California is adapting to increasingly intense storm patterns, largely driven by atmospheric rivers. These narrow bands of concentrated water vapor transport immense amounts of moisture from the tropics, often resulting in heavy rain or snow when they reach land. By utilizing advanced weather forecasting tools and improved monitoring, Forecast Informed Reservoir Operations (FIRO) optimizes reservoir operations to manage runoff more effectively to increase water storage while mitigating flood risks to maximize the performance of California’s existing infrastructure. At the April meeting of the California Water Commission, Dr. Marty Ralph, Director of the Center for Western Weather and Water Extremes (CW3E), discussed how FIRO is being implemented in California.
ATMOSPHERIC RIVERS
Atmospheric rivers are long, narrow bands of water vapor that transport moisture from the tropics near the equator to regions like the West Coast of the United States. But it’s not just a West Coast phenomenon: atmospheric rivers occur worldwide, with an average of four to five active at any given time.
Atmospheric rivers carry immense amounts of water vapor; an average atmospheric river transports as much moisture as the flow of the Mississippi River at its mouth. The strongest atmospheric rivers can carry up to 15 times that amount. When these rivers reach land, they release their moisture, often resulting in heavy rain or snow, depending on the conditions.
“It’s like a river, literally a river in the sky, but it’s a water vapor blown by the win, rather than a terrestrial river, which is liquid water drawn by gravity,” said Dr. Ralph.
Atmospheric rivers have a dual nature, bringing both benefits and challenges. They range in strength from mild to extreme, with the strongest ARs often responsible for significant winter floods along the West Coast. Conversely, smaller ARs play a vital role in supplying much-needed water, producing anywhere from 30 to 50% of the state’s water supply.
The slide below illustrates how atmospheric rivers will make or break California’s water year. The arrows show where the atmospheric rivers made landfall, the color indicating the strength. The graphic on the left shows only one atmospheric river making landfall over California from October 2019 through September 2021, which was during the driest three-year period on record. In contrast, the graph on the right shows Water Year 2023, when many atmospheric rivers made landfall, including nine back-to-back atmospheric rivers from December 2022 to January 2023.
The number, size, and strength of ARs varies; in Northern California, 85% of the variance in annual precipitation is in the 5% wettest days each year, which are almost all atmospheric river events. In the central part of the state where Sacramento and San Joaquin rivers combine, if the 5% wettest days are separated from the 95% remaining wet days, 95% of the storms are almost steady every year.
“It’s as if we can count on the run-of-the-mill storms, but the top 5% days, they vary wildly from year to year,” said Dr. Ralph. “85% of the variation in annual precipitation in the San Joaquin and Sacramento is because of those 5% wet days. If you get about 100 days where it’s somewhat wet, it means five days a year is what makes or breaks water supply in the state.”
DUAL PURPOSE RESERVOIRS
Most of the reservoirs in California are designed and operated for both flood control and water supplies, which presents a significant challenge because these two objectives often conflict with one another. For water supply, the goal is to store as much water as possible for use during dry periods or droughts, which requires keeping reservoirs at higher levels. On the other hand, reservoirs must maintain enough empty capacity for flood control to capture and store runoff from heavy storms to reduce the risk of downstream flooding.
Reservoir operations are governed by Water Control Manuals, which are prepared by the U.S. Army Corps of Engineers upon the completion of a dam. These manuals outline detailed operational guidelines for managing water levels to minimize flood risks and ensure public safety. The Army Corps is responsible for creating and revising these manuals, not only for dams they own and operate directly but also for Section 7 dams, where they provide oversight and updates.
A key element of the manual are the reservoir rule curves, which is the maximum allowable reservoir storage to meet flood control objectives. A typical reservoir rule curve reflects seasonal needs: lower levels in winter and early spring for storm runoff and flood control, rising in late spring and summer to store snowmelt for irrigation, urban use, and recreation. The curve includes a flood control zone for storm inflows, a conservation pool for water supply, and dead storage at the bottom, which is unusable due to design constraints.
