California's Water Year Starts in the Fall. The Winter Storms Will Decide Everything.

California keeps its water accounts on a different calendar than everyone else. The state's official water year begins Oct. 1, just before the first Pacific storms, and runs through the following September — a schedule built around the Mediterranean rhythm of wet winters and bone-dry summers that defines the state's climate.

The bank account that matters most is not a reservoir. It is the Sierra Nevada snowpack, which in an average year holds roughly a third of the water California uses, releasing it slowly through the spring melt exactly when farms and cities need it most. The Department of Water Resources measures it all winter — by remote sensors statewide and by the famous manual surveys at Phillips Station near Echo Summit, where officials wade into a meadow with an aluminum tube and a scale.

November's role in this system is prologue. The first surveys are still weeks away, the reservoirs sit at levels set by last year's storms and this summer's drawdown, and the state's water managers watch the Pacific for the atmospheric river patterns that will make or break the year. A handful of landfalling storms can supply a third of an entire season's precipitation.

San Diego sits at the far end of this plumbing. The County Water Authority blends imported supplies — from the Colorado River and the State Water Project's Northern California reservoirs — with local sources, so a wet Sierra winter is felt, eventually, even at the end of the aqueduct in Southern California.

The state's recent history argues for humility about forecasts: the driest three-year stretch in California's recorded history was followed, in 2023, by one of its wettest winters. The water year now underway will file itself somewhere on that spectrum. By the April 1 survey — traditionally the snowpack's peak — Californians will know which kind of year they got.