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.
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.
These California indices were not created "through a systematic statistical analysis of historic 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.