Water Year
The USGS uses the 12-month period, October 1 through September 30 to designate the "water year". The water year is named for the calendar year in which it ends and which includes 9 of the 12 months. Thus, the year ending September 30, 2001, is called the "2001 water year."
The origin of the water year in the USGS stems from the implementation of streamgaging methods to control drought in the arid Southwest, and is largely based on recognizing that snow accumulation after Oct. 1 is the primary source of runoff to streams during the following calendar year.
For more definitions, please vist the USGS glossary page.
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