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Water Year 2020: 92% of Average Supply

Updated: Jan 12, 2022

  • Climate: Record-breaking cold in October, a generally dry winter, a 9th-inning rally that brought snowpack up to average just in time for it to melt, and a very dry summer. Mean temperature for water year 2020 was equal to the 1989-2019 average, but high variability in springtime temperatures melted snow rapidly during warm periods around May 1 and June 1. Water-year precipitation ended up at 90% of average, but July-September precipitation was only 52% of average.

  • Natural streamflow: 92% of the 1978-2019 average: 88% in upper Henry’s, 94% in Fall River, and 96% in Teton River. Streamflow dropped rapidly in all three subwatersheds after snowmelt peaks in early May and early June and after late June rain. Since 2009, natural flow in the upper Henry’s Fork has exceeded the 1930-2020 average in only three years: 2011, 2012, and 2019.

  • Irrigation Management: Draft of Island Park Reservoir started on July 4 (11 days later than average) and ended on September 9 (6 days earlier than average). Delivery of water to the Teton River through the Crosscut Canal started on July 8 and ended on September 24. Over the period of draft, streamflow in the Henry’s Fork downstream of all diversions averaged 469 cfs, compared with this year’s target of 350 cfs. Using the target downstream of all diversions rather than the St. Anthony gage saved 1,226 ac-ft in Island Park Reservoir while increasing streamflow downstream of diversions by 5.4%.

  • Island Park Reservoir: Winter flow during water year 2020 was 532 cfs, compared with 560 cfs in 2019 and a 1978-2019 average of 356 cfs. The reservoir reached a minimum of 54% full but ended the water year 56% full, compared with 46% full on average. Outflow during the upcoming winter is expected to be around 400 cfs.

  • Predictive Model Performance: Based on April 1 conditions, predictions were within 10% of actual values for hydrologic and water-management parameters that depended on spring and early summer conditions. However, the model under-predicted need for Island Park Reservoir draft by 20-30%, depending on the particular metric. Given observed flow in the lower Henry’s Fork, the model over-predicted Island Park Reservoir carryover by 27.2% (97,135 ac-ft vs. 76,366 ac-ft).


Water year 2020 started off wet and cold, with numerous low temperature records set October 29-31. Some individual station temperatures were over 20 degrees F colder than average. However, after the cold, wet start to the year, November and December were warm and dry, leaving snow water equivalent (SWE) at less than 80% of average on January 1. A series of storms increased SWE to average in early February, but dry weather dropped that below 90% of average in March. Only some very wet weather in April saved the snowpack, which peaked at 100% of average on April 17, within a few days of average peak timing. However, SWE varied greatly across the watershed. When SWE peaked on April 17, SWE was 111% of average in the Teton subwatershed, 98% in Fall River, and 96% in upper Henry’s Fork.

The spring of 2020 can best be characterized by high variability in both temperature and precipitation. Averages do not tell the whole story. Mean temperature for the spring ended right up at average. At the nine SnoTel stations, mean April-June temperature was 42.6 degrees F, compared with 42.6 degrees on average and with 42.4 degrees F last year. The past two springs have provided relief from a four-decade trend of increasing springtime temperatures at these nine stations. Based on that trend, about 1.1 degrees F per decade, last spring’s mean temperature was predicted to be 44.1 degrees F and this spring’s was predicted at 44.2 degrees. So, in both springs, temperatures were about 1.6 degrees below expectations.

However, daily variability in springtime temperatures was much greater this year than last, which is readily apparent in the temperature graphs. A useful measure of variability is the coefficient of variation, defined as standard deviation divided by the mean. Daily coefficient of variation in temperature this spring was 24%, compared with 19% last year and 21% on average. Starting with the week before Memorial Day, we had four episodes during which temperatures dropped from 5-10 degrees above average to 5-20 degrees below average over the course of 24-36 hours. These very strong and unseasonable cold fronts were generally accompanied by high winds and brought heavy precipitation in three of the four cases—those being the Memorial Day weekend, June 16-18 and June 28-30.

Despite average snowpack and average temperatures, the early melt was a result of two extended periods of well above average temperatures, one around May 1 and the other around June 1. During the first extended period of warm weather at the end of April and beginning of May, all of the low-elevation snow melted rapidly, and SWE remained below average the rest of the spring except for a day or two around the Memorial Day weekend. The warm period around June 1 melted the mid-elevation snow and most of the high-elevation snow. Thus, the high variability in temperatures accelerated snowmelt over what would be expected strictly by looking at averages.

In comparison to last year, snowmelt this spring was about two weeks earlier than last year. This was due in part to the high temperature variability mentioned above but was also due to a much lower snowpack this year than last. More snow takes longer to melt, regardless of temperature. This year’s snowpack peaked at the same time as last year’s, but peak SWE was 4.6 inches (14%) lower than last year. As can easily be seen from the SWE graphic, that difference was maintained throughout the spring. During the mid-April to late-June melt period, the difference between last year’s SWE and this year’s ranged between 2.9 inches and 7.5 inches, averaging 5 inches, pretty close to the initial difference in peak SWE between the two years.

