Water-year 2018 ended up close to average, at 102% of average precipitation and 105% of average streamflow.
Upper Henry's Fork subwatershed was below average in precipitation and streamflow, vs. above-average values in Fall and Teton rivers.
However, Upper Henry's Fork water supply improved from 70% of average in 2016 to 91% in 2017 and 94% in 2018, indicating recovery of deep aquifers from 2013-2016 drought.
May and June rain compensated for early snowmelt and resulted in below-average irrigation diversion.
Despite very dry conditions and below-average streamflow during July, August and September, Island Park Reservoir ended the water year at 73% full, compared with 43% full on average, thanks to careful and precise water management.
May 22-24 precipitation totals were over 1 inch at most locations; water-year precipitation jumped from 105% of average to 109%.
Snowmelt continues at average rates, and SWE remains at 102% of average.
Watershed-total natural flow has increased to its highest level so far this year and higher than last year’s peak.
Inflow to Island Park Reservoir is around 1,600 cfs, and outflow is currently just a hair over inflow, allowing the reservoir to drop very slowly. Current reservoir content is a little higher than full pool.
In 2005, when I was a professor at Idaho State University, the Henry's Fork Foundation Board of Directors asked me to present an overview of hydrology and water management in the upper Snake River basin. Since then, the hydrology and water-management short course has taken on a life of its own, and I give this presentation in some form or another a few times each year. Every time I give the presentation, I update it with new information, particularly as related to climate change and the rapidly changing work of water management and admininstration.
Cool, wet weather from mid-February to mid-April turned an average water supply into one that is decidedly above average.
As of May 9, the Henry’s Fork reservoir system is 94% full and filling rapidly.
Based on early-April conditions, summertime water supply in the Henry’s Fork watershed is forecast to be above average.
More storage water will be delivered from Island Park Reservoir this summer than in 2017, but higher inflows will compensate, resulting in a very high probability of better-than-average carryover at the end of the irrigation season.
Despite a forecast for conditions that could have resulted in large loss of snowpack yesterday, atmospheric conditions lined up just right to not only avoid the loss but actually gain a very large amount of snow-water-equivalent (SWE). Read on for the details, as well as for an example from the spring of 2010 that illustrates a very large rain-on-snow event.