Water year

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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. The goal is to ensure that as much as possible of the surface runoff during the water year is attributable to the precipitation during the same water year.^[1]

Due to meteorological and geographical factors, the definition of the water years varies. The United States Geological Survey (USGS) defines it as the period between October 1 of one year and September 30th of the next,^[2][3] as late September to early October is the time for many drainage areas in the US to have the lowest stream flow and consistent ground water levels. The water year is designated by the calendar year in which it ends, so the 2024 water year started on October 1, 2023, and will end on September 30, 2024.^[1]

One way to identify a water-year is to find the 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).^[4] Usually, the time when the variation of storage from year to year is the smallest is the time with the minimum storage level and minimum flow. However, the practical considerations also affect the water year definitions. For example, in Canada the water year starts in October, apparently to coincide with the US one, although better measurement conditions exist in winter.^[5]

To accommodate the regional and climatic variations, some researchers use a per-gauge local water year that starts in the month with the lowest average streamflow.^[6]