Ideally, these manuals should be regularly updated to reflect advancements in technology and changing climate conditions. However, a lack of appropriations has hindered meaningful updates for many reservoirs.In the 1960s, when dam construction was at its peak, skill in weather forecasting was limited, so the procedures for developing guide curves depended on observations (water on the ground) and the likelihood of future weather. The manual codifies these procedures, and reservoir operators are compelled to use them.
FORECASTING ARs
Our ability to forecast atmospheric rivers in California has significantly advanced in recent years, thanks to initiatives like the Atmospheric River Reconnaissance (AR Recon) program, which utilizes the Hurricane Hunter aircraft to study atmospheric rivers in the hurricane off-season. The program, led by the Center for Western Weather and Water Extremes (CW3E) at UC San Diego, employs cutting-edge technologies such as storm-hunting aircraft, dropsondes, and ocean buoys to collect critical data directly from atmospheric rivers. This year, 58 missions were flown over about three months.
The schematic shows how it works. Sensors are dropped from the plane that precisely measure the temperature, pressure, wind, and moisture. Buoys on the ocean surface measure sea level pressure. They are also experimenting with GPS satellites that can be processed onboard the aircraft the information about the temperature and water vapor from the satellite. These observations have improved precipitation forecasts by up to 12% for extreme events, providing water managers and emergency planners with more accurate and timely information.
“It’s sort of like surgically precise sampling of this storm offshore,” said Dr. Ralph. “Then we transmit that data via satellite into the weather models, and it is demonstrably improving the forecast skill for AR landfall and the precipitation.”
Dr. Ralph pointed out that satellites are important but don’t show everything. Satellites can be blocked by high clouds and by water vapor. “ARs happen to be the most sensitive area for when you get it wrong, the forecast errors are going to grow fastest, so we are doing a lot to try to improve our skill there,” said Dr. Ralph. “Atmospheric river reconnaissance is the key linchpin for getting more skill in the forecast to help FIRO be more successful.”
HOW FIRO WORKS
FIRO is defined as ‘a reservoir operation strategy that better informs decisions to retain or release water by integrating additional flexibility in operation policies and rules with enhanced monitoring and improved weather and water forecasts.
The core idea is to integrate forecasts more effectively into operational decision-making. Historically, Corps of Engineers-managed dams have focused on actual water levels rather than predictive data. This applies to both the dams they own and operate directly, as well as Section 7 dams, where they are responsible for revising the water control manuals.
“What our studies have done very rigorously is assess the possibility of giving the operator a little more flexibility to ride the reservoir a little bit higher after a storm in case there’s no storm coming ahead and to dig into the conservation pool in case there’s a big storm coming and enhance flood risk mitigation.”
The graphic shows how FIRO works. The vertical axis is reservoir storage; the green shaded area is a predicted storm that actually happens. The thin black line in the middle is the top of the conservation pool, which is the normal operating level of the reservoir; above this level, the reservoir is designed to store floodwaters.
Starting on the left, the water level is above the top of the conservation pool and in the FIRO pool. A few days in, there is a major atmospheric river. So, the reservoir operator, with forecasting insight, can release water ahead of the storm to create space in the reservoir to absorb the big storm and reduce flood risk downstream. The reservoir fills, and the operator brings the level down to the top of the FIRO pool and rides out the storm.
“After the storm is done, they’ve released the water safely at lower rates than would have been necessary before because they released some before the storm. And then they can look ahead to the next two days to see if another storm is coming, and they could do the same thing again.”
EXAMPLE: LAKE MENDOCINO
The first FIRO project at Lake Mendocino utilized forecasting advancements to increase water supply without reducing Lake Mendocino’s existing flood protection capacity or downstream flows for fish habitat. The project was a partnership between the Sonoma Water Agency, the US Army Corps of Engineers, DWR, the National Weather Service, and others.
