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 1 of one year and September 30th of the next.United States Geological Survey, "Explanations for the National Water Conditions", http://water.usgs.gov/nwc/explain_data.html, Retrieved 16 October 2011.
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). 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.

Classification

Water year types (or ''indices'') are used to present the historical hydrological data in a simplified form. These indices help to categorize similar water years for the planning of the rule-based water operations. A typical set includes: ''very dry year'', dry year, normal year, wet year, ''very wet year''. The years are characterized through setting numerical thresholds for the water 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:
* Palmer Drought Severity Index (PDSI). Proposed by W. C. Palmer in 1965, PDSI is extensively used in the US since then;
* Standardized Precipitation Index (SPI) was proposed by McKee et al. in 1993;
* Reclamation Drought Index;
* '' deciles.
Many practically used indices were created ad-hoc. For example, California River Indices is a weighted average of the estimates of spring melt, runoff for the rest of the year, and the result for the previous year, calculated for few river basins separately to classify the water year as a ''wet'', ''above normal'', ''below normal'', ''dry'', and ''critical'' ("normal" years in California are extremely rare). These California indices were not created "through a systematic statistical analysis of
historic basin conditions and river flows".

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.

Uses

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.

See also

*Seasonal year

References

Sources

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Hydrology
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