A shrinking Lake Powell

By Eric Kuhn

The latest Bureau of Reclamation monthly Colorado River modeling runs show an even bigger drop over the next year in Lake Powell’s elevation that previously projected. But this is not an example of bad news getting worse. Instead, a change toward a drier baseline hydrology more accurately reflects the drying of the Colorado River basin in the 21st century.

Usually, during the non-snow accumulation and non-run-off forecast seasons (August – December), the Bureau of Reclamation’s 24-month studies change very little from month to month. Today, however, the basin’s nerds (I’m on that list) that closely follow these studies were in for a sobering surprise. All three of the October studies (most probable, minimum probable, and maximum probable) showed a consistent shift to a drier future. The reason for this shift is disclosed in footnote #1 – “The October 2021 24-Month Study includes the Colorado Basin River Forecast Center shift to the 1991-2020 period of record.”  In simple terms, out are the wet years from the 1980s and in are the more recent dry years from the 2010s.

This shift to the more recent years is important for several reasons. First, the 1991 -2020 period is likely to be much more representative of the future under climate change driven aridification of the basin.  Second, given the depleted state of the system reservoirs, seemingly small changes in storage can have big operational impacts. For example, the September most probable study showed Lake Powell ending Water Year 2022 at elevation 3545’, the October most probable study now shows elevation 3534’. The change is only 11’, but it’s enough to push Lake Powell into the Lower Elevation Balancing Tier for 2023. This means that the projected most probable release from Glen Canyon Dam for Water Year 2023 has gone up from 7.48 maf to 7.818 maf. When Lake Powell is in this bottom tier, the goal is to release between 7 and 9.5 maf/year so as to equalize end of water year storage between Powell and Mead.

For another example, look at the minimum probable studies. The September version shows Lake Powell bottoming out at 3482’ while in the October version it’s 3467’.  Again, only a 15’ difference, but since the minimum power elevation is 3490’ it has a big impact on how long the power plant will be unable to produce power. Which in turn has an enormous impact on the Upper Basin Fund cash flow.  The September minimum probable showed Glen Canyon Dam not producing power for about 21/2 months. See the graph below, under the October version a Glen Canyon Dam power plant outage could be as long as ten months (if not all year).  From a cash flow perspective, the difference is about $200 million.

The shift in the period of record traditionally only happens once per decade. It is a good thing it happened this year. Given what we’ve seen over the last 20 years, the shift will add needed conservatism and urgency to the Basin’s efforts to bring our water use in balance with the available supply.

One of the central questions dimly visible in the early discussions around the upcoming renegotiation of the Colorado River’s water operations and allocations rules is the question of how bad a “worst case” scenario should be considered.

This is crucial, because it constrains what sort of questions must then be confronted. The lower the future flows considered, the more likely it is that the negotiators will have to stare down the third rail question of how much water the Upper Basin can delivery hydrologically, and must deliver legally, at Lee Ferry, the dividing point between the Upper and Lower Basins.

For the century since the Colorado River Compact was signed, we’ve avoided dealing with that central question – what happens if the river’s flows are so low that the Upper Basin cannot deliver the 7.5 million acre feet per year (or 8.25 million acre feet, we can’t even agree about which number to argue about) contemplated by the compact’s Article III.

This question is so untouchable that in work done for the 2012 Basin Study, the Bureau of Reclamation’s modelers famously added what came to be called “miracle water” at Lee Ferry every time one of their model runs dropped below the threshold that might have otherwise triggered this legal argument.

Under the low flows possible under climate change, we face a stark choice – either we reduce the Upper Basin’s Lee Ferry deliveries below 7.5/8.25 maf, or we will have to curtail existing Upper Basin uses. Advocates of modeling such low flows in the planning scenarios are essentially saying – Let’s have that conversation now.

At the tale end of yesterday’s (Friday 10/15/2021) House Natural Resources Sucommittee on Water, Oceans, and Wildlife, it was California Rep. Jim Costa, a congressman from outside the Colorado River Basin, who asked the question pointed at the heart of the matter – how do we redo water allocations that make no sense in a river much smaller than contemplated in our hallowed Law of the River?

He was addressing a panel of representatives from each of the Colorado River Basin states (his comments start around 2:30 here):

The Law of the River and the quantification of the Upper and Lower Basin states amounted to some 17 million acre feet of water that was determined at that time was the annual flow of the Colorado River, and we know that in the last two decades its been more like 12.4 million acre feet, and that doesn’t account for other Native American tribes that have reserved water right claims that have yet to be resolved. So there’s just a tremendous amount of demand. And with climate change, we know the yield is only going to decline.

This is the question I’d like to submit to all of you, and if you want to provide written statement to your answer I think we would appreciate that.

Let’s say the annual yield over the next 30 years is 10 million acre feet. I don’t know, with climate change, maybe it’s plus or minus. How do we take into account how we got to the original allocation, with the Upper and Lower Basin States and the Native tribes, the sovereign nations, and then reallocate that on a lot less water.

At this point all we can see through the public windows into discussions about next-step Colorado River management guidelines is shadow boxing on this question.

But testimony yesterday from Southern Nevada’s John Entsminger suggests the public shadowboxing we’re seeing on this question is representative of disagreements in the private discussions. (I quote here from John’s written testimony.)

