In January, the National Weather Service forecast offices in California changed from a “rainfall year season” to a “water year” designation.
A water year is defined by hydrologists as the 12-month period that starts Oct. 1 and continues through Sept. 30 the following year. A rainfall year season is defined as the 12-month period beginning July 1 that continues through June 30 of the subsequent year.
The rainfall year season is designated as the year it started. For example, this rainfall year season is 2014. On the other hand, the water year is labeled by the calendar year in which it ends; which is understandable since nine of the 12 months fall in that year. For example, this water year would be referred to 2015.
According to the National Weather Service, “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.”
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Since we live in a Mediterranean climate with a wet and dry season, most other California organizations with an interest in rainfall totals, such as Jan Null’s Golden Gate Weather Services, Chris Arndt’s SLOweather.com, Cal Poly Irrigation Training and Research Centers — which maintains the university’s rain gauge and records and archives rainfall data back to 1870 — and PG&E’s Diablo Canyon Power Plant will continue to use the historical rainfall year season (July to June) designation.
“This convention is based on over 100 years of sound meteorological/climatological practice in California,” Jan Null stated. Chris Arndt likes the rainfall year because it is bookended by the dry season.
The state’s water managers and hydrologists tend to like the water year designation because October usually has the least amount of stream and river flows and tends to centered on the months in which California receives most of its rainfall.
To add another wrinkle, some groups like the Weather Underground bypass both conventions and simply use the calendar year to archive annual rainfall totals. Unlike our Mediterranean climate with a wet and dry season, many locations east of the Rocky Mountains receive equal amounts of rainfall each month.
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