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Despite its reputation for year-round sunshine and mild temperatures, California has one of the most varied climates in the United States.
For Yuba County and neighboring Sutter, these variations have manifested most notably in catastrophic flooding from the Yuba, Feather and Bear rivers, often after years of dry conditions. Since 1950, five major floods claimed 43 lives, caused hundreds of millions of dollars in property damage and resulted in decades of social and economic impacts.
In recent decades, researchers have identified atmospheric rivers – long, narrow bands of concentrated water vapor that produce substantial amounts of rainfall as they move from the Pacific toward the west coast – as the cause of more than 90 percent of recorded floods in Northern California. Already one of the most productive watersheds in the state in terms of runoff per square mile, the Yuba River watershed is especially vulnerable to the impacts of atmospheric rivers and the heavy, prolonged rainfall they bring.
“If you look at the major weather events over the last century, particularly here in Yuba County, nearly all of the recorded floods are connected to these powerful atmospheric river storm events,” explained John James, an atmospheric scientist and water operations project manager with Yuba Water Agency. “Conversely, we also now know that atmospheric rivers provide up to half of the annual precipitation, benefitting our water supply.”
Recognizing the central role that atmospheric rivers play in both flood risk and water supply – two of Yuba Water’s core mission areas – the agency is investing in new research and tools to better understand, forecast and manage for these powerful storms.
After the 1997 New Year’s Day floods devastated Northern California, Yuba Water initiated several actions to reduce flood risk for communities along the Yuba and Feather rivers. This included significant investments in levee improvements and real-time coordination of reservoir operations at New Bullards Bar on the Yuba and the California Department of Water Resource’s Lake Oroville on the Feather.
This coordination, known as the Yuba-Feather Forecast-Coordinated Operations Program (F-CO), has expanded and improved monitoring and information-sharing among federal, state and local agencies in the region. The program also created the foundation for a new, more complex forecast program called Forecast-Informed Reservoir Operations, or FIRO.
Led by UC San Diego’s Scripps Institution of Oceanography Center for Western Weather and Water Extremes, with partners including Yuba Water, DWR, the U.S. Army Corps of Engineers and the California Nevada River Forecast Center, the Yuba-Feather FIRO partnership was formed on the heels of a five-year drought followed by near-record rains in 2017.
“California already has a highly variable climate,” James said. “But these periods of extremes will ping-pong in magnitude and frequency in the future, and FIRO is a perfect tool to help us manage and adapt to these changes.”
Partners like DWR are also optimistic about the application of FIRO and its role in regional water operations.
“Our continued work with Yuba Water and Scripps on potential advances in forecasting and monitoring atmospheric rivers, and the benefits to flood control and water supply that could result, will help DWR be better informed when it comes to water availability, reliability and management,” said John Leahigh, State Water Project Water Operation Executive Manager and co-chair of the Yuba-Feather FIRO steering committee guiding implementation of the program.
When implemented together, F-CO and FIRO combine water management operations during potential flood events with improved atmospheric river forecasts. The goal is to substantially reduce flood risk for the Yuba-Feather watersheds by creating more storage in reservoirs ahead of big storms while also ensuring water supply reliability when prolonged dry conditions are forecast.
Using $2 million in funding from Yuba Water and DWR, a Yuba-Feather FIRO workgroup is now preparing to implement FIRO in the region. The funding has helped establish four new atmospheric river monitoring stations that collect continuous meteorological data like air temperature, humidity, pressure, precipitation, wind speed and direction, and solar radiation. Two additional monitoring sites have also been identified and are expected to go online in early 2021.
Several stations also collect soil moisture at six depths below the ground surface, helping researchers pinpoint the boundary between snow-covered and snow-free surfaces in the watershed. Two stations also include vertically pointing radar, which can observe the altitude in the atmosphere where snow and ice turn to rain. This is important because compared to other watersheds that have applied FIRO into their water operations – the Russian River and Santa Ana watersheds – the Yuba and Feather River watersheds have to consider snow in their operations.
“These stations have the advantage of not only capturing the measurements of meteorology at the surface but also looking up into the atmosphere to give us a sense of what's coming down,” said Anna Wilson, field research manager for the Center for Western Weather and Water Extremes at Scripps. “This information is transmitted via cell or satellite every hour and gives a ten-second to two-minute resolution of conditions.”
The new monitoring stations not only provide new data points to capture conditions consistently over time, they also ground truth observational data collected from airborne and spaceborne sensors. Along with storm-specific sampling using weather balloons and atmospheric river reconnaissance flights over the Pacific, this information feeds into complex atmospheric river forecasting models used by researchers and natural resource managers around the world.
“We've already started to show that this data has a positive impact on being able to better forecast extreme precipitation in the west,” Wilson said. “It’s been really exciting to immediately apply new knowledge to better managing resources.”
The implementation of FIRO in the Yuba-Feather watersheds is one of three interconnected public safety efforts Yuba Water is undertaking to significantly reduce flood risk and improve climate change resilience for Yuba County and nearby communities.
In addition to implementing the Yuba-Feather FIRO program, Yuba Water is working with the U.S. Army Corps of Engineers to update the water control manual for New Bullards Bar. The current manual and operating procedures were originally written at the time of the dam construction in the 1960s when forecasts were not as reliable. It was developed with the expectation of a second dam downstream that was never built. The water control manual update aims to be adaptive in incorporating current and future advances in science and forecast skill, including operational enhancements informed by the Yuba-Feather FIRO program.
The third and largest undertaking is the design of a secondary spillway at New Bullards Bar Dam with gates 31.5 feet lower than the existing spillway gates. With the new spillway gates at a much lower elevation in the reservoir and new operational procedures in place, Yuba Water will be able to release more water in advance of large storm events when there is enough downstream channel capacity to handle the flows. Yuba Water is currently working with a team of engineers to bring the project to a 60 percent design phase by mid-2021 and is exploring potential partners to collaborate with for this important multi-benefit project.
“Yuba Water has always been forward-looking when it comes to managing this watershed with the best available science and multi-agency collaborations,” James said. “But the implementation of FIRO along with the update to our water control manual and the secondary spillway is a whole new level of investment that will pay off big time in the long-run, and we’re all really proud to be a part of that.”
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