The left graphic shows Lake Mendocino in 2020, a drought year. The dashed lines represent the FIRO pool in the reservoir. The red line shows what would have happened without FIRO, and the black line is what actually happened.
“By then, they had a deviation in place that allowed the reservoir to be operated using our tools,” said Dr. Ralph. “We had 18% more water going into the summer. That was enough for 20 or 30,000 households for a year’s worth of water. This is a beneficial outcome for the communities there.”
The graphic on the right shows a flood year. In the December of 2022, there was an onslaught of ARs, and then in January, nine large ARs hit California.
“We went from way like a hard drought into flooding across the state. This is the nature of our climate here; this happens, and ARs are the storm type that does that.”
The whole flood pool was filled in just three weeks. The black line is what happened, and the red line shows what would have happened without FIRO.
“After the big spike into the flood pool, the operator brought the reservoir back down to the top of the blue line and ran it at that level for a month because no ARS were coming,” he said. “Then an AR was predicted, and they kept it lower as the allowed storage went up, and then they filled that. In March, a storm was in the forecast, and they dropped some of that water out. It refilled after the storm and basically rode that FIRO curve up, and they ended up with 13% more than they would have had, which, once again, the volume of water was about 20 or 30,000 households a year.”
LAKE MENDOCINO IN 2025
The graphic below shows Lake Mendocino for 2025. The FIRO pool is between the orange and the gray line. The series of storms came in, and the reservoir operator used the FIRO rules to add 10,000 feet of additional storage.
“So once again, we’re going to see major benefits to water supply reliability.”
FIRO STUDY ON YUBA-FEATHER SYSTEM
Lake Oroville, located on the Feather River, is managed by the California Department of Water Resources (DWR), while New Bullards Bar, situated on the Yuba River, is owned and operated by the Yuba Water Agency. These two rivers converge downstream of the dams, just north of Sacramento. Together, the reservoirs play a critical role in mitigating flood risks by controlling water flow and reducing the potential impacts of heavy rainfall in the region.
For the Yuba-Feather system FIRO study, they modeled the 1986 flood, making it hypothetically 16% bigger. The vertical axis shows elevation instead of storage, and the horizontal axis is from February 7 to February 25, 1986. The blue line is one set of assumptions; the red line is another. The black line is what would happen without FIRO.
In the FIRO study, reservoir storage started higher but was then reduced far below what would have normally been there because they saw the storm coming and made space. Because there was already more space in the reservoir, the storage level wasn’t as high, so the peak storage was reduced. After the storm, the reservoir levels were slightly higher than the top of the FIRO pool. There were increases in the flow before the storm, but very reasonable increases and lower releases at the peak. And then, because there wasn’t as much water to get rid of after the storm, the high releases to get the water out slowed down sooner.
“Combined with Oroville, these two represent 265,000 acre-feet of additional flexibility, either to dig into the conservation pool or to allow for extra potential water availability,” said Dr. Ralph. “This report just finished about a month ago. It’s been five years, roughly, of very intensive, interactive work.”
NEW TOOLS ENHANCING FORECASTS
There are new tools for forecasting atmospheric rivers; the graphic on the slide below shows one such tool. The blue space is looking 16 days out into the future; the reds and purples show an atmospheric river headed towards the Pacific Northwest and British Columbia; the position vertically on the graphic corresponds to the location on the California coast on the right graphic.
“What this means is nothing on the horizon for California for the next two weeks,” said Dr. Ralph. He cautioned though that in the weather business, we don’t really trust it out beyond about seven days, because you can have nothing in the forecast today for 10 days from now. “Reality is that something changes in the weather, and we end up with a storm or vice versa. But we have a lot of confidence when we get into three to five days out that we’re not going to be surprised. That used to be the case when I was younger; it was a dream that we could be able to do anything close to this.”
