#LakeMead likely to drop below elevation 1,040 by late 2023 — John Fleck #ColoradoRiver #COriver #aridification

Graphic credit: USBR

Last week John Fleck took a look at the April 15, 2021 Colorado Basin River Forecast Center’s 24-Month Study. Click here to read the post. Here’s an excerpt:

I’m choosing my words carefully here. The “likely” in this blog’s post’s title means “based on my analysis of the Bureau of Reclamation’s current ‘most probable forecast’ Colorado River water supply model runs.”

The Bureau’s current “most probable” modeling suggests that in both 2022 and 2023, the annual release from Lake Powell will only be 7.48 million acre feet. This is based on a provision in the river’s operating rules that, under certain low storage level conditions, the Upper Basin gets to hang onto water in Powell.

The last time and only time we had a 7.48 release, in 2014, Mead dropped 25 feet in a single year. We’ve never had two consecutive 7.48 releases.

The headline in yesterday’s release of the Bureau of Reclamation’s “24-month study” (pdf here) is that Lake Mead will drop below elevation 1,075 at the start of 2022 (triggering a “Tier 1” shortage) and could drop below 1,050 by the start of 2023 (that’s the trigger for “Tier 2”).

Tier 1 next year, which primarily hits Arizona with some deep forced reductions, was no surprise. That’s been obvious for a while, and Arizona’s water leadership has been softening folks up for months. The increasing risk of Tier 2 in 2023, which would mean deeper cuts in Arizona, is sorta new, but it’s been foreseeable.

The real “holy shit” for me in yesterday’s release was the trail of breadcrumbs in the Bureau’s data, pointed out by my co-author Eric Kuhn, leading to a “most probable” Lake Mead drop to elevation 1,035 by the end of September 2023.

To be clear, the Bureau isn’t saying this yet. The latest 24-month study stops at the end of March 2023. But internally, the Bureau runs the model out farther in order to determine, among other things, how much water is likely to be released from Powell in 2023. And the published numbers clearly show – the Bureau’s “most likely” scenario would call for another 7.48 release.

From there, it’s just arithmetic. Based on my analysis of the publicly available numbers, the “most likely” scenario puts Mead at elevation ~1035 at the end of September 2023. This is my math, but my understanding is that it’s consistent with what the Bureau’s internal calculations show.

Lake Powell is seen in a November 2019 aerial photo from the nonprofit EcoFlight. Keeping enough water in the reservoir to support downstream users in Arizona, Nevada and California is complicated by climate change, as well as projections that the upper basin states of Colorado, Utah, Wyoming and New Mexico will use as much as 40% more water than current demand. A recent white paper from a lineup of river experts calls those use projections into question.
CREDIT: ECOFLIGHT via Aspen Journalism

Eric Kuhn followed up John’s post with one of his own. Here’s an excerpt:

The release of last week’s Bureau of Reclamation 24-month study felt like very bad news for the Colorado River (See Tony Davis for details.). But a careful reading of the numbers, and an understanding of the process through which they are developed, suggests things are likely even worse than the top-line numbers in the study.

The problem: the assumptions underlying the study do not fully capture the climate-change driven aridification of the Colorado River Basin. Taking climate change into account, it is easy to find evidence lurking in the report to suggest that, in addition to problems for Lake Mead, Lake Powell could drop below elevation 3,525 in 2023, a level that is troublingly close to the elevation at which Glen Canyon Dam could no longer generate hydropower.

The 24-month studies are used to project out two years of monthly inflows, releases, storage levels, and power generation from the system’s large reservoirs in both basins as well as diversions by the large water users on the river below Lake Mead, especially the Central Arizona Project and the Metropolitan Water District of Southern California. Reclamation releases a “most probable” study on a monthly basis as well as “minimum probable” and “maximum probable” studies approximately quarterly. These studies are important because they are used to make critical decisions under the 2007 Interim Guidelines and both the Upper and Lower Basin Drought Contingency Plans (DCPs).

For the first year, Reclamation uses “unregulated” runoff forecasts generated by the Colorado Basin River Forecast Center (CBRFC) model. Unregulated inflow is not the same as natural inflow. The CBRFC does its best to adjust the forecasts for upstream diversions and for the many reservoirs that are not included in the 24-month study model. Inflow forecasts for the second year of the 24-month studies are not based on the CBRFC model. Instead, Reclamation, in consultation with CBRFC, uses statistics from the past and its judgment. Running the 24-month study model then simulates the operation of the upstream reservoirs such as Navajo, Blue Mesa, and Flaming Gorge, turning unregulated inflow to Powell into “regulated” inflow. For example, from the April ‘21 most probable study, the WY 2021 unregulated inflow to Powell is 4.897 MAF, regulated inflow is 4.908 MAF. These numbers are close, but in WY 2020 regulated inflow exceeded unregulated inflow by about 700,000 acre-feet.

