Theme for 2021:
Exploring Floods in California's Central Valley from a Climate Perspective
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About This SymposiumThe 2021 CEPSYM was held as a virtual event over two mornings due to the Covid-19 pandemic response. Five speakers were organized around a focus for each day, followed by a panel discussion by the speakers on the focus for that day:
- June 22: Possible impacts on flood dynamics from future climate changes
- Daniel L. Swain, Ph.D. — Rising California Flood Risk from Stronger, Wetter Atmospheric Rivers in a Warming Climate
- Mike Anderson, Ph.D., P.E. — Historic California Floods Contrasted with Potential Future Floods
- Mary Jimenez, P.E. — Addressing Climate Resilience in 2022 Central Valley Flood Protection Plan Update
- Chris Elias — Flood Risk Resilience: Adapting to Climate Change in Lower San Joaquin River Basin
- David C. Curtis, Ph.D., F.EWRI — Sierra Nevada Mountain Snowpack Trends
- Speaker Panel Discussion
- June 23: Adaptations to future climate impacts on flood events
- Gary Bardini, P.E. — Managing American River Flood Risks and Regional Groundwater Levels Together
- John James — FIRO + New Bullards Bar Secondary Spillway: Enhancing Public Safety and Climate Change Resiliency
- Marie L.E. Davis, P.G. — Megafire + Rain = Sediment: Evolution of French Meadows Forest Restoration Project
- Luca Delle Monache, Ph.D. — Improved Weather and Subseasonal-to-Seasonal Predictions with Machine Learning
- Julia Prokopec — USGS Real Time Flood Inundation Mapping Program
- Speaker Panel Discussion
Special Recognition Award
The 2021 Special Recognition Award was presented to David C. Curtis, Ph.D., F.EWRI. See the award language, biographical information, and hear the presentation on the Symposium's Dave Curtis award page.
Moderator for the day was Anne Heggli, Ph.D. Student, Desert Research Institute, Reno, NV.
Rising California Flood Risk from Stronger, Wetter Atmospheric Rivers in a Warming Climate
California's early 21st century climate is already noticeably different from its 20th century climate — with warmer temperatures, decreased mountain snowpack, and both longer and more severe wildfire seasons. Yet despite the occurrence of severe and prolonged droughts in recent years, California's annual average precipitation has remained essentially unchanged — and parts of the state experienced their wettest year on record in 2016-2017. These wild swings in precipitation over the past decade offer an early preview of what California can expect in the coming decades.
In this talk, I discuss recent research pointing toward an increase in California "precipitation whiplash" as the climate warms — with major implications for both drought and flood risk. In particular, I focus on the rising risk of severe flood events stemming from a substantial increase in the intensity (and warmth) of atmospheric river events in California. Our recent modeling suggests that not only will these storms be associated with higher temperatures and water vapor content than their historical counterparts, but also that the spatial pattern of topography-driven precipitation enhancement may change significantly as well. Collectively, the projected increase in total precipitation, storm maximum precipitation rates, and rain/snow partitioning during the strongest atmospheric river events will likely bring about a large increase in storm-associated runoff — including the very real prospect of historically unprecedented flood events across much of the state.
Historic California Floods Contrasted with Potential Future Floods
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Among the many impacts of climate change on water resources management, extremes are expected to increase including the prospect of larger floods. In this talk, we explore California's historic floods through the lens of atmospheric rivers. Metrics explored include antecedent conditions, integrated vapor transport, duration, freezing elevation, and the clustering of atmospheric rivers. With this perspective in mind, the talk looks ahead to the coming decades and explores the influence of climate change on the aforementioned metrics and how that might translate to floods. Where relevant, recent events are highlighted to provide perspective on the impacts of expected change.