“The Scientific American asked me to write an article for them describing the forecasting of atmospheric rivers, and it weaves together the story around one event in January 2021 that was a billion-dollar weather disaster, and we ran air recon. People evacuated. They closed roads. Not a single person, not a single fatality in that storm, and it was the AR recon flights that upped the forecast from being sort of a run-of-the-mill storm to being a strong AR five days ahead. That’s our target. That’s our that’s our goal over time, to continue building that lead time and that skill.”
IN SoCAL, FIRO ADDING TO GROUNDWATER RECHARGE
In Southern California, FIRO is used at Prado Dam to increase groundwater recharge. The Corps of Engineers allows them to keep some water in the bottom of the reservoir which is released slowly to increase recharge, but if too much water backs up behind the dam, they have to release it faster, and they can’t recharge at all.
“Our program showed that they could safely keep some of that water longer to substantially increase the recharge,” said Dr Ralph. In the case of Prado dam, we calculated over time it would average about 7000 acre-feet to the groundwater recharge.
CW3E is working on a FIRO study for a viability assessment on Seven Oaks dam with San Bernardino Valley Municipal Water District, who are planning for a successful FIRO viability assessment by building recharge basins.
“Our FIRO program is really communicating with the managed aquifer recharge community, so I’m professionally optimistic about FIRO being effective in many more places around the state and the West. And I’m personally optimistic that there will be more opportunities for the managed aquifer recharge to be coupled with that.”
THE FUTURE OF FIRO IN THE FACE OF BUDGET CUTS
One of the Commissioners asked Dr. Ralph how operations were going with federal partners.
Dr. Ralph answered, “I also serve on a national committee that was legislated by Congress to advise NOAA on innovations. We’ve been around for almost 10 years, and I’ve been on it for four years, and now I co-chair it. Our role traditionally has been to push NOAA to be more innovative and to push pretty hard. Officially, as part of the legislative directive for our committee is to then report to Congress on what NOAA did or didn’t do with our recommendations.
“The changing policies associated with the new administration have created a lot of concern in the meteorology community and the weather community, including those businesses that also are in the weather business and water business and the agencies that are related to NOAA, that NOAA’s capabilities represent a foundation of what we call the ‘weather and water enterprise.’ It’s a term in the field of Meteorology and Hydrology. It’s an enterprise, its businesses, its universities, its agencies, and all of that combined work together in a remarkable symbiotic way, often founded on the data and the outputs that NOAA produces. That enterprise is 10 times the budget of NOAA or the weather part of NOAA.
“Economic studies have shown that the benefit of the weather and water enterprise to our nation’s economy is about $100 billion a year – 10 times what the weather and water enterprise itself represents. So our committee has taken a very careful look at this. We’re a committee of people from outside NOAA, universities, businesses, NGOs, users of forecasts, et cetera. We’ve prepared a report just posted on the NOAA Science Advisory Board website as a draft report … that the advisory board will consider over the next two weeks and then vote on at the end of the month. That report contains our professional viewpoints, vetted by many people in our field.
“Our report is on what changes we’re aware of and how the enterprise works together as sort of this organism that is very symbiotic and depends heavily on NOAA being healthy and producing the outputs that are required. But remember, our committee’s job is to nudge NOAA towards being more innovative to change so our report also recognizes and acknowledges the need for that fundamental thing to happen.
“One of the things we concluded is that because the weather and water information flows through our nation’s economy and into public safety, sort of like the blood through our veins, we don’t see it every day, but oh my God, do we need it, and some of the changes are endangering that. It’s our committee’s job to report on that, So we have some of that in there. We’re recommending that our committee be used as an independent, objective source of input on any changes that are happening or being considered that would ensure that we don’t damage the system. At the same time, we recognize the need to change and innovate. I’m hopeful this report will help people better understand how this whole weather water enterprise works in this symbiotic manner.”
You can read the Committee’s draft report here, and review the presentation here.
Note: The majority of this article draws on Dr. Marty Ralph’s presentation at the California Water Commission, but also includes information from Dr. Ralph’s presentation at the ACWA Fall Conference in December of 2024, and some additional research.
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