#ColoradoRiver Forecasts Not a ‘Crystal Ball’: Computer models inform key decisions in the Colorado River basin. But they cannot predict the future — Circle of Blue #COriver #aridification

From Circle of Blue (Brett Walton):

Every month the Bureau of Reclamation attempts to peer two years into the future of the Colorado River and its reservoirs.

Reclamation’s 24-month study is a staple forecasting product for the federal agency that manages a chain of dams in the watershed, including those that control lakes Mead and Powell, the country’s largest reservoirs — and currently two of its most consequential. The reservoirs are a key source of drinking water for about 40 million people, plus they store water that irrigates millions of acres of farmland and generates electricity for the Southwest. The reservoirs are also alarmingly dehydrated right now — about one-third full, the lowest since they were first filled. The entire basin is on alert.

The 24-month study, in the simplest terms, projects water levels for the next two years at 12 federal reservoirs in the Colorado River basin. Produced monthly, it’s one of several forecasting products that give water managers a sense of possible futures. It is also the foundation of essential water management decisions in the basin. Reclamation’s other forecasts, updated less frequently, look at mid-term (five years out) and long-term (multiple decades) scenarios.

Typically nested in wonkish obscurity, the 24-month study acquired newfound public prominence in recent weeks. The August results are the most important of all the months because they determine how much water will be released in the following year from Mead and Powell. Because Mead is so low, the August results triggered the first-ever Tier 1 shortage on the lower Colorado River, a declaration that means mandatory cuts in water deliveries in 2022 to Arizona, Nevada, and Mexico. Because Powell is so low, dam managers will release a comparative trickle of water next year, so little that Mead is likely to plunge even lower.

More eyes than usual on a technical product that was designed to guide reservoir operations means more potential for misinterpretation, especially by people unfamiliar with the study and its assumptions. Carly Jerla, Reclamation’s senior water resources program manager, said that the study has its defined uses but also its limits.

“It’s important to understand that we’re not saying that this is what we think is going to happen this year,” Jerla told Circle of Blue about the reservoir levels outlined in the 24-month study. “We’re not saying, ‘Plan for this and only this because we have crystal ball knowledge of what is going to happen.’”

Reclamation’s models, in fact, are not a crystal ball. Critics say that they are not pessimistic enough about the potential for extremely dry years. But as the Colorado River basin dries due to a warming planet, Jerla and others are actively considering how best to convey to the public and water managers alike the looming risks to water supplies and to prepare people, at least mentally, for the possibility that reality could turn out much worse than the forecast had projected…

Accurate mid-term weather forecasts, those that extend out a couple weeks and up to a year, are notoriously difficult to achieve, said Jeff Lukas, an independent climate researcher in Colorado who has worked in the basin for 20 years. It’s especially true in the mountainous terrain of Colorado and Wyoming, where the Colorado River and its main tributaries have their headwaters. Well-known seasonal patterns like the cyclical warming of the eastern Pacific during El Nino years can indicate wetter or drier, but without substantial precision.

Because the future is hazy, the models instead rely on the recent past as a guide, Lukas said. “We’re basically saying in the absence of real prognostic information, we’ll substitute history.”

Here’s how that works. The 24-month study process begins with the Colorado Basin River Forecast Center, a team of scientists operating within the National Oceanic and Atmospheric Administration. Their task is to assess what the rivers might do.

The River Forecast Center starts with current land and water conditions: soil moisture, snowpack, stream flow. From that baseline hydrologists feed their model, one by one, with historical weather observations from the years 1981 to 2015. In effect, it’s as if the temperature and precipitation from each year were repeated in today’s world. That produces 35 possible hydrological futures, each representing the past laid on top of the present.

The Bureau of Reclamation team takes those hydrological futures and uses them in its model of the Colorado River system. The aim of this system model, which includes water inflows and water uses, is to simulate reservoir levels as well as hydropower generation. Fed by the output from the hydrological model, the system model also produces future scenarios, called runs.

