• April 1 snow-water-equivalent was 106% of average in the Henry’s Fork watershed.
• Based on April 1 conditions, summer water supply is predicted to be 108% of average for the whole watershed:
  • 99% of average in the Henry’s Fork upstream of Ashton
  • 117% of average in Fall River
  • 114% of average in Teton River
• Snowmelt is starting at its normal time this spring due to seasonable temperatures.

Early-April climatic conditions favorable for water supply

In 2016, the spring was both warm and dry, following several dry years, and the result was one of the worst water years on record in the Henry’s Fork watershed. In 2017, April-1 snow-water equivalent (SWE) was well above average in Fall River and Teton River subwatersheds but only average in the upper Henry’s Fork subwatershed (upstream of Ashton). One reason for the relatively poor April-1 SWE in the upper Henry’s Fork was a warm March, which had melted most of the snowpack below 7,000 feet by April 1. Fortunately, the overall water supply in 2017 ended up well above average because of cool, wet weather in April and May.

While not as strong in the Teton and Fall river subwatersheds, this year’s April-1 SWE was higher in the upper Henry’s Fork subwatershed than in 2017 and much more uniformly distributed across elevations and subwatersheds that it was last year. The table below summarizes SWE accumulation over the month of March and gives statistics by subwatershed for April 1. You can download a guide to the sites here.

By comparison, April-1 SWE values in 2017 were 101% of average in the upper Henry’s Fork, 112% of average in Fall River, and 116% of average in Teton River.

In contrast to 2017, cool temperatures in late February and March have kept most of the winter’s snow accumulation on the ground so far in 2018. The figure below shows 7-day watershed temperature relative to average. Positive values indicate warmer-than-average temperatures and negative values indicate cooler-than-average temperatures.

You can easily see after a period of well below-average temperatures in late February and early March, temperatures have been right near average for the past month, whereas temperatures during March of 2017 were well above average. This temperature difference is responsible for much of the difference in low- and mid-elevation snowpack between 2017 and 2018.

Across the West, April 1 SWE provides a good index of water supply for the upcoming summer because April 1 is roughly the date on which SWE accumulation reaches its maximum. In the Henry’s Fork watershed, date of peak SWE accumulation is actually today–April 10. The graph below shows mean SWE accumulation over the entire Henry’s Fork watershed.

The graph shows that this year’s SWE followed average pretty closely up until the last few weeks, when heavy precipitation fell across the watershed. You can see the average peak on April 10 and how much higher SWE is compared to that peak. SWE on April 10 this year is 112% of average and almost identical to that on April 10 in 2017. Given current weather forecasts for cool temperatures and more precipitation, SWE will probably continue to increase for the next week or two, perhaps ending up about where last year’s SWE peaked–but with the difference that more of the snow on the ground this year will be in the northern half of the watershed and more of it will be at lower elevations.

Streamflow and reservoir supplies also favorable

Natural watershed-total streamflow stayed near or above average all winter. The graph below shows natural flow, by subwatershed.

Over the October-March time period, which is the time of lowest streamflow in the watershed, natural flow was 95% of average in the upper Henry’s Fork, 124% of average in Fall River, and 125% of average in Teton River. This fall/winter flow is called “baseflow” by hydrologists because it results from long-term storage of water in soils and aquifers rather than from direct rain or snowmelt. With the exception of two recent streamflow spikes caused by minor rain-on-snow events, streamflow has tracked average very closely since early March, indicating that the onset of runoff this year has occurred at its average timing. Compare that to last year, when runoff started in early March, 3 weeks earlier than average. The combination of high baseflow and average onset of runoff indicates optimal streamflow conditions heading into the spring–soil and aquifer moisture is good, and most of the winter’s snow is still yet to melt.

Meanwhile, because reservoir carryover from 2017 was so high, both Henry’s Lake and Island Park Reservoir stayed constant all winter, despite above-average outflow from both reservoirs. The reservoirs are currently very near average for this time of year and will fill easily over the remainder of the spring without any outflow decreases. The following graphs show contents to date of both reservoirs.

Above-average summer streamflow expected

What can we expect for the upcoming summer? In the Henry’s Fork watershed, streamflow over the period from April 1 to September 30 is predicted fairly precisely from knowledge of April-1 SWE and baseflow over the preceding winter. Baseflow indicates status of water contained in soils and aquifers, and SWE indicates water yet to enter the rivers from snowmelt over the upcoming spring and summer. The table below shows the relative contribution of each of these two measures to prediction of summer streamflow in the three subwatersheds.

Percentages in the baseflow and SWE columns indicate the fraction of total variability in summertime streamflow explained by these two respective variables. The percentages in the “TOTAL” column indicate fraction of variability explained by the two variables together. The remainder is variability due to random variables that can’t easily be measured and included in a predictive model. Much of this random variability is due to precipitation that falls between April 1 and September 30, which could be used to explain streamflow after the fact but cannot be used to predict it with information at hand on April 1.

Notice that precision in the predictions is highest in the upper Henry’s Fork, mostly due to the large dependence on baseflow, which is maintained in the upper Henry’s Fork primarily by discharge from the deep Yellowstone Plateau aquifers. As fraction explained by baseflow decreases and fraction explained by SWE increases, overall precision decreases. This is because changes in aquifer discharge occur only very slowly, whereas changes in additional snow accumulation, melt timing, and melt rates can be quite substantial after April 1. In general, the more streamflow depends on groundwater inputs, the more predictable it is. Nonetheless, predictive ability in all three subwatersheds is quite good–at least 73% as measured by the R-squared statistic.

So, with good predictive models and April-1 data in hand, what is in store for the upcoming summer? The table below summarizes the results of the predictive model, including model inputs, predictions, the 90% exceedance values, and comparison with 2017. The 90% exceedance value of a prediction is the value that will be equaled or exceeded with 90% probability, given the inputs and the unexplained random variability.

(NOTE: Slight descrepancies in the SWE values between this table and the SWE table above are due to slightly different averaging periods used in the SWE reporting versus streamflow predictions. The SWE averaging period is a little shorter, weighted toward more recent years with lower snowpacks.)

Predicted water supply is near average in the upper Henry’s Fork and above average in Fall River and Teton River, although not nearly as much above average as the flows observed in 2017. Expected streamflow this summer is more uniformly distributed across the three subwatersheds than it was in 2017. With 90% probability, streamflow will be at least 89% of average across the watershed this summer.

Taken together, all measures of water supply are average at worst right now, and most are above average, giving us a good chance of enjoying another above-average water year.