Headlines:
Peak snow water equivalent (SWE) was 71% of average and 3rd lowest in the 1989-2022 record, but SWE peaked on April 24, 12 days later than average.
Temperatures were well below expected values in the spring but set numerous record highs in July and September. For the whole year, mean temperature was 1 degree F above average.
Water-year total precipitation was 91% of average—much higher than peak SWE—thanks to heavy precipitation in June and August.
Short-term drought indicators improved very modestly over the water year, while medium- and long-term indicators worsened. Most of the watershed is in “moderate drought,” where it started the water year.
Details:
After a promising start to the snow accumulation season, an extended dry period lasting from early January until early April ensured that the 2022 snowpack would be well below average. Snow water equivalent (SWE) was 10% above average on January 7 but had dropped to 35% below average by April 10—second lowest only to 2001 at that point in the spring. However, a cold, wet weather pattern set up for the remainder of the spring, providing at least a little recovery. Peak SWE turned out to be 71% of average, third lowest in the 1989-2022 record behind 2001 and 2015. More importantly, cold temperatures preserved that snowpack well into the spring. SWE reached its annual maximum on April 25, 12 days later than average.
Speaking of the cold spring, mean April-June temperature in the snow-accumulating areas of the watershed—roughly 6,000 feet in elevation and above—is a strong predictor of snowmelt timing. This important climatic characteristic has been increasing at a rate of a little over 1 degree F per decade for the last 40 years. This year’s value was 42.2 degrees, far below the value of 44.5 degrees expected based on that trend, and one that was so far below the trend that it had less than a 5% chance of occurring—hence the late date of peak SWE. The last spring this cold was in 2011, which seems like a long time ago, especially because springtime temperatures in many recent years—notably 2015, 2016, 2017, 2018, and 2021—were 3-6 degrees warmer than what we experienced this year. However, in the big picture, this year’s springtime temperature was only 0.5 degrees below the 1989-2022 average and ranked 13th coldest in that record. Nine of the 12 years with colder springtime temperatures occurred between 1989 and 1999, illustrating that even a seemingly cold spring by today’s standards was warmer than the average year in the 1990s.
The cold weather pattern broke at the end of June, and temperatures quickly exceeded average for most of the rest of the summer. Mean July-September temperature was the warmest in the 1989-2022 record, and new record high daily mean and daily maximum temperatures were set on numerous different days in July, August, and September. These records were set not only in the short 1989-2022 record I use for this report but in very long records all over eastern Idaho. Record highs were set on numerous days within a 10-day period in late August and early September. The month of September was over 4 degrees warmer than average and the warmest on record. For the water year as a whole, warm temperatures this summer—along with warm temperatures last fall—more than outweighed the cold spring; mean temperature for the water year was 1 degree above average.
Precipitation for the water year turned out to be much better than the SWE figures would indicate, thanks to heavy rain in the spring and summer. Precipitation in April and May was just a little above average, and that in June was well above average. Springtime precipitation greatly favored the northern part of the watershed, which had received less precipitation than other areas in 2020 and 2021. Areas along the Continental Divide received 3 inches or better in the mid-June storm that caused catastrophic flooding a little farther north and east in the Yellowstone River drainage. After what turned out to be the driest July in the 1989-2022 record, the monsoon season was relatively wet.
Precipitation was above average in August and locally heavy. Most notably, the Ashton area received 2-3 inches in less than two hours on August 13, setting a new one-day precipitation record. Heavy showers fell in Ashton on several different days during August and September, making Ashton the only of the 12 stations in the watershed to receive above-average precipitation for the water year. Ashton’s precipitation came in at 106% of average for the water year as a whole, while that only 25 miles away in Rexburg was 78% of average. Precipitation for the watershed as a whole ended the water year at 91% of average: 88% in the valley areas, 90% in the Teton headwaters, 92% in the upper Henry’s Fork, and 94% in Fall River.
The short-term drought indicator I use improved somewhat over the water year, primarily due to the cool, wet spring. The one-year cumulative moisture availability in the lower-elevation areas of the watershed (precipitation minus evapotranspiration) is a useful surrogate for long-term soil moisture conditions and also relevant as a measure of irrigation need. This index started water year 2022 at over 6 inches below average, following the very dry, warm summer of 2021. Cool, wet weather in the spring reduced that deficit to around 1.5 inches below average due to both lower evapotranspiration and higher precipitation. Following the hot, dry weather at the end of August and early September, the moisture availability index ended up at 2 inches below average—still indicative of drought but much better than it was a year ago.
The medium-term drought indicator I use is the 3-year rolling average of watershed-wide precipitation. I use three years because that is roughly the response time of the deep Yellowstone Plateau aquifers in the upper Henry’s Fork headwater areas to precipitation and because it is the generation time of most trout populations in the watershed. That index has been in steep decline for the past three years, falling from 18% above average to 12% below average since October 1, 2019—a net drop of 11 inches. The heavy December snow referenced above temporarily moved this index back up to average, but it quickly fell to about where it is now by April. Because rain in June and August was fairly localized, neither of those wet periods had much of an effect on the 3-year average over the whole watershed.
These two specific measures of drought I use coincide well with continental-scale drought indices calculated by a partnership of federal agencies and updated weekly at https://droughtmonitor.unl.edu/. One year ago, the central portion of the Henry’s Fork watershed was classified in an area of “moderate drought” but surrounded by areas of “severe” to “extreme” drought to the west, north, and east that spilled over into the watershed. All but the eastern edge of the watershed is currently in moderate drought, but surrounding areas to our west and north have been downgraded from severe or extreme drought, indicating overall improvement in our area. However, most of the northwest—including the entire upper Snake River was drought-free on October 1, 2019, consistent with the steady decline in 3-year average precipitation observed here.
Long-term drought is best reflected in streamflow, which I will cover in the next installation.
Spoiler alert: the cool, wet spring had major positive effects on reservoir storage but very little on overall water supply. As measured by natural watershed streamflow, water year 2022 was the driest in the 1978-2022 record. Furthermore, natural flow was above the 1978-2022 average in only 8 of the 23 water years since 2000—an indicator of long-term drought persistence in the West.
Comentários