Our weather station uses the traditional "rainfall season" for our reported annual rainfall, which is the period running from July 1st through June 30th of the following year. This is the same period used by the Los Angeles Almanac and many other California organizations who have tracked accumulated rainfall going back to the 19th century. It is a useful period because California's mediterranean climate, with a wet fall through spring and a dry summer, allows such a period starting in mid-summer to begin from a "clean slate" after the previous season's rainfall. The "rainfall season" is designated by the year it started in July.
In 2015, after more than a century of using the "rainfall season" for its annual rainfall reports, the National Weather Service forecast offices adopted the "water year" for its reporting period. This "water year" period begins on October 1st and runs through September 30th of the following year. Since most of the period of the "water year" lies within the year in which it finishes, it is designated by that year it ends. The National Weather Service explained that "this change will keep precipitation reports in the daily NWS climate reports consistent with the U.S. Geological Survey, the State of California water agencies, and most other weather offices nationwide who utilize the October 1st "water year" definition."
Adding to the confusion of reported annual rainfall, our own Santa Barbara County uses what is called the "Normal Water-Year" rainfall, which calculates annual rainfall between September 1st and August 31st and Weather Underground reports annual rainfall simply using a calendar year period.