Rainfall Season vs. Water Year


Unlike most parts of the United States, California’s Mediterranean climate is defined by its “summer drought” given that there is a natural break between one rainy season and the next.  This was recognized by settlers as far back as the Gold Rush and since that time the most common metric to quantify California (and other western states’) rainfall has been a July 1 to June 30 “rainfall season”. Consequently, thousands of reports, studies and analyses related to California rainfall have been based on the rainfall season, including the following from is some of the earliest chronicling of California weather.

Conversely, because there is lag of several months to when streamflows in the state’s rivers are at their lowest, hydrologists have historically (and logically for their purposes) used a Water Year (WY) metric from October 1 to September.

However, about two years ago, for reasons that have never been adequately explained, the National Weather Service (NWS) Western Region decided, without the opportunity for comment from the rest of the meteorological community or the public, to start calculating their rainfall products (i.e., Climate Station Precipitation Summary , and others) in regards to the hydrologist’s Water Year. (The remainder of the NWS, by the way uses the calendar year for summarizing rain data, as they do not have a natural summer break in rainfall.)  While this might be nice for consistency with the hydrologic community it puts them out of step with the vast wealth of historic data and others in the rest of the meteorological community.  This was even highlighted by a recent article in the LA Times, “Ideologies clash as weather service realigns rainfall calendar

The numerical differences between rainfall season and water year are slight (i.e., typically only about 3% of the annual amount) given the small amount of rain that typically falls in the months of July, August and September. And that rain is not lost, it is either counted at the end of one methodology’s season or the beginning of the other’s.

As stated about, the problem comes about when trying to compare data published by the NWS in terms of the hydrologist’s WY, with data from past events which have been characterized by meteorologists’ rainfall season.  [To their credit, NWS San Francisco Bay Area automatically generates a table which has both the rainfall season and water year, and it is my understanding the software used was made available, but unfortunately not adopted, by other NWS offices.]

An effort has been made, outside the NWS, to make available to the public and meteorological community data showing rainfall in terms of the July 1 to June 30 rainfall season with products like California Rainfall Season Totals. But this does not address the amount of unnecessary effort expended by everyone, but a very small group, to keep the data consistent and meteorologically logical.
Jan Null, CCM
Golden Gate Weather Services