Water Year 2022 Summary: Natural Streamflow



Headlines:

  • Natural streamflow was 72% of average, ranking as the driest year in the 1978-2022 record.

  • By subwatershed, natural flow was 73% of the 1978-2021 average in upper Henry’s Fork, 73% in Fall River, and 70% in Teton River.

  • In the long upper Henry’s Fork record, natural flow was 77% of average, ranking 87th out of the last 93 years, just ahead of 2016.

  • April-September natural flow was 69% of the 1978-2021 average, better than my April-1 prediction of 60% of average, due to spring rain.

  • For the watershed as a whole, natural flow has been above the 1978-2022 average in 8 of the past 23 years, compared with 12 of the 22 years prior.

Details:

Watershed-wide natural streamflow was 72% of the 1978-2021 average in water year 2022: 73% of average in upper Henry’s Fork, 73% in Fall River, and 70% in Teton River. The watershed total ranked as the driest in the 45-year 1978-2022 record, behind 2001 at 74% of average and 2016 at 75% of average. In the longer 93-year record of natural watershed inflow between Henry’s Lake and Ashton, water year 2022 came in at 77% of average, ranking 87th, ahead of 2016 by a fraction of one percent. That is well within stream gaging error, and the rank order of 2022 and 2016 may change when final USGS streamflow data are published. The other five years with natural flow lower than this year occurred back in the 1930s.

The water year started out poor as a result of very dry conditions during the summer and fall of 2022. Winter (October-March) natural flow was the lowest in the 1978-2022 record, at 79% of average. Because about half of the watershed’s winter base flow comes from the groundwater-fed upper Henry’s Fork, the low watershed-wide figure was driven primarily by the upper Henry’s Fork, where winter flow was 79% of average and second lowest in the 1978-2021 record, ahead of 2016.

Based on well below average snowpack in early April and on poor winter base flow, April-September natural flow was predicted to be only 60% of average and in a dead heat with 2001 for the lowest in the 1978-2022 record. However, above-average precipitation in May, June and August improved streamflow somewhat, and April-September precipitation came in at 69% of average, ranking 4th driest, ahead of 2001, 1992, and 2021.

As mentioned in the climate summary yesterday, the wet spring brought a modest improvement in drought conditions over the course of the water year, but it was not enough to keep water year 2022 from ranking as the driest overall in the last 45 years. In the context of long-term drought, natural flow has been above average in 8 of the past 23 years, compared with 12 in the previous 22 years. Looked at in terms of ranks, the median year (1980) divides the last 45 years into two 22-year periods—one with natural flow greater than that observed in 1980 and one with natural flow lower. Fourteen of the 22 years in the “dry” half have occurred since 2000, indicating persistence of dry conditions for the better part of the last two decades.

While the spring rains had only a modest effect on streamflow—mostly turning an extremely bad outlook for the summer into just a very bad one—cool temperatures had a much greater positive effect on summertime water supply. As mentioned in the climate summary, snow water equivalent peaked 12 days later than average. As a result, the center of mass of spring/summer natural streamflow—a measure of timing of streamflow--occurred 5 days later than the 1978-2021 average, 6 days later than it did last year, and 9 days later than in 2016. This delayed runoff made a meager snowpack last much longer into the summer than in other recent years with similar water supply. This had a substantial positive effect on the need for reservoir storage to meet irrigation demand this year.

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