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 nearly all this Water Year’s precipitation has fallen already.
Precipitation
Statewide precipitation this wet season has once again been unusually average overall (97%), with unusual uniformity across this large state, so the map below (Figure 1) is much less colorful than last year, with similar statewide precipitation. There were also fewer major wildfires and unusually low snow accumulations compared to last year!
This year’s “whiplash” was within the “wet” season, as seen in Figure 2. If you have enough wet and dry periods within the wet season, the season turns out about average overall.
Snowpack
Snowpack was unusually low in 2026, but less bad than lows during the 2012-2016 drought.
The figure below shows we can have a wide range of snow accumulations and melting patterns. These are changing with a warming climate. This year also had large amounts of early snow-melt in an unusually warm and dry March within the “wet” season.
Reservoir Storage
For water stored instate for California, storage levels are about the same as last year and are pretty good. Because the last three years have not been dry, California retains an unusually large amount of water in its reservoirs. Most surface water storage within California is in the Sacramento and San Joaquin basins, and these basins are well connected to California’s statewide water conveyance projects.
But Colorado River reservoirs continue 43 years of depletion since 1983, when they filled for the first and perhaps last time. Their overdrafting was 2.4 million-acre-ft this last year.
Table 1. California reservoir storage on May 28, 2026. Soruce: https://cdec.water.ca.gov/reportapp/javareports?name=STORAGE
| Area | Total Capacity 1000 AF | Historical Average 1000 AF | 2025 1000 AF | 2026 1000 AF | % Average |
| Intrastate | |||||
| North Coast | 3,096 | 2,346 | 2,689 | 2,683 | 114 |
| San Francisco Bay | 710 | 529 | 495 | 453 | 86 |
| Central Coast | 982 | 643 | 694 | 702 | 109 |
| South Coast | 2,106 | 1,454 | 1,429 | 1414 | 97 |
| Sacramento | 16,038 | 12,919 | 14,922 | 14,795 | 115 |
| San Joaquin | 11,483 | 7,877 | 9,343 | 9,638 | 122 |
| Tulare Lake | 2,087 | 1,063 | 1,401 | 1,620 | 152 |
| North Lahontan | 1,073 | 541 | 884 | 957 | 177 |
| South Lahontan | 412 | 261 | 310 | 340 | 130 |
| Subtotal | 37,989 | 27,632 | 32,167 | 32,603 | 118 |
| Interstate | |||||
| North Coast | 1,137 | 718 | 846 | 693 | 96 |
| Colorado River (1) | 52,939 | 32,500 | 18,479 | 16,072 | 49 |
| Subtotal | 54,076 | 33,218 | 19,325 | 16,765 | 50 |
| Total | 92,065 | 60,850 | 51,492 | 49,368 | 81 |
| 1 – includes lakes Mead and Powell | |||||
Groundwater
Availability of systematic groundwater data for California is still maturing for statewide and regional assessments but is improving.
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.
Some good news is that the last 10 years have seen more wells improving than deteriorating. (Curiously, the total number of wells in the sample also seems to have decreased.) We might be starting to see a net-positive trend with groundwater after more than a century of large net-negative trends in groundwater overdraft, albeit still with a large annual net-overdraft rate.
Groundwater storage capacities of aquifers are huge (statewide more than 10 times California’s surface water storage capacity). Also, local annual groundwater use is subject to very large fluctuations with local, regional, and statewide hydrologic and regulatory conditions. So year-by-year changes in statewide storage seem rather unstable and unreliable as indicators of long-term groundwater conditions.
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 45% 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 25% for other south-of-Delta contractors (12-year average is 30%). There is a possibility that some allocations might increase further. So it is a good year for urban and senior agricultural water contractors, and a better than average, but still disappointing, year for more junior agricultural water project contractors.
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. They remain fragmented and tend to be mostly available annually, with significant lag. 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 and assessments of 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 2026 water year has had a good “wet” season overall. Neither floods nor droughts overall.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://cdec.water.ca.gov/reportapp/javareports?name=TAB_ESI.pdf
https://cdec.water.ca.gov/reportapp/javareports?name=PLOT_SWC.pdf
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.
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