Addressing Climate Resilience in 2022 Central Valley Flood Protection Plan Update
The Central Valley Flood Protection Plan (CVFPP) is California's strategic blueprint for improving flood risk management in the Central Valley. The first plan was developed by DWR and adopted by the Central Valley Flood Protection Board in 2012; and subsequent updates to the plan are developed every 5 years. Consistent with recommendations from the 2017 plan and Governor's Water Resilience Portfolio, the CVFPP is being updated for 2022 with a focus on three key themes:
- Climate resilience
- Performance tracking
- Integration and alignment with other State water management plans and efforts
Future climate projections indicate potential for increased peak flows and flood volumes, which are likely to affect flood risk in the Central Valley. Substantial changes to flood risk are anticipated under future climates throughout the Central Valley, but the magnitude of changes is strongly dependent on the characteristics of individual watersheds. Flood system vulnerabilities and risk are being assessed through new climate scenarios in the Update, and new content is being developed to further explore climate adaptation strategies for both flood management and ecosystem conditions. This presentation will summarize the work underway, including CVFPP's contribution to DWR's overall resilience goals.
Flood Risk Resilience: Adapting to Climate Change in Lower San Joaquin River Basin
Serving as the drainage basin for the San Joaquin River and its many tributaries, San Joaquin County is projected to face increased flood risk over time. The frequency and size of extreme precipitation events are expected to increase, and rising sea levels are predicted to move up into the Delta. The 2017 Central Valley Flood Protection Plan (CVFPP) estimates Lower San Joaquin River flows would increase three fold to 310,000 cfs at Vernalis. The CVFPP study also estimates that within 50 years a 200-year storm event would send New Hogan Dam into an uncontrolled spill resulting in a 38,000 cfs flow at its downstream control point of Bellota. This magnitude of flow is approximately 3 times the 12,500 cfs channel capacity at Bellota, and about 2.4 times the 16,000 cfs channel capacity of the lower Mormon Slough system which runs through Central Stockton.
The San Joaquin Area Flood Control Agency (SJAFCA) is advancing a program for increasing the level of flood protection provided by the Mossdale Tract levees through:
- development of locally-funded improvements,
- partnering with the State on potential State funding of improvements, and/or
- the pursuit of a Federally-authorized set of improvements.
When selecting improvements it is necessary to select the appropriate future hydrology and consider climate change estimates to inform design-level work. Unfortunately, the uncertainties (magnitude, timing, and nature of frequency) associated with accurately predicting that future hydrology requires balancing the flood risk associated with understating that hydrology with the financial costs associated with overstating that hydrology.
Recognizing climate hazards may cause far-reaching impacts to the communities, economy, and environment of San Joaquin County, the SJAFCA Board developed the policy to manage the uncertainty associated with climate change and to minimize the risk associated with unnecessary and expensive expenditures as well as the possibility of constructing a project which is quickly found to be inadequate in light of climate change. Adopting this policy will not eliminate the possibility of future modifications being necessary; but it does allow a project to proceed with good information and then to be updated as new information is available.
Sierra Nevada Mountain Snowpack Trends
[Minor technical problem at start of audio/video causes skips and jumps for the first minute or so.]
Analysis of historical snow course observations in the Sierra Nevada Mountains suggests that current April 1 snowpack, on average, contains approximately 2.4 million acre-feet less water than 60 years ago. The results suggest that general warming in the region was sufficient to cause decreases for April 1 snowpack water content despite the countertrend of increased precipitation during the period.
A key variable in assessing the condition of California's summer water supply is the April 1 snow water equivalent (SWE). SWE is an important driver for water management in the state. A warming climate appears to be changing the total volume and timing of snowpack water needed to meet summer demands.
The Sierra Nevada Mountain Range extends northwestward from the Mojave Desert in southern California to the Cascade Range in far northern California. The California Department of Water Resources (DWR) along with several local agencies and utilities operate long-term snow courses along the range.
The snow course data obtained for this analysis were grouped in three basins: Sacramento River, San Joaquin River, and the Tulare Lake Basin. There are 204 snow courses across these basins. Of these, 166 locations with more than 60 years of data each were analyzed for April 1 SWE trends.
