The water year, explained

A water year runs October 1 through September 30, and it is named for the year it ends in. Water Year 2026 began October 1, 2025 and ends September 30, 2026. If that feels off by a quarter from the calendar, that is the point: the water year is built around the West's wet season, not the tax year.

Why it starts in October

The West gets almost all of its water in winter, as snow and rain from about November through April, then spends the rest of the year drawing it down. Starting the accounting year on October 1 puts a clean break at the driest point, just before the new precipitation season begins. Each water year then tells a whole story from empty toward refill and back, instead of splitting a single winter across two calendar years.

Why the naming trips people up

Because a water year is named for its ending year, most of Water Year 2026 actually happens in calendar 2025. The winter that fills the reservoirs, October 2025 through spring 2026, is the front half of WY2026. Once you know the convention it is simple, but it is a common source of confusion when a report says water year and you read calendar year.

How it anchors the statistics

Almost every water number is pinned to this calendar. Percent-of-normal figures compare a date to the same date in prior years, cumulative precipitation and runoff are tallied from October 1, and carryover is measured at the September 30 close. This site compares each reservoir and gauge to the trailing five-year median for the same date, so the season is always matched like for like: July against July, not July against a wet March.

On the Oroville chart above you can see the water-year rhythm directly: storage climbs through the winter and spring of each year, peaks near the start of summer, then falls through the demand season toward the September 30 low before the next cycle begins. Every recurring dip is a water year ending.

For the number measured at that September 30 close, see the carryover storage guide.