New Report Confirms Benefits of Forecast-Informed Reservoir Operations at Lake Mendocino

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Modern forecasting methods fueled by advances in understanding and predicting atmospheric river storms have enabled U.S. Army Corps of Engineers (USACE) operators to better optimize water resources at Lake Mendocino, a Northern California reservoir. A multi-agency report issued Feb. 4, 2021, describes how these forecasting tools have helped operators increase the lake’s dry season stores of drinking water, improve its ability to alleviate flood risk, and enhance environmental conditions in the downstream Russian River to support salmonid species. 

A key change to USACE water management policy allowing for weather forecasts to be used in planning future reservoir operations opened the door for this program. The report, referred to as the Final Viability Assessment for Forecast-Informed Reservoir Operations (FIRO) at Lake Mendocino, was produced by a partnership of federal, state and local agencies and led by Sonoma Water and the Center for Western Weather and Water Extremes (CW3E) at Scripps Institution of Oceanography at UC San Diego. 

This work on Lake Mendocino in Sonoma County establishes an approach that could be tested at suitable additional reservoirs elsewhere in California and other regions where water supply can vary widely. Climate change is exacerbating the feast-or-famine nature of precipitation even further in the state, scientists say, making critical the need to assess the potential for forecasts of atmospheric river storms to be used to operate reservoirs more flexibly than had been possible before these forecast improvements. 

In addition to Sonoma Water and CW3E, the partnership includes the U.S. Army Corps of Engineers, National Oceanic and Atmospheric Administration, California Department of Water Resources, and the U.S. Bureau of Reclamation, and other key agencies. 

FIRO was demonstrated successfully during the course of two very different water years—Water Year 2019 was a relatively wet year, while Water Year 2020 was the third driest year over a 127-year record. In both years, FIRO increased water supply benefits and managed flood risks. In Water Year 2020, FIRO enabled a 19 percent increase in water storage, totaling more than 11,000 acre-feet, by the end of winter. 

The Russian River basin experiences some of the most variable climate in the U.S., with atmospheric rivers and their extreme precipitation driving this variability. Atmospheric rivers are phenomena that transport large amounts of water vapor in veritable “rivers” in the sky that trigger heavy precipitation in the Russian River region. These storms cause 25-50 percent of annual precipitation in key parts of the West, which can replenish water supply, but can also lead to hazardous and costly flooding, with atmospheric rivers causing 84 percent of Western U.S. flood damages, and 99 percent of damages in Sonoma County. Conversely, a lack of atmospheric rivers has been shown to lead to drought conditions. In a warming climate, atmospheric rivers are anticipated to increase in intensity becoming even bigger contributors to California’s annual precipitation total and posing greater flood risk hazards, however storms not related to atmospheric rivers will contribute less to total precipitation. This means California could vacillate even more wildly between extremes of drought and flooding. 

“The Lake Mendocino FIRO project is an example of how multiple agencies can collaborate to collectively explore the potential of emerging technologies in observations and forecasts and create an adaptive strategy with multiple benefits for water management in a changing climate,” said Michael Anderson, state climatologist with the California Department of Water Resources.

Many dams in the West are strictly regulated by USACE issued water control manuals based on historical long-term averages of winter storms and spring runoff. These manuals do not rely on forecasts and have traditionally operated on directives to “manage water on the ground” and typically specify lower reservoir levels in the fall to make room to prevent winter storm runoff floods and raise levels in the late spring. The variability of when rainfall occurs from year to year is not directly considered. Many water control manuals were developed prior to modern technology such as satellites, radar, and numerical models that have led to significant improvements in forecasting skill.  

To address these challenges, FIRO leverages current and improved forecasts of atmospheric rivers and their associated heavy precipitation and streamflow, through tools developed as part of the project. These data and tools inform reservoir operations, allowing more proactive and adaptive adjustments to variable weather conditions in decisions to retain or release water. FIRO does not require reservoir operators to employ information provided by FIRO; it merely provides additional information to inform operational decisions. The program represents an innovative use of science, technology, and observations for operators to adapt to variable conditions without costly reservoir infrastructure improvements. 

“Sonoma Water’s interest in innovation, and our partnership with federal and state agencies including the U.S. Army Corps of Engineers and the Center for Western Weather and Water Extremes, generated a breakthrough in water management for Sonoma Water,” said Lynda Hopkins, chair of the Sonoma Water Board of Directors and Sonoma County Board of Supervisors. “This comprehensive report demonstrates significant regional benefits for people, the environment, and the economy.”

