Welcome to the first week of 2022, at least according to your desk calendar.
For hydrologists, we are already one quarter of the way through the 2022 water year, which began on October 1, 2021.
In the United States, hydrologic water years run from October 1st through September 30th, and are named by the calendar year in which it ends. So, the 2021 water year ended on September 30th 2021 and the 2022 water year began on October 1st, 2021.
There are a couple hydrological things happening in early autumn that warrant celebrating New Water Year’s Day on October 1st.
The first involves snow. Particularly in high mountain settings, snow that falls from October through December might not melt and become stream flow until spring the following year. If water years were aligned with the calendar, the annual water budget would not be balanced. The hydrologic income (i.e., annual precipitation) would not equal hydrologic expenditures. A simplified water budget can be expressed as:
P = RO + ET + ΔS
P = precipitation,
RO = runoff (stream flow),
ET = evapotranspirtation (combined evaporation and transpiration from plants), and
ΔS = change in soil storage (groundwater).
The second reason for celebrating New Water Year’s Day on October 1st instead of January 1st is stream flow. On average, early autumn is when stream flow is at its lowest. The blue line in the image below shows the mean daily discharge of the Esopus Creek at Allaben for water years 2014-2021 (October 1, 2013 to September 30, 2021). The vertical red lines represent October 1st of each year.
It is clear in this chart that early autumn is typically when the Esopus Creek is at its lowest flow of the year. The hot summer growing season, when evaporation and transpiration were at their peaks, has depleted the groundwater storage. Stream baseflow decreases in response to the lowering of the groundwater table. This makes October 1st the perfect time to celebrate New Water Year’s Day.