New Year’s Eve is still months away, but we are on the cusp of the new water year. The United States Geological Survey defines the start of the water year as October 1 of the current year–just as fall rains are ramping up here in the Pacific Northwest–and it continues all the way through September 30 of the following year. Hydrologists, meteorologists, and other scientists can then compare the amount of precipitation from each water year, and get a decent idea of year-to-year trends as well as longer-term patterns.
Within the water year, though, there are more localized details. For example, just saying that “we got X amount of precipitation over the past water year” doesn’t tell how that precipitation was distributed. How much of it fell as rain versus snow? How much fell at once, and how long were the dry periods, and how numerous? The amount is important, yes, but its delivery is even moreso.
For example, west of the Cascades it’s normal for the rainy season to begin in September, and persist well into May, with a mild, dry summer. In recent years this pattern has been increasingly disrupted; 2021, for example, not only saw the hottest temperatures on record in many places, but the rain essentially dried up in April, and didn’t return until less than two weeks ago. The few extra dry weeks, coupled with the higher temperatures, led to increased drought conditions on both sides of the mountains. And the uncharacteristic heat wave that hit in late June depleted many places of any spare water they might have had, scorching trees and other plants.
Compare that to the past couple of weeks as rain has returned. As I mentioned in last week’s post, the rain that broke our dry spell here on the coast bucketed over three inches on us in less than 72 hours. This past weekend saw almost an inch added to that. Now, it’s not abnormal for us to get rain this time of year, but it is unusual to be getting it in larger quantities. We’re seeing more frequent severe weather events, to include storms rather than simple rain. And when larger quantities of rain pour down in short periods of time, it washes immediately into streams and rivers rather than soaking into the soil like our usual steady, calm rain does. This means that the land holds less water to hold into the dry season, exacerbating drought conditions not just where I am, but across the entire American west.
Since we’re getting several inches of rain here at the very end of the water year, this can make it appear like we’re getting a more normal amount of precipitation, which could cause some people to claim there’s nothing to worry about. But the drought is in the details, and if we’re going to use the water year to educate people about the effects of climate change on hydrology, we’re going to need to look at individual “water months” and even “water days” in order to get the whole story of what’s going on.