The 2010/2011 snow season in California’s Sierra Nevada: Role of atmospheric rivers and modes of large-scale variability

American Geophysical Union (AGU) | October 17th, 2013

The anomalously snowy winter season of 2010/2011 in the Sierra Nevada is analyzed interms of snow water equivalent (SWE) anomalies and the role of atmospheric rivers (AR

The anomalously snowy winter season of 2010/2011 in the Sierra Nevada is analyzed interms of snow water equivalent (SWE) anomalies and the role of atmospheric rivers (ARs)—narrow channels of enhanced meridional water vapor transport between the tropics and extratropics. Mean April 1 SWE was 0.44 m (56%) above normal averaged over 100 snow sensors. AR occurrence was anomalously high during the period, with 20 AR dates during the season and 14 in the month of December 2010, compared to the mean occurrence of nine dates per season. Fifteen out of the 20 AR dates were associated with the negative phases of the Arctic Oscillation (AO) and the Pacific-North American (PNA) teleconnection pattern. Analysis of all winter ARs in California during water years 1998–2011 indicates more ARs occur during the negative phase of AO and PNA, with the increase between positive and negative phases being90% for AO, and50% for PNA. The circulation pattern associated with concurrent negative phases of AO and PNA, characterized by cyclonic anomalies centered northwest of California, provides a favorable dynamical condition for ARs. The analysis suggests that the massive Sierra Nevada snowpack during the 2010/2011 winter season is primarily related to anomalously high frequency of ARs favored by the joint phasing of AO and PNA, and that a secondary contribution is from increased snow accumulation during these ARs favored by colder air temperatures associated with AO, PNA, and La Nina.

The Big Water Supply Shift: Groundwater Key to Water Security in California's Changing Climate

Union of Concerned Scientists | November 1st, 2015

For more than a century, California has relied on its snowmelt-fed reservoirs, rivers, and streams for the majority of its water, but drought and climate chang

For more than a century, California has relied on its snowmelt-fed reservoirs, rivers, and streams for the majority of its water, but drought and climate change are depleting those traditional supplies. Snow is already melting as many as 30 days earlier than in the mid-twentieth century, meaning less water is available during the hotter months when water demand is highest. California is increasingly turning to groundwater to meet its water needs. Today, groundwater supplies up to 50 percent of California’s water, but California’s prolonged drought has led to the over-pumping of groundwater, overdrafting the Central Valley’s aquifers. Sustainable groundwater management will allow the state to adapt to climate change while increasing water reliability in the future.

The California Water Atlas

Governor's Office of Planning and Research (OPR) | October 24th, 1979

This book sets out to tell the biggest story in the richest and most populous state in the Union. Water lies at the basis of the modern prosperity of California, and th

This book sets out to tell the biggest story in the richest and most populous state in the Union. Water lies at the basis of the modern prosperity of California, and the history of the state is in large part the history of water development. The problems of water supply and delivery for the future are emerging among the critical issues facing not only California but the entire American Southwest over the next ten years. And yet, at a time when environmental consciousness is high and complex problems of world energy supply and international finance are part of the normal fare in our daily newspapers, water remains probably the least popularly understood of our natural resources. For a higher resolution copy, click here to be taken to Digital Commons of California State University, Monterey Bay

The California Water Plan: Projected Use and Available Water Supplies to 2010

California Department of Water Resources (DWR) | December 15th, 1983

The case for state-level drinking water affordability programs: Conceptual and empirical evidence from California

Utilities Policy (Elsevier) | April 1st, 2020

Informed by the design of a program in California, we make a three-part argument for why state-level governments might consider implementing direct bill-assistance progra

Informed by the design of a program in California, we make a three-part argument for why state-level governments might consider implementing direct bill-assistance programs for drinking water service to address growing affordability concerns for low-income households. State-level operation of bill-assistance programs represents a departure from the global trend of allowing water utilities to operate bill-assistance programs at their own discretion. We first explain the motivation for enhanced public sector support to help households pay for drinking water service. In state-level contexts such as California, we find that the basic household affordability equation has worsened due to an increase in the numerator of residential retail bills and a stagnation in the denominator of household incomes. We next demonstrate why direct bill assistance to households is necessary even with the presence of other system-level financial support or retail rate structure policies. Finally, we make the case that direct bill-assistance programs are best situated at the state level, due to substantial variation in systems’ capacity and demonstrated willingness to run their own robust affordability programs for vulnerable customers.

