The official water year runs October 1st to September 30. It's the officially recognized dates for tracking yearly water totals. USDA, USGS, and most folks use it for planning purposes, reservoirs, and agriculture. The warm season ends the water year, "calendar" flips as vegetation starts to go dormant.
CoCoRAHS has this:
While the definition may vary regionally, a common definition of "water year" is October 1 through September 30th. In the cool months, evaporation rates are very low, snow accumulates in cold regions, storms tend to be widespread, and most areas see accumulation of moisture in the soil. Then comes spring with it's accompanying snow melt and high runoff from mountainous regions and higher latitudes. The summer months bring high evaporation rates. Precipitation (much from thunderstorms) becomes much more localized. Soil moisture may be gradually depleted during summer as vegetation uses large volumes of water. Streams run slower and clearer.
https://www.cocorahs.org/wateryearsummary/