Henry’s Fork, May 24th, 2022
It looks like a late year for the giant stone fly event beginning in good numbers. Let’s see what the warming weather of the next few days brings. Looking at what Dr. Rob Van Kirk observes in the Henry’s Fork drainage with respect to effect of cold weather on the drainage, it is no wonder that insect activity is behind for the date. See below
His snow-water analysis for Henry’s Fork drainage of yesterday follows.
Headlines:
- Another cool, dry day dropped water-year total precipitation to 83% of average but kept snow melt relatively low.
- Natural flow continued to drop yesterday and was 45% of average for the date and 70% of average cumulatively for the water year.
- Diversion held steady at 93% of average.
- At an average outflow of 389 cfs, Island Park Reservoir gained 77 ac-ft and is 99.6% full.
Details:
Mean temperature yesterday was 6 degrees F below average, continuing what has been a very long run of below-average temperatures. Precipitation dropped to 83% of average for the water year so far. Snow melt was a little below average for the date, and 65% of this year’s peak snow water equivalent (SWE) remains yet to be melted, compared with 53% on average. Current SWE at the mid-elevation stations is about average for the date. Cold weather continues to keep aquatic insect hatches delayed by about 5-6 days in the lower watershed. Crop development also continues to lag average due to cooler temperatures.
A couple of warmer days are in store tomorrow and Thursday before cooler weather and precipitation return on Friday and persist through the Memorial Day weekend. Temperatures will be around 5 degrees below average. Precipitation forecasts have not been very consistent, due to a pretty strong gradient between a relatively dry forecast to our southwest and a very wet forecast to our northeast. The latest 7-day quantitative forecast calls for around 0.1 inch along the western edge of the watershed to around 1 inch along the Montana border. Areas just to our northeast, in Yellowstone National Park and southern Montana, could receive up to 2 inches. So, slight changes in moisture feed could mean large differences in actual precipitation amounts. At this point, rain in the valleys would be beneficial, but current forecasts suggest a continuation of the recent pattern of showers in the mountains but little if any in the valleys.
Natural stream flow continued to drop yesterday in response to cooler weather. Natural flow is currently 45% of average for the date and 70% of average for the water year so far. These numbers are truly astounding. In the 1978-2022 record, natural flow for the water year to date in the next driest year to this one (2002) is 79% of average, illustrating just how far this year is away from anything close to “normal.” Diversion stayed constant at 92% of average, dropping the difference between supply and demand down to only 600 cfs above the indicator of need for reservoir draft to meet irrigation demand. With warmer weather on the way—as well as some precipitation—natural flow will meet demand by a small amount at least into next week, although stream flow will be far less than half of average for this time of year throughout the watershed.
At an average outflow of 389 cfs yesterday, Island Park Reservoir gained a very small amount of water and is 99.6% full. Current outflow is just at the right level to keep the reservoir within a few hundred ac-ft of full for the next several days. Outflow may need to be increased a hair at the end of the week to accommodate increased inflow in the form of rain, should forecast amounts creep upward. On a full reservoir, 1 inch of rain adds 625 ac-ft, which is less than the difference between full pool and current reservoir content. Right now, forecasts call for around 0.5 inch in Island Park later in the week.
Rob Van Kirk, Ph.D.
Senior Scientist
P.O. Box 550
Ashton, ID 83420
208-881-3407 CELL
208-652-3568 FAX