By Jay Lund
October 1 marks the beginning of California’s new Water Year (WY). Water years here run from October 1 until September 30 of the next calendar year and are named for the calendar year of the bulk of the water year (January-October).
October 1 is also the nominal beginning of California’s wet season. California’s hydrology has two basic seasons, wet and dry, with the wet season nominally from October 1 – April 1 (Figure 1). The first sizable storm sometimes arrives in late September and sometimes not until November (recently in 2021). Sizable storms rarely arrive after April.
As we leave California’s long dry season, and prepare hopefully for the wet season, it is a good time to reflect on the last water year and prepare for what is also California’s flood season. At this time, it also is customary for people to make largely futile predictions and speculations of precipitation for the coming 12 months.
The 2025 water year was near-average when averaged across the state (Figure 2). Not too wet. No major floods. Not a drought. Both WY 2024 and 2025 were unusually normal (averaged statewide) for the last highly variable decade.
Figure 3 shows that northern California in WY 2025 had near-average total precipitation (about 108% of average). But, as is typical, some months were unusually wet (November and February) and others were unusually dry (January). (Drought years tend to have more dry months and fewer wet months, as the mathematics of statistics and averages require.)
2025 was highly unusual in southern California, with an extraordinarily late start to the wet season; remember the unusually late and devastating January 2025 wildfires in southern California. Overall, southern California had a very dry WY 2025 (Figure 3).
The San Joaquin Valley tended to be on the drier side of average overall.
Figure 3 also shows several other common features of California’s hydrology. First, northern California tends to be wetter and southern California to be drier. Second, California is large with much local variability – weather and climate is often spotty within regional tendencies.
What is the California water system’s current condition in terms of water storage? What is the likelihood of floods or drought in WY 2026? What should we do now?
Storage going into 2026 Water Year
For the Central Valley, Water Year 2025’s precipitation was scattered, a bit above average in the north, drier but not drought in the San Joaquin Valley, and distinctly dry in southern California. https://cdec.water.ca.gov/snow_rain.html
Most major California reservoirs today have a little above average water storage pretty much statewide, https://cdec.water.ca.gov/reportapp/javareports?name=RESSW. If the coming water year is moderately dry, this storage will be helpful for cities and agriculture.
Groundwater is by far the largest supply of stored water in California, particularly for droughts. Despite passage of the Sustainable Groundwater Management Act (SGMA) and a few wettish years, some San Joaquin and Tulare basins continue to deepen their groundwater overdraft and land subsidence, worsening prospects of compliance with SGMA by 2040. DWR’s groundwater data site (Groundwater Live) provides some useful data on groundwater levels and subsidence: https://sgma.water.ca.gov/CalGWLive/ Hopefully this site will continue to mature to give more consolidated indications of groundwater trends, comparable to data available for surface water storage.
What will happen in 2026?
Nobody really knows, but expect extensive speculation from now until March. Only by March (and sometimes mid-April) is it late enough in California’s mercurial wet season to have already seen and reasonably predict the water year’s precipitation.
Statistically, there is almost no correlation of unimpaired runoff in northern California from one year to the next, as seen in Figure 4. Similarly, there is almost no simple correlation between El Niño conditions and runoff from northern California (Figure 5). However, El Niño correlations are a bit stronger in southern California. (There is a little more correlation in streamflow from one year to the next, mostly driven by overyear groundwater storage.)
Forecasting storms more than a few days becomes increasingly uncertain. Sometimes in life, and always with California water, all we can do is to prudently prepare for contingencies and surprises. Preparing for a range of future conditions is central to California water management.
The Future
The future seems likely to have less water availability for California than it has become used to historically. This will come mostly from the unavoidable elimination of overdrafting of California’s groundwater and Colorado River reservoirs, as well as climate change and needs for additional environmental water dedications (Lund et al. 2024). This will be a growing challenge.
What to do?
Every year, water managers and users must be prepared for both flood and drought, in addition to their other duties.
Californians should pay serious attention to water and likely climate changes, without complacency or panic over our remarkably effective but substantially flawed water management system and institutions. Changes are needed in the development of coordinated (perhaps consolidated) technical work by the DWR and SWRCB in groundwater modeling, regional water balances, and other areas for SGMA implementation and other challenges (particularly regarding ecosystems). Complacency and panic are convenient rhetorically and for fund-raising, but they also are expensive and potentially life-threatening reactions to situations that deserve serious and more constructive thought, analysis, and deliberations.
Today, most of our water deliberations and analyses remain relics of the history of water infrastructure and allocation for agricultural and urban growth. They are not without value but need improvements to help us adapt to a changing climate, ecosystems, economy, and social concerns.
Take advantage of less urgent average years to think hard and discuss such challenges soberly with others, especially those outside your current advocacy identity. These are long-term conversations we should not seek to control for short-term advantage.
So, have a drink (of water) to celebrate the new water year. Even average years are never entirely average here and are becoming less frequent. We must prepare for floods, droughts, and middling water years that give opportunities to make reasoned preparations and investments.
About the Author
Jay Lund is an Emeritus Distinguished Professor of Civil and Environmental Engineering and Vice Director of the Center for Watershed Sciences at the University of California – Davis.
Further Reading
California’s DWR has (perhaps too many minimally coordinated but very) informative websites:
DWR Water Watch https://cww.water.ca.gov/ A summary of some interesting conditions, but a bad short introduction.
California’s Groundwater Live: https://sgma.water.ca.gov/CalGWLive/ A useful beginning for a great California groundwater website, which does not seem to have been built.
CDEC – California Date Exchange Center https://cdec.water.ca.gov/ Probably the richest website, except for groundwater. A geek can learn lots about California water from this site, in so many ways.
Sacramento Valley historical unimpaired runoff data. http://cdec.water.ca.gov/cgi-progs/iodir/WSIHIST . For the geeky.
Lund, J., J. Medellin-Azuara, and A. Escriva-Bou (2024), The Magnitude of California’s Water Challenges, Commissioned by the California Municipal Utilities Association, May 2024
Lund, J. (2024), “Happy New Water Year 2025! – Wet, dry, or just plain weird?” California Water Blog. September 29, 2024.
Lund, J. (2023), “Happy New Water Year 2024! – from 2023’s wild ride to the wilderness of 2024.” . California Water Blog. October 8, 2023.
Lund, J. (2022), “Happy New Water Year 2023!“ California Water Blog. October 2, 2022.
Moyle, P. (2023), “Future Ancestors of Freshwater Fishes in California.” . California Water Blog. September 17, 2023.
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