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A water year (also called hydrological year, discharge year or flow year) is a term commonly used in hydrology to describe a time period of 12 months for which precipitation totals are measured. Its beginning differs from the calendar year because part of the precipitation that falls in late autumn and winter accumulates as snow and does not drain until the following spring or summer's snowmelt.
Due to meteorological and geographical factors, the definition of the water years varies; the United States Geological Survey defines it as the period between October 1st of one year and September 30th of the next. The water year is designated by the calendar year in which it ends, so the 2010 water year (USGS) started on October 1, 2009 and ended on September 30, 2010.
One way to identify a water-year is to find that successive 12-month period that most consistently, year after year, gives the highest correlation between precipitation and streamflow and negligible changes in storage (i.e., soil water and snow). 
Examples of how water year is used:
- Used to compare precipitation from one water year to another.
- Used to define a period of examination for hydrologic modeling purposes.
- Used in reports by the United States Geological Survey (USGS) as a term that deals with surface-water supply.
- The end of the water year is used by the CoCoRaHS project as an opportunity for observers to audit and verify data for their site.
- United States Geological Survey, "Explanations for the National Water Conditions", http://water.usgs.gov/nwc/explain_data.html, Retrieved 16 October 2011.
- "The hydrological year". Water UK. 31 October 2012.
- Likens, G. E. (2013). Biogeochemistry of a forested ecosystem. Springer Science & Business Media.