California’s climate is changing, with atmospheric rivers and extreme events occurring more frequently. This is not the new normal; we’re just getting started, says state climatologist Dr. Michael Anderson in this presentation at the California Irrigation Institute’s 2024 annual meeting.
In this presentation, Dr. Anderson talked about how climate change is altering water availability, the rise in extreme events, the increasing variability of the climate, and the importance of flexible water management strategies.
How a snowpack does (or does not) develop
As with everything, geography is important, and California’s water story depends on where the state sits on the planet along with the features of the landscape. For example, 95% of the Sacramento watershed is below 7500 feet, so when a storm comes in with a freezing elevation of 9000’, almost all of the watershed contributes to runoff right away. 
In contrast, half of the San Joaquin basin is above 9000’, which is why it is a snow melt-dominated watershed with peak runoff in May, and the infrastructure is built around metering out snowmelt rather than managing the large floods that Sacramento River can produce. Another key feature is the Golden Gate, which has a one-mile terrain gap, which is important when large amounts of water vapor are moving near the earth’s surface. All of that plays a key role in the timing, pace, and scale of storms that build the water year.
“We’re managing for benefit versus mitigating hazard and how that might play out as we keep moving through the water year,” said Dr. Anderson.
The new water year starts on October 1st. Sometimes, the precipitation comes early, sometimes late, sometimes very late. When the storms come later in winter months, it can be problematic because California is tilting away from the sun; the days are getting shorter, the sun angle is lower, and the higher elevations are colder. If the snow falls on dry soil, that will diminish the runoff. Dr. Anderson said that’s good to know, but quantifying that is very difficult.
On average, half of the annual precipitation will occur in December, January, and February usually in only 14-21 days of rain. If it’s an atmospheric river, it can deliver a tremendous amount of water, which is where balancing hazard mitigation with capturing water comes into play.
The spring can bring a ‘March Miracle’ or an early shutoff, such as in 2021 when very little precipitation fell after New Year’s Day. Knowing when the peak snowpack happens and how big it is is vital for understanding how much water will be available for beneficial use as the state heads into the dry season.
In the dry season, the concern is how quickly the landscape dries out and the timing of the dry down. If it’s slow, rangeland peaks in May and turns golden as it nears fall. Managing the landscape is a bigger challenge if it’s already into deep dryness by June.
Looking ahead to next year and the coming decade, forecasting remains a challenge. Dr. Anderson noted the difficulty in predicting outcomes even 90 days in advance. The uncertainty surrounding future changes makes it crucial to consider flexibility in our planning efforts.
The narrative of climate change
With climate change, the state must prepare for warmer temperatures, greater variability, more extremes, longer dry seasons, rising sea levels, and ecosystem changes.
“But it isn’t the new normal,” said Dr. Anderson. “This is the jumping-off point. It doesn’t stop here; we’re just getting moving. So things are going to get really interesting.”
The US climate normals are calculated for a 30-year period, consisting of annual/seasonal, monthly, daily, and hourly averages and statistics of temperature, precipitation, and other variables from about 15,000 weather stations across the country. The normals are recalculated every ten years. The chart below shows two different climate normals: 1901 to 1930 and the latest, 1991-2020.
“What we see is temperature-wise for the state, we’ve warmed two degrees Fahrenheit,” said Dr. Anderson. “Precipitation-wise, though, close enough to call it the same. But if we look at how closely the circles are to the triangle versus the squares to the diamond, it’s not the same – there’s more variability. We’re wandering further afield from the average value, meaning if you’re planning to average, you’re finding it’s not as successful as it used to be. And we might need to develop ways to better understand how we’re going from dot to dot, so to speak, year to year, of the progression.”
Below is a chart showing the minimum temperature for the statewide water year. The one hot year in the 1930s was an outlier that hadn’t been seen in the observed record since 1896. It didn’t happen again for another 50 years, and then increasingly until 2011, when it became commonplace.
“If we aren’t adapted to being that warm, we’re in trouble because it’s already here all the time,” said Dr. Anderson. “We’ll get a new opportunity for new extremes that’ll become episodic and then common. And this is the progression, particularly in a system like ours, where we have crazy variability.”
California is blessed with the highest year-to-year variability in precipitation anywhere in the US, so we know what we’re dealing with; we just have to amp it up. “Think of the people whose experience has been pretty much the same every year; the seasons walk by and only vary 10% or so. Imagine if you drop our climate on them, which is what will happen – not necessarily they go dry for four months, but the fact that year to year is an adventure.”