The Case of Shifting Snow

Climate Central | February 5th, 2020

Whether you live among palm trees or pine trees, snow plays a critical role in our climate. Snow keeps our planet cooler, significantly affects water resources, and is a

Whether you live among palm trees or pine trees, snow plays a critical role in our climate. Snow keeps our planet cooler, significantly affects water resources, and is a revealing indicator of climate change. Forecasting snowfall and determining long-term trends of snow climatology are inherently challenging, but the research team at Climate Central has produced an analysis of snowfall trends across the United States. While no single overall national trend in snowfall can be discerned from the results, clear regional and seasonal patterns do emerge. In almost all areas of the country, snow is decreasing in the “shoulder” seasons—fall and spring. Results from 145 locations show that 116 stations (80%) had decreased snowfall before December, and 96 stations (66%) had decreased snowfall after March 1. Winter showed a mixed record, with more snow in northern climates, and decreasing snow in the southern regions. We also compared total snowfall from the 1970s to the 2010s and ranked the 20 cities with the biggest percentage gains and losses, using endpoint analysis. The changing patterns of how much, when, and where snow falls have significant impacts on our climate, our economy, and our lives. This report provides a primer on the climatology of snow and includes resources on how to report on snow—or the lack of it—in your area.

The Central Valley Project - Federal or State?

University of California, Berkeley (UC Berkeley) | June 1st, 1955

The challenge of closing the climate adaptation gap for water supply utilities

Nature Portfolio (Springer Nature) | June 29th, 2024

The Changing Nature of Atmospheric Rivers

American Meteorological Society (AMS) | March 15th, 2025

Atmospheric rivers (ARs) are expected to strengthen in a warming climate, largely due to the thermodynamic (moistening) effect. Here, we show that this trend is already e

Atmospheric rivers (ARs) are expected to strengthen in a warming climate, largely due to the thermodynamic (moistening) effect. Here, we show that this trend is already evident in historical reanalysis data using the AR Tracking Method Intercomparison Project (ARTMIP) tier 2 AR detection tools (ARDTs) and variants of our own global AR detection applied to ERA5, MERRA-2, and JRA-55 reanalysis data. Over the 1980–2019 (ARTMIP) and 1980–2023 (variants) periods, total AR area increased by 6%–9%. AR integrated water vapor (IWV) increased by 1.5%–2.5% while integrated vapor transport (IVT) increased by less than 1% and 850-hPa wind speed (|V850|) and vertically integrated moisture flux convergence (VIMFC) both decreased. IWV increases were the most robust overall. All trend magnitudes were sensitive to subsetting, with fixed-frequency subsets consisting of the most intense AR grid points showing larger increases of 3%–4% IVT, 4%–6% IWV, and 6%–10% VIMFC, this last opposing a decreasing AR-mean VIMFC trend likely associated with large area increases. For individual ARs, maximum IVT and IWV increased at ;3–63 and ;1.5–23 the rate of AR-mean values, respectively. Regional changes were often even larger, particularly for extreme events, though most geospatial trends had low detectability under a false discovery rate control framework. Ultimately, despite considerable ARDT and methodological diversity, we found robust consensus moistening and expansion of ARs between 1980 and 2023. However, further research is required to determine the extent to which these trends are affected by reanalysis observational assimilation changes. For example, previous studies indicate that certain reanalyses misrepresent 1980–94 atmospheric moistening and so may underestimate historical AR expansion and intensification rates.

The Delta Plan

Delta Stewardship Council (Delta Council) | May 16th, 2013

The Delta Plan is a comprehensive, long-term management plan for the Delta. Required by the 2009 Delta Reform Act, it creates new rules and recommendations to further the

The Delta Plan is a comprehensive, long-term management plan for the Delta. Required by the 2009 Delta Reform Act, it creates new rules and recommendations to further the state’s coequal goals for the Delta: Improve statewide water supply reliability, and protect and restore a vibrant and healthy Delta ecosystem, all in a manner that preserves, protects and enhances the unique agricultural, cultural, and recreational characteristics of the Delta. Now the Council is considering updates to the Delta Plan. For information about the amendment process, please click here. Developed through eights drafts, hundreds of hours of public meetings and thousands of public comments over two years, the Delta Plan is guided by the best available science and is founded on cooperation and coordination among federal, State and local agencies. The Delta Plan is enforceable through regulatory authority in the Delta Reform Act that requires state and local agencies to be consistent with the Delta Plan. The Delta Plan was unanimously adopted by the Delta Stewardship Council on May 16, 2013. Subsequently its 14 regulatory policies were approved by the Office of Administrative Law, a state agency that ensures the regulations are clear, necessary, legally valid, and available to the public. The Delta Plan became effective with legally-enforceable regulations on September 1, 2013.