San Joaquin River

Wet Antecedent Soil Moisture Increases Atmospheric River Streamflow Magnitudes Nonlinearly

American Meteorological Society (AMS) | June 4th, 2025

Atmospheric rivers (ARs) drive most riverine floods on the U.S. West Coast. However, estimating flood risk based solely on AR intensity and duration is challenging becaus

Atmospheric rivers (ARs) drive most riverine floods on the U.S. West Coast. However, estimating flood risk based solely on AR intensity and duration is challenging because precipitation phase, antecedent conditions, and physical watershed characteristics (e.g., slope and soil depth) can influence the magnitude of floods. Here, we analyze how antecedent soil moisture (ASM) conditions contribute to variability in streamflow during AR events and how that changes across climatic regimes and physiography in 122 U.S. West Coast watersheds. We identify a robust nonlinear relationship between streamflow and ASM during ARs in 89% of watersheds. The inflection point in this relationship represents a watershed-specific critical ASM threshold, above which event maximum streamflow is, on average, 2–4.5 times larger. Wet ASM conditions amplify the hydrologic impacts of more frequent but weaker, lower moisture transport AR events, while dry ASM conditions attenuate the hydrologic impacts that stronger, higher moisture transport AR events could otherwise cause. Our research shows that watersheds prone to ASM-amplified streamflows have higher evaporation ratios, lower cold-season precipitation, lower snow-to-rain ratios, and shallower, clay-rich soils. Higher evaporation and lower precipitation lead to greater ASM variability during the cold season, increasing streamflow during wet periods and buffering streamflow during dry periods. Lower snow fraction and shallower soils limit the antecedent water storage capacity of a watershed, contributing to greater sensitivity of streamflow peaks to ASM variability. Incorporating ASM thresholds into hydrologic models in these regions prone to AR-amplified streamflow could improve forecasts and decrease uncertainty.

Wetland geomorphology and tidal hydrodynamics drive fine‑scale fish community composition and abundance

Environmental Biology of Fishes (Springer) | January 19th, 2024

What Can We Learn From How the State Responded to the Last Major Drought?

Legislative Analyst's Office (LAO) | May 13th, 2021

For the second consecutive year, the state is experiencing extremely low rates of precipitation. As we prepare for what could be an extended period of dry conditions, it

For the second consecutive year, the state is experiencing extremely low rates of precipitation. As we prepare for what could be an extended period of dry conditions, it is helpful to review how the state responded to the last major drought. Such information can inform—and thereby potentially improve—the state’s current and ongoing response to developing conditions. In this report, we summarize the major activities, spending, and policy actions undertaken by the state to respond to the severe drought that occurred from 2012 through 2016. We also describe current conditions, and highlight some key lessons the Legislature can learn from previous efforts to help guide its response to the emerging drought.

What If California’s Drought Continues?

Public Policy Institute of California (PPIC) | August 20th, 2015

California is in the fourth year of a severe, hot drought—the kind that is increasingly likely as the climate warms. Although no sector has been untouched, impacts so

California is in the fourth year of a severe, hot drought—the kind that is increasingly likely as the climate warms. Although no sector has been untouched, impacts so far have varied greatly, reflecting different levels of drought preparedness. Urban areas are in the best shape, thanks to sustained investments in diversified water portfolios and conservation. Farmers are more vulnerable, but they are also adapting. The greatest vulnerabilities are in some low-income rural communities where wells are running dry and in California’s wetlands, rivers, and forests, where the state’s iconic biodiversity is under extreme threat. Two to three more years of drought will increase challenges in all areas and require continued—and likely increasingly difficult—adaptations. Emergency programs will need to be significantly expanded to get drinking water to rural residents and to prevent major losses of waterbirds and extinctions of numerous native fish species, including most salmon runs. California also needs to start a longer-term effort to build drought resilience in the most vulnerable areas.

When is Groundwater Recharge a Beneficial Use of Surface Water?

University of California, Berkeley (UC Berkeley) | August 1st, 2018

Any diversion and use of surface water in California requires a water right. Today, to receive a new water right permit, an entity that wants to appropriate surface water

Any diversion and use of surface water in California requires a water right. Today, to receive a new water right permit, an entity that wants to appropriate surface water must file an application with the Board. The Board may only approve an application if it determines that the proposed use of the water is for a reasonable and beneficial purpose. Currently, there is uncertainty about whether—and, if so, under what circumstances—the Board will consider groundwater recharge to be a beneficial use of water. California law makes clear that the act of recharging groundwater, alone, is not a beneficial use of water. Instead, the specific purpose of the recharge is key.

Where the Wild Things Aren't: Making the Delta a Better Place for Native Species

Public Policy Institute of California (PPIC) | April 5th, 2012

This report proposes a reconciliation approach for addressing 160 years of accumulated problems and for managing the Delta’s ecosystem in the future. Reconciliation eco

This report proposes a reconciliation approach for addressing 160 years of accumulated problems and for managing the Delta’s ecosystem in the future. Reconciliation ecology seeks to improve conditions for native species while recognizing that most ecosystems have been altered irrevocably by human use and will continue to be used to support human goals. Improving ecosystem conditions for native species must, therefore, happen in a context of continuing use of land and water by humans and continuing physical and biological change.

