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Awater year (also calledhydrological year,discharge year orflow year) is a term commonly used inhydrology to describe a time period of 12 months for whichprecipitation totals are measured. Its beginning differs from thecalendar year because part of the precipitation that falls in late autumn and winter accumulates assnow and does not drain until the following spring or summer'ssnowmelt. The goal is to ensure that as much as possible of thesurface 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. TheUnited States Geological Survey (USGS) defines it as the period between October 1 of one year and September 30 of the next,[2][3] as late September to early October is the time for manydrainage 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; for example, the 2025 US water year runs from October 1, 2024 through September 30, 2025.[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-gaugelocal water year that starts in the month with the lowest averagestreamflow.[6]
Water year types (orindices[7]) are used to present the historical hydrological data in a simplified form.[8] These indices help to categorize similar water years for the planning of the rule-based[7] water operations. A typical set includes:very dry year,dry year, normal year,wet year,very wet year.[8] The years are characterized through setting numerical thresholds for thewater runoff in the water year. The methods of calculation (and the set of types) naturally vary by the region, therefore many indices exists, for example:[7]
Many practically used indices were createdad-hoc. For example, California River Indices[12] areweighted averages of the estimates ofspring melt, runoff for the rest of the year, and the result for the previous year, calculated for fewriver basins separately to classify the water year as awet,above normal,below normal,dry, andcritical ("normal" years in California are extremely rare[13]). These California indices were not created "through a systematic statistical analysis ofhistoric basin conditions and river flows".[14]
All indices by nature reflect the historic values and therefore cannot capture the variations in climate that are known to cause the distribution of water year types to be non-stationary in time.[15]
Examples of how water year is used: