, there are questions about these modeling exercises on the demand side as well. Are water uses across the basin overstated? Might that at least partially offset overly optimistic supply estimates?

Pat Mulroy, after being burned by false probabilities, famously said that as a water manager she was only interested in possibilities, not probabilities.  The hydrology from 2000-2004 is a possibility. Let’s learn the lessons it holds.

Not gonna lie – watching Colorado River reservoirs decline so precipitously has been painful.

But it is important to cultivate optimism, and there is, in fact, reason to be hopeful about our ability to deal with the challenges. That’s the message the University of Arizona’s Bonnie Colby and I shared in a recent conversation with Sarah Bardeen at the Public Policy Institute of California:

Bonnie Colby: Everybody knows we’re moving into a serious situation. State and federal officials have been tracking reservoirs and groundwater levels, and tribal nations are involved in a way that they never have been before. That’s much needed, from a social justice perspective, and because they’re holders of the most senior rights in the system. In Southern California, 20 years ago, all the water users were much more likely to lean on their legal entitlements and litigation. We see much more flexibility nowadays—there’s been big progress.

Brad Udall on twitter yesterday ran through a striking series of graphs of the current state of the Colorado River. With his permission, I’m posting them here along with a slightly polished version of his accompanying commentary. Some key points that grabbed my attention:

• Second-lowest Powell inflow in a period of record we use dating to 1964.
• Risk of Powell dropping next year to levels that could jeopardize power production
• Risk of Mead dropping low enough in the next 18 months to trigger much deeper “Tier 2” reductions to Lower Basin water users in 2023.

By Brad Udall

Reclamation’s ‘unregulated inflows’ into Lake Powell show that 2021 will be the 2nd worst year after only 2002 going back to 1964. 2021 will be the RED bar most likely. This is a really grim year for runoff.

Lake Powell inflows

2021 inflow will be only ~3 maf, compared to the 1981-2010 average of 10.3 maf or the 2000-2021 average of 8.3 maf (20% less than 1981-2010 average).(maf = million acrefeet)

Considering that Powell will release or lose to evaporation ~ 8.5 maf, the lake will lose ~ 5 maf this year or ~55 feet of elevation.

April 2021 snowpack above Powell peaked at ~85% of normal but will generate about 25% of normal river flow. This comes on top of April 2020 snowpack of 100% of normal that generated about 50% of normal flow.

Declining runoff efficiency has been noted in multiple peer-reviewed studies. For a recent overview of recent climate change studies on the Colorado River see this written with Jonathan Overpeck:

Multiple studies since 2016 have now found human fingerprints on the nearly 20% loss in flow since 2000 and attribute up to half of that loss to the approximately 1.2°C or more warming that has occurred during the last century.

Jeff Lukas points out that the twitter thread implied that the low runoff efficiency this year as measured by runoff as a percent of snowpack is all due directly  to warming. I did not mean to imply that. The low runoff percent numbers are much more a function of (1) very low spring precipitation in both 2020 and 2021 and to a lesser extent (2) low soil moisture from the previous year.  It may be that there is a human-caused connection to the low spring precipitation although there’s no real evidence of this yet.  Low soil moisture in the springs of 2020 and 2021 is definitely connected to dry and very warm late summer and early fall from the previous years.  Teasing this apart to obtain the actual driver(s) is not simple. That said, no one should doubt that climate change is reducing the flows of the Colorado. Multiple peer-reviewed papers have now supported this finding.

More from Jeff on this here.

Here’s what’s going to happen to the nation’s 2 largest reservoirs because of this measly inflow:

Losses to Mead and Powell

Note that combined contents will drop below 30% by late next year.

Here’s that decline for all years back to 1935 when Mead first filled. These two reservoirs will hold less water than Mead did alone in many years before 1964 when Powell was built.

Combined storage, Mead and Powell

By next April Powell will hit 5.4 maf, ~185 feet below full. See Red dots. This will be the lowest since its initial fill in 1964. Since 1999, Powell will have lost ~18 maf, 75% of its contents.

At 5.4 maf Powell could be low enough to lose the ability to generate power. (We’re uncertain about how penstocks operate when lake gets low — water in penstocks can not be aerated or turbine damage will occur.)

Loss of power, while not calamitous, is concerning. Power revenues fund environmental compliance and other important items in the basin.

As part of the 2019 agreement, the UB can release flows from reservoirs upstream of Powell to prop it up. But there is only about 5 maf for that all together. It is a one-shot deal.

We’ll have to wait on next winter to understand what happens after April of 2022. But 5.4 maf is very little water in a 25 maf reservoir.

So what about Lake Mead, the nation’s largest reservoir?

Reclamation’s current forecasts show January 1, 2022 elevation at 1065’ feet (8.8 maf) , well below the 1075’ needed to avoid a ‘Tier 1 Shortage’.

Decline of Lake Mead

If it ends up below 1050’ on Jan 1 (as projected in August 2022), that will lead to a Tier 2 Shortage (total cutbacks of 721 kaf). Otherwise, Mead will face a 2nd year of Tier 1 shortages. Either way, this is not good.

Thanks to Megan Kamerick and KUNM, our New Mexico public radio juggernaut, for offering the platform and leverage to help boost our message about climate change response on the Colorado River:

[N]obody’s going to sort of voluntarily raise their hand and say, ‘Yeah, we’re happy to have less.’ And so negotiating those agreements where everybody agrees to live with less and agrees on a set of numbers, or what that might look like, is really going to be the hard part. Because I’m really confident that, you know, once we come to agreement on what the allocations are going to be, we’ll learn to live with them.

We’re really adaptable.

Here’s the Fleck/Udall editorial in Science magazine that triggered Megan’s piece.