Who makes decisions about California's water?

Restore the Delta | January 9th, 2023

This project collected and created data on California's water managers at the state, local and individual level to determine their race and gender. At the State level, we

This project collected and created data on California's water managers at the state, local and individual level to determine their race and gender. At the State level, we used the State Workforce Analysis to examine the race and gender of water-related state executives. At the local level, we coded about 1500 directors of water agencies for race and gender and then compared each board to the race and gender of the agency service areas. At the individual level, we coded nearly 14,000 individual rights holders for race. The results were not surprising. At every level, the people who make decisions about California's water are more white and more male than the population of California. At the local agency level, boards of directors are 86% White and 79% male. At the individual level, 91% of small named water rights holders have last names that code as White.

Why People Adopt Climate Change Adaptation and Disaster Risk Reduction Behaviors: Integrated Model of Risk Communication and Results from Hurricanes, Floods, and Wildfires

American Meteorological Society (AMS) | November 1st, 2022

With climate change, weather and climate disaster risks are increasing. At-risk individuals can take climate adaptation and disaster risk reduction behaviors to mitigate

With climate change, weather and climate disaster risks are increasing. At-risk individuals can take climate adaptation and disaster risk reduction behaviors to mitigate and prepare for disaster risks, reduce costs from damage, and save their lives. However, previous fragmented studies have not provided an integrated model to directly compare the factors and identify factors that are most influential in at-risk community members’ behaviors. I present the Integrated Model of Risk Communication by consolidating major theories. This study uses structural equation modeling of quantitative surveys to simultaneously test the impacts of 15 factors on 15 adaptation behaviors for the two most common federally declared disasters (wildfires and hurricanes with floods) in three disaster-prone U.S. states (California, Florida, and Texas) (N = 3,468). Specifically, this study examines 15 behaviors including preparedness, nonstructural mitigation, structural mitigation, insurance purchase, and adaptation policy support. Social norms perceptions, self-efficacy, response efficacy, and resource constraints strongly affect behaviors. Response efficacy strongly affects policy support. Risk perception, knowledge, and climate change perception—commonly argued to be key drivers—are insignificant or weak. The models explain 55%–86% of the variance in adaptation behaviors. Results suggest that the focus of adaptation efforts may need to shift from risk perception and climate change perception to efficacy and social norms.

Widespread Increase in Atmospheric River Frequency and Impacts Over the 20th Century

American Geophysical Union (AGU) | December 11th, 2025

Atmospheric rivers (ARs) are long, narrow regions of atmospheric moisture transport that provide water resources to many regions, but can also cause hazards such as extre

Atmospheric rivers (ARs) are long, narrow regions of atmospheric moisture transport that provide water resources to many regions, but can also cause hazards such as extreme rainfall and moist heatwaves. Despite their importance, there is uncertainty about how the frequency and impacts of ARs may be changing with time. Our study extends the global record of AR activity back to 1940. We find that the frequency of ARs has increased broadly across the midlatitude regions, rather than shifting polewards as observed in recent decades. This increase in AR frequency leads to more rain and snow associated with these features in both the mid‐ and high‐latitudes. We also find that there is an increase in humid heatwaves associated with ARs over the same period

Wildfire Induces Changes in Receiving Waters: A Review With Considerations for Water Quality Management

American Geophysical Union (AGU) | September 15th, 2022

Wildfires have increased in frequency in many ecosystems, with implications for human health and the environment, including water quality. Increased fire frequency and ur

Wildfires have increased in frequency in many ecosystems, with implications for human health and the environment, including water quality. Increased fire frequency and urbanization also raise the prospect of fires burning into urban areas, mobilizing pollutants few have considered to date. As a result, water quality managers lack information to anticipate, respond to and potentially mitigate wildfire impacts. Here, we reviewed the scientific literature to assess wildfire effects on response endpoints of a conceptual model linking fire to water quality, quantifying response directionality, magnitude and duration. Physically, water yield, sediments, and temperature all increased post-fire. Chemically, nutrients, ions, organic chemicals, and metals increased in burned watersheds, sometimes by orders of magnitude over pre-fire or reference conditions. In select cases, post-fire concentrations exceeded aquatic life criteria or drinking water standards, at times even in the finished drinking water. Biological assemblages commonly declined after post-fire runoff events. The duration of effects was less than 5 yr for most endpoints (e.g., metals) on average following fire, although effects did extend 15 yr or more in some individual cases. We found only a few studies on pollutants mobilized from wildfire impacted urban areas with benzene contamination in drinking water and high metal concentrations in ash prominent exceptions. Overall, this review provides a resource for understanding wildfire impacts on water quality endpoints, with the goal of informing the response of managers and other decision makers to this growing problem.