The media buzz over the April 24-month study primarily focused on the projected Tier 1 shortage for the Lower Basin in 2022 – an event that is newsworthy, but one that also was totally expected. Perhaps more interesting and alarming is what the 24-month studies suggested for 2023. As pointed out by John in his recent blog, the most probable study shows two years of 7.48 MAF releases from Lake Powell, Lake Mead elevations on the cusp of a Tier 2 shortage in 2023, and by inference, Lake Mead dropping to a level of about 1035’ by the end September 2023, which by implication would trigger a third straight shortage year and California’s possible participation sharing shortages under the Lower Basin DCP.

For Lake Powell, the most alarming results come from the minimum probable study, not the most probable study. Under the minimum probable inflow forecast to Powell, which, in theory, represents an unregulated flow that would be exceeded in 90% of years, by March of 2023 Lake Powell drops well below the 3525’ target that would trigger supplemental releases from the upstream CRSP reservoirs under the Upper Basin DCP. There is also a real possibility that Lake Powell could end up in the Lower Elevation Balancing Tier. If this happens, the April minimum probable study shows that Lake Mead gets more water in the first six months of WY 2023 than under the most probable study.

The term “minimum probable” implies an outcome that is very unlikely to occur, therefore, why should we be that concerned? My answer is that given the abundance of recent science concluding that the Colorado River Basin is not in a classic drought, but rather, it is undergoing aridification where the flows seen in the last two to three decades may be the new abnormal and may continue to decline (see for example Overpeck and Udall, and the latest Utah State Future of the Colorado River white paper White Paper). The April studies show a most probable Powell unregulated inflow for WY 2022 of 9.998 MAF and a minimum probable inflow of 7.208 MAF. For comparison, the mean unregulated annual inflow to Lake Powell over the last ten years, including WY 2021, was only 8.04 MAF and five of the individual years; 2012, 2013, 2018, 2020, and 2021, were well below the 7.208 MAF. The average of those five dry years was 5.08 MAF, over two MAF less than the assumed minimum probable inflow for 2022. If you take the record back to 2000, the results are similar. In 11 of 22 years, unregulated inflow to Lake Powell was less than 7.2 MAF/year.

Based on the last 20-plus years and the recent science, I conclude that both the minimum probable and most probable 24-month study year two unregulated inflows to Lake Powell are overly optimistic. The likelihood that in the next few years Lake Powell storage will fall below the 3525’ target or even the minimum power elevation (3490’) and that Lake Mead storage will approach 1025’, the level that triggers the maximum annual cutbacks under the Interim Guidelines and DCP, about 1.4 MAF, is much greater than what is conveyed by these studies.

In a photo from 2020, a distinct line around the rocky shore shows how much the water level has decreased in Nevada’s Lake Mead. Mead is expected to drop 15 feet in 2021 Photo credit: Roberto (Bear) Guerra/High Country News

Finally, Click here to read Tony Davis’ article at Tucson.com. Here’s an excerpt:

The Central Arizona Project seems almost certain to suffer its first significant shortage in water deliveries next year.

Reservoirs are expected to fall so low by the end of 2021 to warrant cutting nearly two-thirds of the CAP water that Pinal County farmers now get. At that point, CAP deliveries used by the state to store water in the ground for future use by cities and tribes would also be cut. So would CAP water supplies sold to the Central Arizona Groundwater Replenishment District, an agency that recharges water into aquifers across the state’s urban centers to compensate for groundwater pumped elsewhere for new development.

The loss for farms has been expected for years. But possible cuts for other water customers now loom sooner than anticipated, as the Colorado River’s situation worsens.

For the first time, a federal agency’s river forecast predicts that at the end of 2022, the Lake Mead reservoir will be at or very near a point where CAP must cut deliveries to other categories of water users.

Those cuts would fall upon Phoenix-area cities and on Arizona tribes, including possibly the Tohono O’Odham whose reservation is south and west of Tucson.

If they happen, the cuts would also start slicing deliveries of relatively small amounts of CAP water to Rosemont Copper and Freeport McMoran Copper in the Tucson area and to Resolution Copper in the Superior area.

Tucson depends on CAP for drinking water, but its supplies wouldn’t yet be affected.

The cuts to farmers will be required if Lake Mead falls below 1,075 feet at the end of this year. The Bureau of Reclamation’s new forecast — announced Thursday for the river — puts the expected level at 1,067 feet by then. The bureau will likely decide in August whether to declare a shortage for 2022.

The additional cuts to tribes and to Phoenix-area cities would be required in 2023, if Lake Mead falls below 1,050 feet. The new forecast is for the lake to be at 1,050.31 feet by December 2022.

Tucson’s CAP supply wouldn’t be cut unless the lake fell below 1,025 feet.

Leave a Reply