South Coast

Hourly storm characteristics along the U.S. West Coast: Role of atmospheric rivers in extreme precipitation

American Geophysical Union (AGU) | June 21st, 2017

Gridded hourly precipitation observations over the conterminous U.S., from 1948 to 2002, are analyzed to determine climatological characteristics of storm precipitation t

Gridded hourly precipitation observations over the conterminous U.S., from 1948 to 2002, are analyzed to determine climatological characteristics of storm precipitation totals. Despite generally lower hourly intensities, precipitation totals along the U.S. West Coast (USWC) are comparable to those in southeast U.S. (SEUS). Storm durations, more so than hourly intensities, strongly modulate precipitation-total variability over the USWC, where the correlation coefficients between storm durations and storm totals range from 0.7 to 0.9. Atmospheric rivers (ARs) contribute 30–50% of annual precipitation on the USWC and make such large contributions to extreme storms that 60–100% of the most extreme storms, i.e., storms with precipitation-total return intervals longer than 2 years, are associated with ARs. These extreme storm totals are more strongly tied to storm durations than to storm hourly or average intensities, emphasizing the importance of AR persistence to extreme storms on the USWC.

How are Western water districts managing groundwater basins?

California Agriculture (UCANR) | March 13th, 2018

Making the transition from open-access groundwater rights to sustainable groundwater management is a formidable task for newly formed groundwater sustainability agencies

Making the transition from open-access groundwater rights to sustainable groundwater management is a formidable task for newly formed groundwater sustainability agencies in California. As agencies begin to decide how to make equitable water allocations, how to monitor groundwater use and what mix of supply- and demand-side mechanisms to adopt to satisfy sustainability criteria, the groundwater management strategies in place across other basins in the western United States are worth studying. We surveyed 18 groundwater districts in California and other Western states to identify the management approaches and practices they have instituted. The conclusions we draw suggest a correlative rights framework of water allocation with phase-ins for heavy users; metered pumping; flexible arrangements for trading and carrying over allocations for multiple years; and incentivizing groundwater recharge, including recharge from deep percolation from crops. Rigid formulas for significantly reducing groundwater use in medium- and high-priority basins are likely to have significant negative effects on the regional economy.

Hudson v. Dailey

Supreme Court of California | December 1st, 1909

Human emissions drive recent trends in North Pacific climate variations

Nature Portfolio (Springer Nature) | August 14th, 2025

Click here for a plain language discussion of the article by co-authors Pedro N. DiNezio and Timothy M. Shanahan

Click here for a plain language discussion of the article by co-authors Pedro N. DiNezio and Timothy M. Shanahan

Human-induced changes in wind, temperature and relative humidity during Santa Ana events

Climatic Change (Springer) | November 24th, 2011

The frequency and character of Southern California’s Santa Ana wind events are investigated within a 12-km-resolution downscaling of late-20th and mid-21st century time

The frequency and character of Southern California’s Santa Ana wind events are investigated within a 12-km-resolution downscaling of late-20th and mid-21st century time periods of the National Center for Atmospheric Research Community Climate System Model global climate change scenario run. The number of Santa Ana days per winter season is approximately 20% fewer in the mid 21st century compared to the late 20th century. Since the only systematic and sustained difference between these two periods is the level of anthropogenic forcing, this effect is anthropogenic in origin. In both time periods, Santa Ana winds are partly katabatically-driven by a temperature difference between the cold wintertime air pooling in the desert against coastal mountains and the adjacent warm air over the ocean. However, this katabatic mechanism is significantly weaker during the mid 21st century time period. This occurs because of the well-documented differential warming associated with transient climate change, with more warming in the desert interior than over the ocean. Thus the mechanism responsible for the decrease in Santa Ana frequency originates from a well-known aspect of the climate response to increasing greenhouse gases, but cannot be understood or simulated without mesoscale atmospheric dynamics. In addition to the change in Santa Ana frequency, we investigate changes during Santa Anas in two other meteorological variables known to be relevant to fire weather conditions—relative humidity and temperature. We find a decrease in the relative humidity and an increase in temperature. Both these changes would favor fire. A fire behavior model accounting for changes in wind, temperature, and relative humidity simultaneously is necessary to draw firm conclusions about future fire risk and growth associated with Santa Ana events. While our results are somewhat limited by a relatively small sample size, they illustrate an observed and explainable regional change in climate due to plausible mesoscale processes.