The middle result — the most probable — is the one that is presented in the main 24-month study report. It’s the result that determines how Mead and Powell will be operated. It’s called the most probable because it’s in the middle, if each of the runs was ordered from wettest to driest. It means that historically half the time it was wetter than the middle result and half the time it was drier.

There are drawbacks to this approach. The runs are not assessed as to how likely they are to occur, which means that a repetition of each of the past years is considered equally likely. The problem: there are more wet years in the 1981 to 2015 period than dry ones. (Runoff in 1983, for instance, was so extremely high that it almost broke Glen Canyon Dam.) Because of this imbalance, the middle result is arguably skewed toward wetter conditions, Lukas said.

Jerla said there is no scientifically valid way to privilege the likelihood of one outcome over another.

An update of the River Forecast Center’s data sets will soon help reduce the skew. Cody Moser, a senior hydrologist at the Center, told Circle of Blue that data from the years 2016 through 2020 will be added this fall. Instead of 35 historical hydrologies fed through the models, there will be 40. Adding the drier recent years will push the most probable outcome to a drier result, with reservoir projections in the 24-month study likely pushed downward at the same time…

The 24-month study is most associated with the most probable scenario. But recently Reclamation has expanded its offerings to include two other reservoir scenarios, now produced monthly: the minimum probable and the maximum probable. The minimum probable is the tenth percentile, meaning the third or fourth driest of those 35 historical hydrologies. The maximum probable is the ninetieth percentile, or the third or fourth wettest historical hydrology.

For the minimum probable, the tenth percentile of flows is used in the first year of the 24-month study, but the second year of the study is calculated with the twenty-fifth percentile, under the assumption that consecutive extremely bad years would not happen.

If you look only at the most probable result, you’re not seeing how bad things might plausibly get. Lake Mead today, when it is one-third full, sits at elevation 1,068 feet. The most probable elevation for July 2023 is 1,037 feet, when Mead would be 26 percent full. The minimum probable elevation for that date is 1,027 feet, when the gasping reservoir would be 23 percent full.

That’s a significant difference in elevation, and if the drier scenario came about it would change basin operations. But even the minimum probable has a flaw. It is not as pessimistic as it could — or maybe should — be.

“The minimum probable does not represent a worst-case scenario,” Jerla said. “If you wanted to be the ultimate pessimist, which I think probably makes sense given where the system is, things could be worse than what is provided in that minimum probable because it is only the tenth percentile.”

In other words, the minimum probable is not the minimum possible. There have been years in the last two decades with worse river flow conditions than what is represented by the minimum probable. Things could be drier still.

As Jerla puts it regarding the minimum and maximum probable: “There is an area above and below those that have futures that folks should be aware of.”

[…]

Reclamation already runs its five-year projections with what it calls “stress test” hydrology. This scenario is a replication of the years 1988 to 2019, and represents hotter and drier conditions that have settled over the basin. Udall and others argue that Reclamation should consider running its models with an even more stressful test: the years 2000 to 2004, the last time that Lake Powell almost crashed.

“That five-year period is really unique,” Udall explained. Annual runoff averaged 9.4 million acre-feet, a fraction of what coursed through the river throughout the last century. The average annual runoff in the years for the stress test hydrology is about 13.3 million acre-feet. That makes the first years of this century unique, Udall said. And frightening. “There’s nothing like it in the 20th century. It’s stunning how bad a period it is, and we could be in the middle of that right now.”

[…]

The next update of the five-year projections will come out in early September. Five years is a tricky time frame to analyze, Jerla said. It’s far enough in the future that current conditions lose their predictive power. But it’s close enough to be relevant for farmers, city utilities, and marina operators — all of whom need to plan for near-term water supply. Jerla and her colleagues are trying to thread that needle, thinking hard about how to “provide the public with a good understanding of future outcomes and future risks without confusing the heck out of them.”