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Following the commonly-known USGS definition of Water Year (or Hydrological Year) we know that it starts on the 1st October and ends on the next 30th September.

Is this definition valid only for temperate/mid-latitudes? What's the water year e.g. in the Equatorial, Polar and Arid regions?

Can I keep using the USGS definition for these areas or shall I just use the calendar year (1st January-31st December)?

Thanks

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The water year does vary depending on the region, for example in Australia the water year begins on July 1. I am not sure about other regions, I believe the October 1st water year is quite standard across the northern hemisphere although may shift as you go far north to polar and more arid climates. I would recommend searching for similar links to the one for Australia to find the government defined water year, if it exists in your region of interest.

The purpose of the water year is to use, in lamens terms, the least exciting hydrologic time of year to avoid missing large events or fluxes based on the start time of simulation or data consideration, so depending on the annual hydrologic patterns in the given region shifting the water year start generally makes sense. For this reason as well, the calendar year is generally a bad idea for hydrologic models or data considerations, since you are then starting during a time of accumulated snowpack and can have mid-winter melts around that time (at least in the northern hemisphere, generally speaking).

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    $\begingroup$ In spite of these official definitions, consider the locality and the purpose of your work. In southeast Australia areas of winter rains, it often makes more sense to pick (southern hemisphere) mid summer rather than mid winter, since streamflow is at a minimum in summer. In that case use a water year that starts January 1 or perhaps March 1. $\endgroup$
    – haresfur
    Commented Nov 22, 2019 at 9:59
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October 1st is mostly used, but it varies from country to country.

In Norway, the hydrological year start on September 1st, mostly because groundwater levels are at their lowest. At that time of the year, all snow in the mountains has melted, and it is before temperatures fall and the autumn rain settles in.

It is probably better to use October 1st than the calendar year. You probably want to avoid starting new periods in the middle of a snow season (or a heavy rain season near equator). If you are only dealing with data from one country, it is perhaps best to use the local standard.

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