Water year

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.