by Jay Lund and Alvar Escriva-Bou

October 1 marks the beginning of the new Water Year in California. 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).  It is a good time to reflect on the last year and make largely futile predictions of precipitation for the coming 12 months.

The 2024 water year was blissfully normal.  Not too wet.  No major floods.  Not a drought.  The year was unusually normal, for the last decade.  See Figure 1.  Little to complain about, except that farmers an environmental interests would like average flows to be higher.

What is the California’s water system’s current condition in terms of water storage?  What is the likelihood of the coming year having floods or drought?  What should we do now?

Status of storage going into 2025 Water Year

For the Central Valley, Water Year 2024’s precipitation was near average, making it highly unusual for recent decades. https://cdec.water.ca.gov/snow_rain.html

Most major reservoirs have higher than average water storage at the end of the irrigation season, https://cdec.water.ca.gov/reportapp/javareports?name=RESSW.  This storage will go a long way to dampening a drought for cities and agriculture if the coming water year is moderately dry.  But as we saw in 2020, after the wet 2019, drought conditions in reservoirs could start to worry many by the summer of 2025 if dryness is significant.

Groundwater is by far the largest supply of stored water in California, particularly for droughts.  Despite 10 years since the passage of the Sustainable Groundwater Management Act (SGMA) and a couple of wettish years, some areas of the San Joaquin and Tulare basins continue to deepen the overdraft and land subsidence that they will need to address by 2040.  DWR has a groundwater data site (Groundwater Live) that provides copious data on groundwater levels and subsidence: https://sgma.water.ca.gov/CalGWLive/  Over the coming years, this site will hopefully mature to give a more consolidated indication of annual and seasonal groundwater trends, comparable to indicators available for surface water storage.

What will happen in 2025?

Nobody really knows, but you can expect endless speculation from now until March.  Only by March (and sometimes mid-April) is it late enough in California’s mercurial wet season that we have seen most of 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 2.

Similarly, there is almost no simple correlation between El Niño conditions and runoff from northern California, as seen in Figure 3. However, these correlations are a bit stronger in southern California.

This is your annual reminder on the difficulty of forecasting storms more than a few days into the future.  Sometimes in life, and always with California water, all we can do is to prudently prepare for contingencies and surprises, while we await the future.

Strategic changes and climate handwringing

Many aspects of California water are eternal from our human perspective – a Mediterranean climate with large seasonal and interannual variability, a tendency to be dry, great internal geographic variability in hydrology and water demands, large structural changes in water uses and demands driven by technological changes and global markets, and recurrent introductions of non-native species.  California water management has always had to deal with large changes (both variability and structural changes).

Today’s climate (and more) is changing.  It is important to prepare for multiple interacting changes altogether. 

Fortunately, California’s ever-restructuring economy continues to become less dependent on abundant supplies of water.  This is especially true for cities, where more than 90% of the state’s population and economy are now largely (but not entirely) decoupled from water issues. 

Agriculture (less than 5% of California GDP) remains the exception. The recent prosperity of agriculture has increased its vulnerability in some ways, especially from climate swings, as well as from market drivers (as recent reductions in nut prices show).  Although California’s agriculture is much more tied to climatic patterns, farmers’ economic output has grown during the recent decades with the help of various adaptation mechanisms—such as switching to more profitable crops, some additional groundwater banking and trading, and selective fallowing, among others.

But groundwater-dependent communities are still suffering the consequences of groundwater depletion. Even in a relatively-good water year as we had, over 200 domestic wells went dry (https://mydrywatersupply.water.ca.gov/report/). If dry conditions return, farmers and cities will increase groundwater pumping, causing hundreds, if not thousands, of domestic wells to go dry. Identifying vulnerable communities and taking preventative actions can reduce such crises. 

California’s native ecosystems are becoming more vulnerable to changes in climate, the stresses of water and temperature affecting habitat reduction, fragmentation, and disruption combined with invasive species introductions promise continued declines.  There seems to be no plausible scenario where California’s future ecosystems will be like its past ecosystems.  Yet we have barely begun discussing realistic futures for managing California’s forest and aquatic ecosystems.

We need to think about and discuss how to adapt to these changes more strategically.  These discussions will be hard, but should be exciting if they open broad opportunities for more sustainably and resiliently achieving human and ecosystem objectives.

What to do?

Every year, water managers and users must be prepared for both flood and drought.  It has always been thus, and it is becoming more like this with a warmer and more variable climate. These trends indicate that water leaders and managers need to be serious about planning and pre-planning discussions.

Californians should pay serious attention to water and its likely changes from climate change, without complacency or panic over our remarkably effective yet 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.  Complacency and panic are rhetorically convenient, but expensive and potentially life-threatening reactions to situations that deserve serious thought, analysis, and deliberations.

Today, most of our 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.

Now, go have a drink (of water) to celebrate another water year’s completion.  In WY 2024, we were reassured that average years can still happen, even if they are becoming less frequent.  As usual for this time of year, we must prepare for floods, droughts, and even average water years, as well as weird combinations in a context of many changes.

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.

Moyle, P. (2023), “Future Ancestors of Freshwater Fishes in California,” CaliforniaWaterBlog.com, September 17.

Lund, J. “Happy New Water Year 2024!  – from 2023’s wild ride to the wilderness of 2024,” October 8, 2023.

Lund, J. “Happy New Water Year 2023!“ October 2, 2022.

Alvar Escriva-Bou is an Assistant Professor of Civil and Environmental Engineering at the University of California – Davis.  Jay Lund is an Emeritus Distinguished Professor of Civil and Environmental Engineering at the University of California – Davis.