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. (No significant rain yet, by the way.) 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 2026 dry season and prepare, hopefully, for the wet season, it is good to reflect on the last water year and prepare for what is also California’s flood season. This is commonly a time for largely futile predictions and speculations of precipitation and runoff for the coming 12 months.
The 2026 water year was near-average when averaged across the state (Figure 2). Not too wet. No major floods. Not a drought. Slightly drier than the 2025 water year. The 2024 water year was also unusually average. California has now had three unusually normal water years (averaged statewide), after a fairly wild decade of dry and wet years. This is the longest run of near-average years since the late 1990s.
Figure 3 shows that for WY 2026, most of California had near-average total precipitation (about 95% on average). But as is typical, some months were unusually wet (December and April), and others were unusually dry (March) (Figure 1). (The mathematics of averages usually require drought years to have more dry months and fewer wet months.)
2026 WY had less precipitation variability across the state than usual – unusually average in time and space (Figure 3), but still with substantial variability across months.
Northern California tends to be wetter and southern California to be drier. Also, California is large with much local variability, but seemingly with fewer deviations than usual from local averages in 2026 than in most years – California’s weather and climate are 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
The paths of snow accumulation in recent years of near-average precipitation (including 2026) tend to be well below statistically average snow levels in the spring. This has some implications for reservoir and groundwater recharge operations. In almost any year, there is almost no carryover of snowpack across years (except some as groundwater recharge).
Storage in most major California surface reservoirs today (at the end of the irrigation and dry seasons) tends to be a small percent of storage capacity. However, as a percent of average reservoir storage at this time of year, most large reservoirs are a little below last year at this time, but still near or above average: 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 California’s largest supply of stored water, 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, challenging compliance with SGMA by 2040. But some groundwater levels have been improving substantially. DWR’s groundwater data site (Groundwater Live) provides some useful data on groundwater levels and subsidence: https://sgma.water.ca.gov/CalGWLive/.
Spencer Cole at PPIC recently published some encouraging results on changes in Central Valley groundwater levels since 2021 (Figure 5). Dry years and average years have major overdraft, with wet years having substantial net recharge. Overall, there is some reduction in the overdraft rate (but sizable overdraft continues).
What will happen in 2027?
Nobody really knows, but expect much speculation from now until March 2027. 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 overall precipitation and water availability conditions.
Statistically, there is essentially no correlation of precipitation in northern California from one year to the next, as seen in Figure 6. Similarly, there is only about 1.2% simple correlation between El Niño conditions and runoff from northern California (Figure 7). El Niño-runoff 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.
Today, with an extreme El Niño forecast, California flood management is benefiting from the popular and political attention, even if it does not actually result in flooding. (Although the variance in northern California runoff might increase with El Niño.)
Sometimes in life, and always with California water, all we can do is prudently prepare for extremes and surprises. Preparing for a range of future conditions is central to California water management. Sometimes useful political and popular attention is based more on fashionable and scienterrific, rather than technical, thinking. Decision-making is not entirely rational.
The Future
“There is rarely a shortage of water, but often shortages of cheap water.” – common water economist saying (see Note 1).
The future seems likely to have less water availability for California than it has become used to historically. This will be mostly from eliminating overdrafting from California’s groundwater and Colorado River reservoirs, plus climate change and needs for additional environmental water dedications (Lund et al. 2024). Water scarcity will be a growing challenge.
What to do?
Every year, water managers and users must prepare for both flood and drought, in addition to their other duties.
Californians should pay attention to water and likely climate changes, without complacency or panic over our effective but flawed water management system and institutions. Changes are needed to develop 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 risky 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 these less urgent average years to think hard and discuss such challenges soberly with others, especially those outside your current advocacy identity. These long-term conversations should seek to focus on long-term issues and eschew short-term advantage. (Clearly, I’ve retired.)
For 2027, we will have a new Governor and new leaders for many of the State’s water agencies and legislative committees, as well as many regional and local water agencies and stakeholders. These folks will face challenges, and they will need help. So, drink (water) to celebrate the opportunities of this new water year.
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 recent conditions.
- 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. (2025), “Happy New Water Year 2026!” California Water Blog. September 2025.
- 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.
Note 1 – The origins of this quote might come from: ‘David S. Jenkins, the man who heads up the Department of Interior’s Office of Saline Water, points out that there really is no shortage of water. ‘There’s a shortage of cheap water,’ he says. ‘We have all the water anyone will ever need in our oceans. The trick is to desalt it and put it where we want it at reasonable cost.’”
– Popular Mechanics, May 1959, p. 119. [Provided by Gemini]
Leave a Reply