Water Year 2021: Average reservoir carryover with a 2016 water supply




Water year 2021 turned out to be as dry or drier than 2016 by most measures, yet Island Park Reservoir volume on September 30 was just a few percent below average and over twice what is was in 2016. Here are the highlights:

  1. Climate. Water-year precipitation was 19% below the 1989-2020 average, ranking 28th out of 33 years since 1989. Maximum snow water equivalent (SWE) was 22% below average. Peak SWE occurred on March 26, compared with the median of April 14. April and June set monthly low-precipitation records, and numerous high-temperature records were set in June and July, including one for mean temperature over the two-month period. In these measures, 2021 was worse than 2016, the last drought year in the watershed.

  2. Natural flow. Water-year total natural flow was 25% below the 1978-2020 average, ranking 41st out of the 44 water years since 1978, only two spots ahead of 2016. April-September natural flow in 2021 was 23% below average and even below that in 2016.

  3. Irrigation management. Because of dry conditions across the entire upper Snake River Basin, Fremont-Madison Irrigation District accrued only 74% of its storage right. Natural-flow water-rights priorities on the Henry’s Fork dropped about three weeks earlier than normal. As a result of these paper shortfalls and mid-summer rain, diversion during the 2021 irrigation season was 92% of the 2001-2021 average, compared with 96% of average in 2016. Using new water-management infrastructure, streamflow in the lower Henry’s Fork was maintained at a lower but more consistent level in 2021 than in 2016, saving water in Island Park Reservoir while also maintaining more stable fish habitat conditions in the lower Henry's Fork.

  4. Island Park Reservoir management. Reservoir fill was delayed because ice remained until the first of May, by which time the snowmelt peak had already passed. Outflow was reduced to 200 cfs in May, and even then, the reservoir filled prior to June 1 only because of heavy rain May 21-26. The reservoir drafted from June 10 to September 18, except for 13 days in August during rainy periods. September-30 reservoir volume was 45% full vs. 47% full on average and 20% full in 2016. After accounting for natural flow and diversion, reservoir carryover exceeded the statistical expectation by 20,000 ac-ft due to new precision water-management practices and infrastructure.

  5. Island Park stream gaging. HFF made 20 flow measurements during the summer and fall of 2021. Mean absolute error of HFF measurements compared to USGS adjusted flow was 21 cfs or 8.3%, small enough that our measurements provided accurate real-time estimates of streamflow to river and water users in between USGS measurements.

  6. Accuracy of predictive models. Based on springtime temperature and snowpack, predictions of runoff timing were within one day of observations, but predictions of natural flow made based on April-1 conditions were well below observed values due to subsequent dry conditions. Models correctly predicted need for Crosscut Canal delivery and hence magnitude of outflow from Island Park Reservoir, but actual timing of need for reservoir draft was 2-3 weeks earlier than predicted. September-30 reservoir content, as well as savings due to precision water management, were accurately predicted.

  7. Water quality. Water quality parameters followed the seasonal and spatial variations we have quantified over the past seven years, and most parameters were near average over most of the summer. The exception was water temperature, which was higher than average at most locations during the record-setting heat of June and July. Daily mean temperatures remained below 70ºF at all locations upstream of St. Anthony but frequently exceeded 70ºF at and downstream of St. Anthony.


A full report, with tables and figures, is available here for download here as a pdf file.






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