Despite the fervent warnings from internationally renowned scientists like Jonathan Overpeck and Brad Udall that urge us to plan for a future with even less than 12.3 million acre-feet, the river community is far from consensus about how dry of a future to plan for. And, while this panel was asked to talk about drought, on-the-ground evidence suggests the Colorado River basin is not experiencing drought but aridification – a permanent transition to a drier future. If we are to build upon the river’s many successes over the last 25 years, we must confront the magnitude of the challenge in front of us and quickly reach agreement on what future scenario we’re willing to plan for. (emphasis added)

Speaking two weeks ago at this year’s Getches-Wilkinson Center conference in Boulder, Entsminger put a number to Southern Nevada’s thinking. In the next iteration of its long range water resources plan, Entsminger’s Southern Nevada Water Authority will include a “what if” planning scenario for how the agency would deal with an 11 million acre foot per year Colorado River. This is not to say that Southern Nevada expects an 11 million acre foot river, but rather than it believes it needs to have a plan in place should that happen.

I could be wrong, but so far I’ve seen no public evidence that any of the states of the Upper Basin are willing to entertain flows that low in the planning scenarios to be considered in the modeling done to support the upcoming negotiations. I look forward to seeing the written answers the basin states’ representatives submit to Costa’s question.

At last week’s Getches-Wilkinson Center conference in Boulder, attorney Jay Weiner, who represents tribes (but was careful to say he was not speaking on any particular tribe’s behalf) made an important point, which is repeated in this excellent piece by Mark Armao this week in Grist:

“The basin is free-riding off of undeveloped tribal water rights,” said Jay Weiner, an attorney for the Quechan Indian Tribe. Weiner said there is a “fundamental tension” between tribes’ desire to fully develop their water rights and the overarching need for everyone in the basin to consume less water overall.

Courtesy Dave Kanzer and Eric Kuhn, from Eric’s 2013 Colorado River Water Users Association presentation

The Bureau of Reclamation Colorado River team did something remarkable in yesterday’s release of its new 5-year reservoir levels analysis – the “stress test”, a methodology pioneered a decade ago by an Upper Colorado River Basin technical team that included John Carron of Hydros and Eric Kuhn and Dave Kanzer of the Colorado River District is now the “new normal”, to borrow a terrible phrase. From the “5-year projections approach tab” here:

The method used to generate future inflows in the current projections includes resampling a subset of the historical natural flow record (1988-2019) using the Index Sequential Method (ISM), referred to here as “Stress Test” hydrology. In the past, the full historical record (1906-2019), known as the “Full” hydrology, was used to provide 5-year probabilistic projections. The Stress Tests hydrology scenario applies ISM to a shortened period of the natural flow record, 1988-2019, which removes the earlier portion of the natural flow record and focuses on the recent (approximately 30 years) hydrology. This period has a 10% drier average flow than the Full hydrology. Use of the Stress Test scenario is supported by multiple research studies that identified a shifting temperature trend in the Colorado River Basin in the late 1980s that affected runoff efficiency and resulted in lower average flows for the same amount of precipitation (McCabe et al. 2017, Udall and Overpeck 2017, Woodhouse et al. 2016).

The idea is that the traditional approach – using the entire period of record to model the probabilities of future river flows – is not longer valid because climate change is changing the river.

John, Eric, and Dave reasoned nearly a decade ago that using a shorter record, focused on our climate-changed Colorado, might better help managers think about and plan for what to expect next. (Dave also famously provided the memorable Homer Simpson image for Eric’s CRWUA presentation).

The “stress test” has been creeping into basin management discourse for a while, and Reclamation had already begun publishing stress test scenarios alongside. But the new 5-year flow and reservoir level estimates now are all in on the stress test.

The stress test may not be stressful enough, which was one of the implicit messages in the editorial Brad Udall and I published in Science magazine in May, and which Brad and I made more explicit here. But this use of the stress test is nevertheless hugely important, kudos to the Reclamation technical and management team for this important step.

By Brad Udall, posted with permission, from his Aug. 18, 2021 presentation to the Colorado Water Conservation Board

Colorado State University’s Brad Udall has been doing some really interesting thinking about how to conceptualize and communicate climate change risk to water supplies in the Colorado River Basin.

Shown above (and shared with permission) is one of Brad’s “selected averages” graphs. The horizontal lines show the average river flow value for a period of interest – the entire period of record, for example, or the 21st century. (It’s from a presentation Brad gave to the Colorado Water Conservation Board earlier this month.)

While the general pattern will likely be familiar to people working on Colorado River issues – wetter in the long run, drier in recent decades – the value being graphed may not be. It’s “unregulated inflows”, which is the number calculated by the Bureau of Reclamation and the Colorado Basin River Forecast Center to estimate the actual inflows into Lake Powell, minus the impact of upstream reservoir operations.

It’s the best number to use if you want to look at the the actual water flowing into Lake Powell, and therefore available to meet compact-driven Upper Basin commitments to deliver water downstream to the Lower Basin at Lee Ferry. It combines both natural flows, and Upper Basin consumptive uses.

The key number to look at here is the one in the blue box. Since 2000, the average unregulated inflows have been 8.38 million acre feet per year. The Upper Basin’s Lee Ferry compact delivery “obligation” (lawyers please don’t subpoena me on this point, I ) is 8.23 maf, which means that we’re right on the edge of trouble even now, in terms of the Upper Basin’s ability to meet the expected deliveries to users in the Lower Basin. Increased Upper Basin use, or decreased flow (see Brad’s 6.66maf for 2018-2021) would cross that trouble threshold.

What this means is that, based on 21st century hydrology, the Upper Basin is already caught in what Doug Kenney and colleagues dubbed “the Upper Basin climate change squeeze”.

This is what Brad and I were getting at in our May editorial in Science. Lower flow scenarios are entirely credible based on the best available climate science and the hydrology we are seeing. We need analyses – modeling runs – that consider them as we prepare for the next round of river management negotiations, so that we know what will do if Brad’s scary “6.66” is a harbinger of our future.