Hydraulic fracturing near domestic groundwater wells

National Academy of Sciences (NAS) | December 12th, 2017

Hydraulic fracturing operations are generating considerable discussion about their potential to contaminate aquifers tapped by domestic groundwater wells. Groundw

Hydraulic fracturing operations are generating considerable discussion about their potential to contaminate aquifers tapped by domestic groundwater wells. Groundwater wells located closer to hydraulically fractured wells are more likely to be exposed to contaminants derived from on-site spills and well-bore failures, should they occur. Nevertheless, the proximity of hydraulic fracturing operations to domestic groundwater wells is unknown. Here, we analyze the distance between domestic groundwater wells (public and self-supply) constructed between 2000 and 2014 and hydraulically fractured wells stimulated in 2014 in 14 states. We show that 37% of all recorded hydraulically fractured wells stimulated during 2014 exist within 2 km of at least one recently constructed (2000–2014) domestic groundwater well. Furthermore, we identify 11 counties where most (>50%) recorded domestic groundwater wells exist within 2 km of one or more hydraulically fractured wells stimulated during 2014. Our findings suggest that understanding how frequently hydraulic fracturing operations impact groundwater quality is of widespread importance to drinking water safety in many areas where hydraulic fracturing is common. We also identify 236 counties where most recorded domestic groundwater wells exist within 2 km of one or more recorded oil and gas wells producing during 2014. Our analysis identifies hotspots where both conventional and unconventional oil and gas wells frequently exist near recorded domestic groundwater wells that may be targeted for further water-quality monitoring.

Hydroclimate Report Water Year 2018

California Department of Water Resources (DWR) | March 31st, 2020

Water year (WY) 2018 whipsawed from dry to wet and added high heat for the summer. The WY ended with 82 percent of average precipitation in the Northern Sierra and 20 per

Water year (WY) 2018 whipsawed from dry to wet and added high heat for the summer. The WY ended with 82 percent of average precipitation in the Northern Sierra and 20 percent below average April-July streamflow in the Sacramento River and 19 percent below average for the San Joaquin River. Peak statewide snowpack was only 60 percent of average. While precipitation and stream flow were below average, the big story of the 2018 WY were record temperatures that lead to several unprecedented and deadly wildfires in the state.

Hydroclimate Report Water Year 2019

California Department of Water Resources (DWR) | June 30th, 2020

Water year (WY) 2019 continued to demonstrate effects of climate change with greater variability and more extremes. Hydroclimate extremes presented themselves with atmosp

Water year (WY) 2019 continued to demonstrate effects of climate change with greater variability and more extremes. Hydroclimate extremes presented themselves with atmospheric rivers causing both flooding and heavy snowpack. Before the wet winter and spring months, California experienced a very dry fall in where the devastating Camp Fire started on November 8, 2018 in Butte County. Fall transitioned to into a wet winter with 64 percent of the WY total for the DWR Northern 8-station index falling in the three months of January, February and March. Temperatures went from extreme cold in February to hot in August as the year progressed from winter to summer.

Hydroclimate Report Water Year 2020

California Department of Water Resources (DWR) | August 31st, 2021

Water Year (WY) 2020 continued to demonstrate the climate change narrative of greater variability and more extremes. In both precipitation and temperature, there were ne

Water Year (WY) 2020 continued to demonstrate the climate change narrative of greater variability and more extremes. In both precipitation and temperature, there were new extremes with a very dry February and very hot end to the water year. Late fall precipitation onset continued with the first significant storm of the year arriving in late November. The WY ended with 67 percent of average precipitation statewide and 62 percent of average in the Northern Sierra. For runoff, the Sacramento Basin had 66 percent of average April-July streamflow while the San Joaquin Basin had 58 percent of average. Peak statewide snowpack was 62 percent of average reaching that point in early April with a late season storm providing some offset to the winter’s dryness. February was one of the big stories of WY 2020 with record low precipitation. In a year of extremes, August and September also contributed record warmth for the month including a 130-degree Fahrenheit reading at Death Valley on August 16, 2020. This observation, if verified by the World Meteorological Organization, would be one of the hottest daily temperature maximums recorded on Earth. Several cities set new records for days above 90 degrees as well.

Hydroclimate Report Water Year 2021

California Department of Water Resources (DWR) | December 31st, 2022

Water year (WY) 2021 added to the climate change narrative with extreme dryness leading to record setting drought conditions. In both precipitation and temperature, there

Water year (WY) 2021 added to the climate change narrative with extreme dryness leading to record setting drought conditions. In both precipitation and temperature, there were new extremes with a record dry and warm spring being most notable. Late fall onset occurred again this year arriving in late November starting the seasonal snowpack on very dry soils. The WY ended with 48 percent of average precipitation statewide making it the second driest year in 126 years of record. It was also second warmest for statewide average temperature. For two-year totals, WY 2021 set a record, recording less precipitation than WY 1977. There was only one significant storm in the WY in late January which included low elevation snow and very strong damaging winds. The April 1 statewide snowpack registered 62% of average. Peak snowpack at 64% of average occurred the week before April 1. Runoff from the snowpack was far below expectations due to the dry antecedent conditions and record dry and warm spring. The lack of runoff resulted in the need for emergency drought response measures.