Last year, there was a new ‘feature’: within-year variability. At the beginning of Water Year 2023, there had been three years of record-setting drought, with a fourth year looking likely, and until Christmas, it did. However, after Christmas, things changed.
“We had winter in three weeks, which was exciting,” said Dr. Anderson. “In three weeks, 46% of our water year precip and 86% of the snowpack show up. Unfortunately, we were mitigating a lot of hazards and finding out where the system wasn’t quite up to handling such a thing. We got a month off to catch our breath, and then we had cold storms that put a lot of snow in places where the people moved there probably to not have snow. And for the folks that do experience snow, they got a whole lot of snow, to the point that it started crushing the buildings. It’s a new kind of extreme to figure out how to manage. Then right after the cold storms comes a warm storm, and we have enough water to have a summering over Tulare Lake.”
However, in 2023, the wettest part of the state wasn’t the far north; it was the Bay Area and Southern California – the large populations all experienced a very wet year that was record-setting in many ways.
“Where we have our biggest reservoirs is where we expect a lot of water to fall,” said Dr. Anderson. “In 2023, we got to average, which definitely stopped the drought, so it was a big improvement. But it wasn’t the same as everybody experienced. And this is a challenge in California: where you are and what you experience matters. But it also matters if you get your water from somewhere else. And so having that watershed understanding is really key.”
Without data to help tell the story, it’s hard to get people to appreciate what’s happening. In the graph below from Scripps Institution of Oceanography, the orange dotted line shows the maximum temperature; the green dotted line shows the minimum temperature.
“This way, we see how a warmer world manifests itself,” he said. “Is it one to two degrees Fahrenheit each day throughout the year? Or are those punctuated extremes like that Labor Day heat wave where multiple weeks were above temperature? … So how we harness this power is really important.”
Atmospheric rivers
In 1998, microwave satellite imagery revealed that water vapor is not distributed evenly across the globe; about 90% of the transport through the mid-latitudes happens in about 10% of the area. Most of that moisture is in the first two and a half kilometers of the atmosphere, which is not a concern when it is over the ocean.
But when that moisture runs into California, landscape features get in the way. They are all shaped differently, and their interaction with the atmosphere differs. And with atmospheric rivers, that can make a huge difference. Atmospheric rivers are important for both water supply and flooding – they usually bring 40-60% of annual precipitation and are associated with more than 90% of flood damages in Northern California.
In water years 2020, 2021, and 2022, only three atmospheric rivers had an impact on California. In 2023, seven atmospheric rivers hit California.
“The timing, pace, and scale of these storms tell the story of how our water year is built and how we can manage for benefit and mitigate hazard,” said Dr. Anderson.
Understanding the behavior of atmospheric rivers and how that links to large-scale climate variability is critical for water resource management in California. So, Scripps created the Atmospheric River catalog, which has data on atmospheric rivers that have made landfall along the West Coast for the last 70 years, with information on magnitude, landfall location, and orientation.
“This is awesome because each of the latitude dots on the coast of North America shows the time series going from 1950 to the present. So we see how many happened at that latitude. Color-wise, we see their scale. Within a year, we see the distribution and the peaks.”
It also shows the orientation, which is important, because it’s how the moisture is driven in the landscape and how the precipitation manifests itself. “If we get a storm out of the Northwest, it is nowhere near as productive if it’s coming out of the Southwest, shoots through the Golden Gate, and highlights the American, Yuba, and Feather River watersheds. And those two watersheds can produce as strong or stronger than the Colorado River Basin, which is exciting.”
“So this new tool gives the background information to help understand what we’re seeing and what’s an extreme, what’s becoming episodic, and what we better be ready for coming into place.”
Snow on the landscape
How much rain and snow, and where’s the snow line? Dr. Anderson said we have a metric that calculates the percentage of precipitation that falls as rain versus snow annually. However, that boundary is complex and varies wildly during a storm. An atmospheric river can start above 10,000 feet and suddenly crash to 2000 feet after that cold front comes through. So it all depends on where the precipitation is happening and how much rain and snow there is.
“We look at it, though, and annually and seasonally. We’re seeing signs of change, particularly in spring. But every watershed is different. So you have to be careful when you look at the lumped stats for the state perspective versus what’s happening in your neck of the woods.”
A warming atmosphere will put snow on the landscape in an entirely different pattern than before. This became apparent in 2021 when the snow index sites suggested a nice snowpack, but runoff was very low. So what was reliable before isn’t so reliable anymore, as the patterns are changing.