Linear regression was performed on the data to identify an approximate trend in the April 1 SWE data over the periods of record. Of the 166 snow courses, 78% showed a negative trend over 60 years. Below 2,500 m elevation, 88% of the snow courses showed negative trends. Little to no trends were observed above 2,500. Decreased April 1 SWE occurred as precipitation generally increased in the Sierra Nevada over the same period.
Panel Discussion: Possible Impacts on Flood Dynamics From Future Climate Changes
Managing American River Flood Risks and Regional Groundwater Levels Together
Folsom Reservoir plays a critical role in Sacramento's flood risk management and in the operation of the Central Valley Project (CVP) water service contract deliveries.
The draft American River Basin Study evaluated potential climate change impacts on water resources in the watershed. It forecasts a pronounced shift in peak runoff by more than a month earlier by mid-to-late century while the total annual volume remains about the same. This suggests a significant reduction in available water supply and an increase in spills during flood season. Forecast-Informed Reservoir Operations (FIRO) combined with Managed Aquifer Recharge (MAR) was identified as one of six basin-wide adaptation portfolios to address climate vulnerabilities and improve regional water resiliency.
The recent technology advancement in weather forecasting and improved understanding of atmospheric river phenomena as related to flood events in California have increased flood forecast skills for both volume and timing. Reliable seven to ten-day-ahead forecasts are already achievable. This creates opportunities to refine reservoir operation to allow additional conditional storage in flood control space to increase water supply and environmental benefits. The recently completed Lake Mendocino FIRO Viability Analysis demonstrates the maturity of FIRO as the future of reservoir flood operations.
At Folsom Reservoir, the completed auxiliary spillway alleviated dam safety concerns and provided needed release capacity to safely implement conditional storage under FIRO. The associated 2019 water control manual update lowered the institutional barriers previously inhibiting a formalized FIRO operation.
FIRO is a catalyst to unlock additional value and enhance the benefits of planned infrastructural improvements. FIRO implementation at Folsom Reservoir also enables SAFCA to collaborate with upstream reservoir facility operators for additional flood protection and promote MAR with a portion of the winter flood volumes conserved through FIRO. This effort relies on coordination with U.S. Bureau of Reclamation to develop agreements that would allow using water conserved in conditional storage to be released during the winter flood season into the Folsom South Canal, where it could then be diverted to groundwater recharge projects south of the American River basin. This recharge would enhance sustainable groundwater management efforts in the south county while providing the multiple benefits that are a hallmark of water resiliency projects.
FIRO + New Bullards Bar Secondary Spillway: Enhancing Public Safety and Climate Change Resiliency
Yuba Water Agency is a stand-alone special district created for the purposes of flood risk reduction and water supply reliability for the people of Yuba County. To continue to enhance these primary missions the Agency is co-leading the Yuba-Feather Forecast Informed Reservoir Operation (FIRO) program.
As a part of the Yuba-Feather FIRO program, Yuba Water is taking a progressive, forward looking approach to building climate resilience in the watershed and the region by further enhancing operational partnerships. The program includes many state and federal partners including leadership from the State Water Project (Lake Oroville), the Army Corps, and Scripps Institute of Oceanography's Center for Western Weather and Water Extremes (CW3E).
The FIRO program seeks to improve forecasts specific to the Yuba-Feather region and integrate the improved forecasts into more flexible and adaptable reservoir operations under the Corps Water Control Manuals for New Bullards Bar and Lake Oroville.
In parallel with the FIRO program, the Agency is currently planning a new Secondary Spillway at New Bullards Bar that is designed to be operated under FIRO. The additional outlet will have gates lower in elevation in the reservoir and will add significant increases in release capacity. This will allow for increased flexibility in managing the reservoir and downstream watershed. The project will enhance flood protection for more than 100,000 residents and reduces risk to property in Yuba and Sutter counties, by lowering water levels on levees during high flows.
Together "software" FIRO enhancements through updates to the New Bullards Bar and Lake Oroville Water Control Manuals and a new "hardware" infrastructure project at New Bullards Bar will significantly improve public safety and climate resilience in the Yuba-Feather.
