By Jay Lund

California’s Water Year runs from October 1 of the previous calendar year through September 30.  California’s “wet” season is traditionally October 1 – April 1.  The rest of the year (and often parts of the “wet” season) is usually dry.  We can get major storms into April, but often not.

So almost all of this Water Year’s precipitation has fallen already.  

Precipitation

Statewide precipitation this wet season has been unusually average overall (104%), but a bit weird otherwise.  The north was unusually wet (and without major floods), but the south was unusually dry – so extremely dry for months that the south had extensive wildfires in January!  

Like many years, this wet season had some wild swings between months, as well as across regions.  Northern California was unusually dry until mid-November (25% of average), until some good storms began arriving (254% of average November), but January was dry again (12% of average), and February very wet (187% of average), before settling into an average (102%) March.  No wonder we are often confused about how we the water year will be until the end of March.  (In most years California gets some form of monthly whiplash, separately from the recently-famed interannual weather whiplash (Swain et al 2025).

Snowpack

Snowpack is doing well with about 99% for this date, 120% for the north, 91% for the central Sierra and 85% for the southern Sierra.  Not a bad year for snow.

The figure below shows we can have a wide range of snow accumulations and melting patterns.  These are changing with a warming climate.

Reservoir Storage

Because the last two years have not been dry, California has an unusual amount of water in its reservoirs.  Much unfilled storage in the table below is remaining empty flood storage, which will be partially or entirely filled by melting snow in the coming month or so.   Alas, this is not true for the Colorado River reservoirs, which continue to be overdrafted since 1983, when they filled for the first and perhaps last time.  

Groundwater

California’s systematic groundwater data is still maturing for statewide and regional assessments but seems to be improving rapidly.  

Over the last 20 years, of several thousand wells monitored semi-annually, few had increasing groundwater levels, and more had decreased levels than no change.  Areas with the greatest overdraft are concentrated in the southern Central Valley’s Tulare Basin.  This pattern of overdraft is about a century old.

The last three wetter years have partially reversed trend, with more well levels rising than falling.  However, areas with the greatest historical overdraft also seem to have more wells where water levels continue to decrease.  

Is SGMA working, or did we just have three wetter years?  Maybe a bit of both.  What Is certain is that the last three years have been unusually wet. As we move closer to 2040, the state will need a reasoned approach for considering how much of non-compliance is hydrologic luck as opposed to inadequate management (Escriva-Bou et al. 2020).

SWP and CVP Water Deliveries

State Water Project (SWP) allocations are now 40% for this year (the 30-year average allocation is 60%).  The Central Valley Project allocations are 100% for the most senior water contractors (Settlement and Exchange contractors) and for Friant contractors’ Class 1 water, 75% of historical use for urban contractors, and 40% for other south-of-Delta contractors (12-year average is 30%).   There is a possibility that some allocations might increase further.

For most state and federal water contractors, this water year is good or ok, certainly not a drought year.  But there will always be demands for more water from the projects, especially with SGMA bringing groundwater overdraft to an end in some of the driest parts of California.

Biological Water Year Data

As we intensely struggle with slow progress in improving water operations for ecological purposes, we will need more organized and real-time information on biological conditions.  We collect considerable data on salmon returns and juveniles statewide, Delta smelt (alas, mostly zeros), and other fishes, as well as various waterbirds.  

But these data are not yet organized for policy, water management, or public discourse.  Today they are fragmented and tend to be mostly available annually.  There are some efforts to improve this situation, such as https://www.cbr.washington.edu/sacramento/.  

We need better organized and available real time data to support broader and more integrated discussions on ecosystem policy and water management.  Otherwise California’s struggles will be longer, more expensive, more controversial, and less productive. 

(As is often the case, the organization of data on a problem reflects the organization and effectiveness of our institutions.  This is borne out by recent improvements in groundwater data.)

Overall

California’s 2025 water year has had a good “wet” season overall.  Neither floods nor droughts overall, except for great dryness in southern California creating landscape conditions for horrible wildfires.  Even in statistically average years, California water will usually be weird in places and at times.  

As we work to improve water management, we need to improve our data management, and water accounting.  The pace of innovation is often limited by our ability to organize effective data development.

About the Author

Jay Lund is an Emeritus Professor of Civil and Environmental Engineering at the University of California – Davis, and Vice Director at the Center for Watershed Sciences.

Further Reading

https://cww.water.ca.gov

Click to access javareports

Click to access javareports

https://cdec.water.ca.gov/reportapp/javareports?name=STORAGE

https://sgma.water.ca.gov/CalGWLive/#groundwater

Escriva-Bou, A., R. Hui, S. Maples, J. Medellín-Azuara, T. Harter, and J. Lund “Planning for Groundwater Sustainability Accounting for Uncertainty and Costs: an Application to California’s Central Valley,” Journal of Environmental Management, Vol. 264, 110426, June 2020.

Swain, D. L., A. F. Prein, J. T. Abatzoglou, et al. 2025. “Hydroclimate Volatility on a Warming Earth.” Nature Reviews Earth & Environment 6, no. 1: 35–50. https://doi.org/10.1038/s43017-024-00624-z.

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