California's year ends on September 30, and the snowpack is why

The water year runs October to September because the wet season does. A calendar year would cut the state's only rainy period in half.

Almost every hydrological number in California — reservoir inflow, runoff, percent-of-average precipitation — is reported against a water year that begins on 1 October and ends on 30 September. The reason is structural, not administrative.

California's precipitation arrives in a single concentrated block. Across this site's 57 stations, November through April carries about 86% of the annual total, and in 50 of the 57, half the entire year falls within three consecutive months. A calendar year would slice straight through the middle of that block, putting December in one accounting period and January in the next — so a single storm sequence spanning New Year would be split across two years and neither would describe it.

The reservoir with no dam

The other reason the year is drawn this way is the snowpack. California's largest single store of water is not engineered: it is snow on the western slope of the Sierra Nevada, accumulating from November, peaking around 1 April, and releasing slowly through spring and early summer — arriving in the rivers at exactly the time of year when nothing is falling from the sky.

That delay is the whole mechanism. Roughly a third of the state's supply passes through it, which is why the 1 April snowpack survey is the most watched measurement in California hydrology and has been run since 1929.

Why inches are not the whole story

Two seasons with identical precipitation totals can produce very different summers. The same inches arriving warm — with a high snow level — rain on ground that should be collecting snow, and run off within days instead of releasing over months. A wet year delivered warm can still leave summer short.

The inverse is the flood case: a warm atmospheric river arriving over an existing snowpack contributes its own rain plus the meltwater it releases, and the two together produce runoff far beyond what the rainfall alone would suggest.