by Jay Lund
The first four months of California’s water year, which started in October 2024, have been wildly variable over the months and in different parts of the state.
Every year, we never know what to expect of California’s wet season until it ends, usually in late March or early April. This year is no exception. Precipitation in most of California is almost uncorrelated from year to year (even with El Niño), so last year’s rain, snow, and streamflow doesn’t help much in predicting this year or next year’s water availability. More aggravating, there is also very little precipitation correlation from month to month during the wet season, so what we know about this year’s water availability is largely how much precipitation we have gotten so far and how much is storage in the reservoirs and groundwater.
Today’s Northern Sierra precipitation index is about 130% of average for this time of year (but wetter than last year at this time). San Joaquin and Tulare basin precipitation indices are at 57% and 54% of average.
Southern California is almost at zero precipitation this water year, an astonishingly low number for this time of year. The geographic disparity in precipitation this year from north to south is extraordinary, at least so far – helping to explain the region’s extraordinarily late, large, and intense fires. The figure below says much in few words.
The water year began dryish. For the northern Sierra, October precipitation was fairly dry with 25% of its long-term average. November was very wet, with more than 250% of average November precipitation, followed by an averagish December with 112% of the months long-term average precipitation. January was flat-out dry with 12% of average January precipitation.
The San Joaquin and Tulare basins were drier still, and southern California has been almost completely dry in all months.
A very weirdly wet and dry year overall, so far. We could end the rainy season in a drought, and we could still have floods. At this point, it could easily become a wet year in the north and stay extremely dry in the south. We still have some of the wettest months of the water year to come.
Given the precipitation so far, 2025 could be anything, but is unlikely to be among the wettest years of record (which almost requires every month to be wet).
Overall, as usual for this time of year, almost anything could happen.
As for water storage, snowpack is a bit above average for this time of year in the north (thanks to this week’s storms) and about half of average in the southern Sierras. But it is not as low as during the driest years in the last decade.
Fortunately, we also have water in storage in reservoirs and groundwater. Surface water storage is mostly above average for this time of year, which should dampen many local drought concerns. See Figure 3. Groundwater varies greatly across the state and is not so well indexed as precipitation and surface storage. This will help if the dry year continues. (It is usually safe to assume that California is always dry, unless it is too wet.)
So, as with last year (and every year), hold back on judging the water year. Prepare for both floods and drought. Given California’s hydrology, we won’t really know how this will all end until late March.
Water deliveries this year and next will be affected far more by the coming months of uncertain precipitation than the fate of the Delta smelt, or even the political hydrology of Sacramento or the political climate in Washington, DC, which obsessively distract us.
The success of water management in California is driven mostly by local and regional management, supported by the abilities of state and federal agencies to manage and regulate in this decentralized context.
Further reading
Lund, J. (2024) Even when most of California is dry doesn’t mean we can’t have floods, CaliforniaWaterBlog.com January 28, 2024.
Lund, J. (2024) How’s California’s water year developing? – January 2024, CaliforniaWaterBlog.com January 21, 2024.
Data and plots
A nice overall snapshot of California water conditions with some great DWR links: https://cww.water.ca.gov/
Precipitation: https://cdec.water.ca.gov/snow_rain.html
Snow: https://cdec.water.ca.gov/reportapp/javareports?name=DLYSWEQ
Reservoirs: https://cdec.water.ca.gov/reportapp/javareports?name=RES
What is the chance of above normal precipitation this year? Great maps. https://cw3e.ucsd.edu/odds-of-normal-water-year-precipitation/
Jay Lund is joyously failing retirement as a Professor of Civil and Environmental Engineering at the University of California – Davis, where he is also a Vice Director at the Center for Watershed Sciences.
Leave a Reply