Snowpack melting rapidly: below-average water supply likely by mid-summer

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On March 12, snow-water-equivalent (SWE) in the Upper Henry’s Fork subwatershed was 122% of the 30-year median. Unfortunately, the last two weeks brought record high temperatures, and we have already lost a large amount of snow. Today, SWE in the Upper Henry’s Fork is only 104% of median. SWE is 127% of median in the Fall River subwatershed (down from its peak of 137%) and 125% of median in the Teton River subwatersed (down from its peak of 138%). Although some of the early snowmelt has been stored in Henry’s Lake, Grassy Lake, and Island Park Reservoir–and in local and regional aquifers–the vast majority of recent record-high streamflows has flowed out of the upper Snake River basin. This water will not be available later in the summer when we need it most. Check back next week for detailed predictions of streamflow for the upcoming spring and summer.

Current conditions

Graph of snow-water-equivalent in the Henry's Fork watershed as a percent of median.

Graph above shows percent-of-median SWE since November 1 in the three subwatersheds of the Henry’s Fork watershed. Also shown is the percent-of-median SWE for the Henry’s Fork and Teton Basins, as reported in the Natural Resources Conservation Service (NRCS) daily snow/precipitation report. Table below shows data for individual sites, as well as the subwatersheds and the NRCS Henry’s Fork and Teton Basins index. If you would like to receive daily updates of this graph and table via email, send a message to Rob at rob@henrysfork.org with “SWE request” in the subject line.

Table showing current and peak SWE data.

To see how we use raw data from the NRCS SnoTel website to calculate the numbers in the graph and table above, download our SWE methods document. Go to the NRCS interactive map for a great visualization of current snowpack across the western United States. Also, see last week’s blog for a thorough analysis of runoff timing in the Henry’s Fork watershed.

Analysis of Early Runoff

I included this analysis in my daily snowpack report on March 22. I’ve reprinted it here in slightly modified version for those of you who are not on the daily email list.

As snow has melted over the past two weeks, that snowmelt has been translated into high streamflows, providing ecological benefits in stream channels and floodplains throughout the Snake River basin, all the way downstream to the Columbia River. Furthermore, some of this early runoff has been captured and stored in Grassy Lake, Henrys Lake, and Island Park Reservoir. Unfortunately, the vast majority of early runoff in the upper Snake River basin has exited the basin without being stored or recharged into the aquifer. Flood-control releases from the other reservoirs have been necessary to make room for runoff yet to come, and even though managed recharge is occurring at its current capacity, that capacity is far lower than the amount of water available. Irrigation diversion can’t legally begin in most canals in the basin until April 1, and some canals can’t begin diverting until April 15.

I will emphasize yet again that streamflow during the summer is much better predicted by the amount of snow on the ground on April 1, not the amount on the ground in early March. Thus, although total water-year streamflow basin-wide will be well above average, we may still face low natural flows and heavy reliance on reservoir storage late in the summer because a substantial percentage of our total water-year supply will have flowed down the river in March, when most of it could not be used or stored. This is especially true in the Henry’s Fork watershed, which is lower in elevation than the main Snake River headwater areas and has received lower precipitation all winter.

I started drawing attention to the possibility of early runoff in my daily report on January 26, ending the report that morning with “A good snowpack won’t contribute as much to water supply at the right time if it melts early.” Although we are a long way from tabulating the final statistics on the spring of 2017, even at this early date we know the following:

  • Streamflow records were set in late October and early November from an unprecedented autumn rain-on-snow event.
  • Streamflow records were set in the southeastern corner of Idaho in mid-February from a rain-on-snow event that broke records set during the 1962 Willow Creek flood, which prompted construction of Ririe Reservoir.
  • Peak SWE at sites in the 6000-7000 elevation range this year will be 12-20 days earlier than 1981-2010 medians.
  • Our analysis of runoff timing and late-winter/early-spring temperatures provides strong scientific evidence that 2017 (and 2015 and 2016 for that matter) may be unusual among water years experienced during the mid- and late-20th century but are more likely to occur in the future.
  • The State has temporarily suspended normal water-right restrictions to allow the maximum amount of managed aquifer recharge right now.

Without delving further into the complexities of water-rights accounting, ongoing contested cases, and management of the Eastern Snake Plain Aquifer, I will take liberty of suggesting that administrative changes—and private investment in infrastructure to complement what the State is already doing—will be necessary to increase the amount of managed aquifer recharge that can be done during late winter and early spring of years such as 2017. The ability to recharge even 20-30% of the 21,000 cfs currently flowing past Milner Dam would go a long way toward solving many of the current water-management problems we face in the upper Snake River basin without the high economic and ecological cost of building new storage reservoirs.

This post appeared first on www.HenrysFork.org. Donate to Henry's Fork Foundation Today!

2017-03-27T20:09:32+00:00

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