water year / hydrological year

A continuous period of twelve months over which years are counted in Hydrology. In Spain, the river basin authorities count it as beginning on October 1 and ending on September 30. This division has its rationale in the usual behavior of precipitation throughout the year, with the end of this period coinciding with the time when water reserves reach their minimum. That division does not exactly coincide with that of the hydrometeorological or agricultural year.