Why are warmer droughts worse?
Dr. Anderson said that we’ve had two three-year record-setting droughts less than a decade apart, and we’ll have to get better at it because it will keep happening.
Warmer droughts are worse because the warm temperatures and dry conditions increase the soil water deficit that must be filled before runoff happens. Warmer air can hold more water vapor, increasing its thirst and pulling more water from the landscape.
“In the wintertime, we used to think that everything goes to sleep,” he said. “It’s not going to sleep anymore. It’s now warm enough in the winter. Landscapes are transpiring, and the elevations that do that are the parts of the watershed that are drying out and not producing runoff in the same way they used to. That impacts water availability for use.”
The slide below shows the decade from 2013 -2022.
“We’d like to say most of the years fall in that expected range where all the infrastructure and regulatory tools work perfectly,” said Dr. Anderson. “But we find that in a warming world, the same relationships that we built 40 years ago might not hold because the relationships in the ocean, land, and ice systems behave differently.”
“So if you wondered why we kept having to have special rules, it’s because everything’s at the edge of anomalous to extreme where everything we plan to normally do doesn’t work, and we need special accommodations to make it happen.
“We need the ability to visualize and understand this is the world we’re living in, but it can be hard to communicate why you have to keep doing those same things that people thought you would only need to do once in a while. That’s true if Mother Nature would play along, but she’s not.”
New tools for dynamic water management
So what do we do? The answer is dynamic water management, or monitoring and adjusting the system to maximize the amount of water managed for benefit and minimize the water mitigated for hazard. So, the Department is collaborating with research partners to take advantage of emerging technologies, especially in observations and forecasts.
“Some folks are doing some really cool work in forecasting, but are they forecasting something that’ll be actually useful to you?” said Dr. Anderson. “That’s where decision support comes in. It is taking the really cool advances in science and technology and putting it into something useful. Once we get there, the manage volume capability goes up, the amount of water we’re mitigating for hazards goes down, and we manage it better.”
“The key is, though, we have to communicate, collaborate, and coordinate because you have a lot of people working together with a lot of different perspectives. We have to look at this gem from a lot of different facets to figure out what the whole thing looks like to be successful, but it’s possible. How do I know? We got through one year, 2023, and did some cool things.”
In 2023, the AR Reconnaissance missions (or AR Recon) were in place, utilizing the “Hurricane Hunter” planes to increase our understanding of atmospheric rivers. “We found that AR Recon provides the same benefit as the entire land base radiosonde effort; that’s a pretty good return on investment. But it’s also an investment across multiple agencies, coordinating, collaborating, and communicating to pull off. It’s doing some cool things for us. And it’s improving our ability to manage water with increasing weather extremes.”
The Airborne Snow Observatory provides highly accurate and reliable basin-wide distributed measurements of snow water equivalent and forecasts of snowmelt runoff with rapid turnaround. In 2022, DWR began using the Airborne Snow Observatory (ASO) over the Feather, Truckee, Carson and Yuba watersheds.
“Mapping that snowpack lets us know how much is there, how ready it is to melt, and, really important, how much water we can expect from it,” said Dr. Anderson. “This then helps us in our reservoir operations to understand how much water’s coming in and how much we have to meter out for use. How much do we have to manage hazard and balance to the extent we can … if we can manage the timing of the release to stay more of the time under that damaging flow, we’re doing better, especially if we can coordinate it with groundwater recharge.”
Closing thoughts
A warming world has already shown new and more frequent extremes impacting water management actions. Expectations are for these extremes to amplify further, leading to longer dry spells between larger storms requiring adaptation strategies to maintain supply reliability and reduce flood risk.
“Each year is a practice round for a new facet of a new extreme that we get to manage and work our way through, so consider it on-the-job training,” he said. “The challenge is, though, we have longer dry spells that are warmer, where if it’s not raining, it’s dry. And it’s a much more dynamic landscape. So, how do we adapt so that we maintain supply reliability and reduce flood risk? We had hints last year that this is doable.”
More spatially explicit data at more frequent time intervals is key to understanding the state of the watershed, such as how dry it is, how ready it is to produce runoff, and if it has water availability. Forecast improvements provide key information at longer lead times for greater flexibility to meet multiple water management criteria.
“We can get there; we found a way to do it,” said Dr. Anderson. “With each extreme, we get more experience, more confidence, and iron out all the wrinkles that cause challenges. If we understand the characteristics and what’s coming, we can do a lot with it.”