The science of forecasting atmospheric rivers has continued to advance, particularly in understanding the origin and evolution of these storms, through enhanced observations before they make landfall, and through better modeling and prediction. The Center for Western Weather and Water Extremes has a focus on atmospheric rivers. Their research includes atmospheric and soil moisture observations in the Russian River basin, data collection over the Pacific Ocean including measurements from buoys and dropsonde deployments into approaching storms thanks to a program with NOAA’s Office of Marine Aviation and the U.S. Air Force Reserve 53rd Weather Reconnaissance Squadron “Hurricane Hunters,” and advanced modeling that allows for better assessment of uncertainty in forecasts. 

“The skill of forecasting atmospheric rivers and their associated extreme precipitation and runoff, based on scientific advances and modern technology, have been shown by this program to enable Lake Mendocino to be operated more flexibly than in the past,” said Scripps research meteorologist F. Martin Ralph, director of CW3E. “Even in the third driest year on record, this program demonstrated the ability to apply science to save water, which is essential for California given how common droughts are in the region.”

The FIRO assessment initially ran virtual trials of the program using modeling, forecasts and hindcasts. The success on paper led to operators receiving USACE-approved major deviations from the Lake Mendocino Water Control Manual, which was originally developed in 1959 with minor revisions in 1986, to test the viability with real-world application. Four FIRO management options were evaluated, in addition to the current operations, using 16 metrics.

An economic assessment, funded in part by the U.S. Bureau of Reclamation, completed to quantify the benefits of FIRO for dam operations, water supply, fisheries, recreation, and hydropower found that FIRO will lead to positive benefits in all these areas except hydropower, resulting in total estimated annual benefits of $9.4 million. 

“The Final Viability Assessment creates a precedent within the U.S. Army Corps of Engineers for how Forecast-Informed Reservoir Operations can be safely and effectively implemented within the context of the flood risk management mission of the Corps of Engineers,” said Cary Talbot, chief of the Flood and Storm Protection Division at the U.S. Army Engineer Research and Development Center. “It also set a great pattern for bringing together individual stakeholder perspectives in a steering committee comprised of state, local, and federal agencies to assess viability, a model which could be used by any agency that wants to evaluate the applicability of FIRO.”

This report has implications throughout the West. USACE and CW3E are also actively assessing FIRO opportunities in other settings where atmospheric rivers are dominant. Efforts are underway to evaluate the viability of FIRO to Prado Reservoir in Southern California, New Bullards Bar Reservoir in Yuba County, Calif., Lake Oroville in Butte County, Calif., as well as the Howard Hanson Dam near Seattle, Wash. 

The benefits of FIRO using existing science and technology are significant. With further improvements in forecasting skill achieved through continued investment in research, the benefits of FIRO will continue to increase into the future. The Lake Mendocino FIRO partners recommend additional research for continued enhancement of FIRO. This future phase—“FIRO 2.0”—will be important to further improving water supply reliability and adapting to a changing climate. The partnership also recommends that the U.S. Army Corps of Engineers work to update Lake Mendocino’s Water Control Manual to officially include FIRO strategies.   

Additional Quotes from FIRO Steering Committee Members

“I’m really happy to see that the final viability assessment report has been completed. I was really honored to be a part of that process to collaborate with our state, local and federal partners. What I thought really made this a successful effort was the ability for everyone to come to the table to speak their concerns and have them addressed, whether you were bringing up environmental, water supply or flood risk management issues. I look forward to further evaluating FIRO as we try to implement a water control manual update for Lake Mendocino.” -- Patrick Sing, hydraulic engineer, lead water manager, Corps of Engineers, San Francisco District

“I can’t say enough about the completion of the FIRO FEA. Having been a part of this process since the beginning, I’ve experienced true sustained collaboration water management at a high level. The FIRO FEA demonstrates how multiple agencies with different missions can come together to develop a solution in a watershed known for drought, flooding and endangered species. For this, I congratulate all involved for reaching this significant milestone. NOAA Fisheries is proud to be a part of this effort and what it means to manage water in a changing climate.” -- Joshua Fuller, fish biologist, NOAA Fisheries, West Coast Region

“One of the big things that FIRO or Forecast-Informed Reservoir Operations has done for us is to greatly expand our ability to fulfill our mission. We need to balance water supply with flood protection and FIRO’s given us a structure for an unprecedented level of collaboration and we’ve been able to demonstrate that using forecasted information, we can store more water than we would’ve otherwise and we can do it safely.” -- Alan Haynes, hydrologist in charge, California-Nevada River Forecast Center, NOAA National Weather Service

“The Lake Mendocino FIRO effort demonstrates that advances in forecasting open opportunities for new water management and reservoir paradigms. Growing demand for water in the western United States coupled with increasing prevalence of drought will further strain our water resources in the decades to come. As such, the role of forecasts in reservoir operations will only increase in prevalence and importance as we work toward modern, 21st-century water management.” - Kenneth Nowak, water availability research coordinator, Research and Development Office, Bureau of Reclamation