Megafire + Rain = Sediment: Evolution of French Meadows Forest Restoration Project
The 2014 King Fire in Eldorado National Forest incinerated nearly 100,000 acres of headwater forest with unprecedented rapidity and severity. Within the watershed of the Rubicon River, tributary to the Middle Fork American River and a major source of water supply and hydroelectric power generation for the people of Placer County and the region, 40,000 contiguous acres were burned to such high severity that all vegetation and topsoil organic matter were destroyed.
Forest soils provide the base of the terrestrial ecosystem as well as a water storage sponge which releases moisture to vegetation and hydrology throughout California's long summer dry season. The absorptive properties of this soil sponge also moderate storm runoff and flood potential downstream during the wet season. Severe burning of the topsoil oxidizes organic matter into hydrophobic waxy substances which inhibit water absorption, promoting surface runoff and erosion. The severe soil burning during King Fire resulted in the erosion of hundreds of thousands of tons of topsoil into Rubicon River during the first wet season, clogging aquatic habitats as well as water storage facilities and hydroelectric generation infrastructure owned and operated by Placer County Water Agency (PCWA).
Looking to proactively protect the adjacent watershed providing water, power, environmental, and recreational resources, PCWA and Placer County in 2016 joined with the American River Conservancy, Tahoe National Forest, The Nature Conservancy, and University of California Merced to create an avant garde partnership to use ecological forestry to reduce forest fuels and severe fire potential in the headwaters of the Middle Fork American River at French Meadows. The French Meadows Forest Restoration Project is beginning its third operating season, having treated 2300 acres of federal land, employing local contractors, and introducing prescribed fire onto the landscape.
Improved Weather and Subseasonal-to-Seasonal Predictions with Machine Learning
Systematic and random errors in dynamical model forecasts mask valuable information about precipitation patterns in extreme weather events. Machine learning based post-processing methods can be used to tackle both kinds of error.
In this study, we leverage a 34-year reforecast developed by the Center for Western Weather and Water Extremes (CW3E), which is based on the CW3E's version of the Weather Research and Forecasting (WRF) model tailored for the prediction of extreme events over the Western U.S. (West-WRF). We use a U-Net architecture convolutional neural network (CNN) to identify and reduce errors in the 24-hour accumulated precipitation over the North American west coast. The Parameter-elevation Relationships on Independent Slopes Model (PRISM) climate dataset is used as the ground truth in the training phase and for forecast verification. The U-Net CNN training and testing phases show a consistent 15% improvement in root mean squared error (RMSE), a 10% improvement in mean absolute error (MAE), and a 2-5% improvement in correlation compared to the raw West-WRF forecast. We report improvements for severe precipitation events over California, which covers most of the spatial domain considered in this study.
In this talk we will also discuss how machine learning algorithms as random forest and neural networks can be designed for seasonal predictions to skillfully predict large spatial clusters of precipitation anomaly over a three-month period.
USGS Real Time Flood Inundation Mapping Program
The US Geological Survey (USGS) Flood Inundation Mapping Program focuses its efforts at state and local levels to help communities understand flood risks and make cost-effective mitigation decisions. We partner with local communities to assist in the development and validation of flood inundation map libraries. Communities use these maps to help protect lives and property. The USGS works with the National Weather Service, the U.S. Army Corps of Engineers, and the Federal Emergency Management Agency to connect communities with available federal resources thereby ensuring the quality and consistency of flood inundation maps across the country.
The USGS Flood Inundation Mapper combines flood inundation map libraries with real-time USGS river-level data and National Weather Service flood forecasts into a powerful tool that helps communicate when and where it may flood and allows for better tools to inform local responses that can protect lives and property.
Access the Flood Inundation Mapper and available flood inundation map libraries at: https://fim.wim.usgs.gov/fim/.
Panel Discussion: Adaptations to Future Climate Impacts on Flood Events
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