by Jay Lund

The first few months of California’s water year, which started in October 2023, have been pretty dry.

We never know what to expect of California’s wet season until it ends, usually in late March.  This year is no exception.  Precipitation in California is almost uncorrelated from year to year (even with El Nino), 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 essentially, “How much precipitation have we gotten so far this year?”

California’s 2024 Water Year could still be quite dry and/or bring floods, but it seems unlikely to become among California’s wettest years, if only because the water year’s first months have been dry.

Today, the Northern Sierra precipitation index is about 66% of average for this time of year.  San Joaquin and Tulare basin precipitation indices are at 46% and 40% of average, respectively.  Extreme wet years usually require all wet season months to be wet.  So far, October to December have been dry in all major basins.  January has been ok.  But we still have some of the wettest months to go in California’s October – March nominal wet season (which sometimes extends a bit into April).

Given the precipitation so far, 2024 is highly likely to be wetter than the very driest years of record, but is also highly unlikely to be among the very wettest years in the past 100 years or so. 

Overall, it is more likely to be dry than wet, but almost anything could happen.

As for water storage, snowpack is pretty low so far this year, reflecting low precipitation and warmer temperatures this year, but not as low as during the driest years in the last decade.

Fortunately, California’s reservoirs remain mostly full.  This will help if the dry year continues.  (In a sense, California is always dry unless it is too wet.)

Groundwater was helped by a wet 2023, but will not have entirely recovered.

So, hold back on judging the water year.  Prepare for both floods and drought, as Californians should do every year.  Given California’s hydrology, we won’t really know how this will all end until late March.

A bit like Schrodinger’s cat, or Schrodinger’s plates below, each water year is potentially both wet and dry until the end of the nominal wet season in March.

Further reading

Lund, J. (2023) Happy New Water Year 2024!  – from 2023’s wild ride to the wilderness of 2024, CaliforniaWaterBlog.com, October 8, 2023

Data and plots

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

Jay Lund is an imperfectly retired 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.