In 2015, after more than a century, National Weather Service forecast offices adopted the “water year” versus the “rainfall season” as the 12-month period over which to measure accumulated rainfall. A “rainfall season” runs from July 1 through June 30 of the following year. This is the rain season used by the Los Angeles Almanac. This period is generally referred to by the year in which it begins, although we refer to it, for example, as the 2019-2020 rainfall season. A “water year,” on the other hand, as now used the National Weather Service, begins on October 1 and runs through September 30 of the following year. Since most of this period lies within the year in which it finishes, it is designated by that year.
The National Weather Service explains, “This change will keep precipitation reports in the daily NWS climate reports consistent with the U.S. Geological Survey, state of California water agencies, and most other weather offices nationwide who utilize the Oct. 1 'water year definition.'”
Like the National Weather Service, hydrologists who measure water flow and streams, prefer to use the “water year. Nevertheless, the Los Angeles Almanac, like most other California organizations who focus on tracking accumulated rainfall, prefers the “rainfall season” because the California climate typically has its driest months in the summer. This allows us to begin measuring 12 months of accumulated rainfall from a “clean slate.”
Source: 'Rainfall year season' differs from 'water year' designation for forecasters by John Lindsey, Lompoc Record, Oct. 4, 2015.