After the cold, rainy period at the end of June, precipitation was sparse. Total July-September precipitation was only 52% of average. The water year ended with accumulated precipitation at 90% of average: 94% of average in the valleys, 87% in upper Henry’s Fork, 89% in Fall River headwaters, and 92% in Teton River headwaters. Compare that to water-year precipitation in 2019 that ended up at 114% of average over the whole watershed: 124% of average in the valleys, 119% of average in upper Henry’s Fork, 108% of average in Fall River headwaters, and 109% of average in Teton headwaters. As a result, three-year average precipitation dropped from 43 inches (4 inches above average) in late September of 2019 to 37 inches (1 inch above average) in late September of 2020. By the beginning of the new water year, most of the western U.S. was covered by drought, the headwater areas of the upper Snake River basin being a notable exception.

The only two of the 12 individual climate stations in the watershed that received above-average precipitation in 2020 were Rexburg at 103% of average and Ashton at 111% of average. In fact, precipitation in the valleys this year was high enough that grain harvest was delayed by several weeks until grain moisture dropped to desired levels, and dry-farmed (non-irrigated) grain yields were high. In 2019, Ashton was also the winner among all stations, at 133% of average precipitation. However, in 2019, all 12 stations received above-average precipitation.

Summer temperatures started off near average in July but were above average for most of August and September. Mean August-September temperature was 2 degrees F above average. When combined with dry weather, accumulated moisture surplus in the valley areas dropped rapidly during the month of September. I define moisture availability in the agricultural regions of the watershed as the difference between precipitation and evapotranspiration, averaged over the previous year. On average, evapotranspiration exceeds precipitation by 37 inches per year. In theory, this is the amount of irrigation needed to fully irrigate an alfalfa crop. In early September, this difference was only 34 inches, indicating a surplus of around 3 inches. However, that 3-inch surplus had turned into a 1.5-inch deficit by the end of the water year, resulting in above-average irrigation demand during August and September.

Although water year 2020 brought many extremes in temperature—and extreme changes in temperature that brought high winds—one event stood out because of its spatial scale and magnitude. A very strong cold front moved into the watershed from the northeast on Labor Day, dropping temperature in Ashton from 74 degrees at 2:15 p.m. to 35 degrees by 7:00 p.m. This cold front was accompanied by strong northeast winds that persisted for 24 hours. This wind event was rare in three characteristics: 1) direction (prevailing wind, and that accompanying most weather fronts blows from the southwest here), 2) magnitude (northeast winds are rarely very strong), and 3) duration (sustained for 24 hours—rare even for the prevailing southwest winds). Furthermore, this cold front generated strong winds throughout the western U.S., fanning wildfires throughout Washington, Oregon, and northern California, and causing extensive damage along the Wasatch Front.

Closer to home, this event had the effect of pushing around 1,000 ac-ft of water in Island Park Reservoir temporarily out to the west end of the reservoir, where large areas of reservoir bottom were exposed because the reservoir was only 54% full at the time. When that water returned back to the east side of the reservoir, where the dam is located, it carried a large sediment load that increased turbidity and sediment in the river downstream for over two weeks. We are still analyzing data from throughout the watershed to quantify the fate of that sediment as it passed through the watershed. I will provide that information in the form of a blog once we have the analysis done.

Natural Streamflow (water supply)

Watershed natural flow in 2020 was 92% of average: 88% in upper Henry’s Fork, 94% in Fall River, and 96% in Teton River. Natural flow in water year 2020 ranked 26th out of the 43 water years since 1978 and closely mirrored the subwatershed precipitation figures shown in the table above. Last year, natural flow was 98% of average, despite 114% of average water-year precipitation. Some of the discrepancy between precipitation and natural flow in 2019 was due to percolation of fall rain and spring snowmelt into soils and aquifers, deferring some of the effect of 2019’s abundant precipitation until 2020.

In fact, good baseflow was the primary reason streamflow in water year 2020 was as good as it was, considering how low precipitation was. October-March natural flow, which depends primarily on groundwater, was 98% of average over the whole watershed, reflecting good precipitation in 2019. April-September natural flow, which depends primarily on snowpack and direct runoff from spring and summer rain, was only 89% of average, reflecting relatively low precipitation this year. July-September precipitation in 2020 was only 52% of average, which is one reason why natural flow dropped from 98% of average over the fall and winter to only 89% of average over the spring and summer.

As mentioned above, springtime temperature this year was highly variable. Low-elevation snow melted rapidly in the last few days of April and first few days of May, producing a very high but short-duration spike in natural flow in the upper Henry’s Fork watershed. N