HELENA — A water year is a term commonly used in meteorology and hydrology to describe a time period of 12 months for which precipitation totals are measured.
Also known as the hydrological year, discharge year or flow year, its beginning differs from the calendar year because part of the precipitation that falls in fall and winter accumulates as snow and does not drain until the following spring and summer.
It essentially covers an entire year's worth of vegetative activity, with dormancy typically beginning October 1.
The water year is a successive 12 month period that most consistently, year after year, gives the highest correlation between precipitation and streamflow, and is used to analyze the surface water supply.
Montana's water year was below average. The area composite of 10.89 inches is 3.45 inches below normal and the driest since 2003. Bozeman had their sixth driest water year and driest since 2012. Havre also had their sixth driest and driest since 1997. Lewistown had its 2nd driest of record and driest since 1919.
This dry year can be directly related to the very active wildfire season.
Looking ahead, a La Niña winter should mean above-average snowfall in the mountains which would be a jumpstart to this next water year.