• May 2018 saw warm temperatures, heavy rain, and above-average snowmelt.
• Only 30% of this year’s peak snow-water-equivalent (SWE) remains, compared with an average of 43% remaining on June 1.
• Water-year precipitation stands at 108% of average, but SWE has dropped from 117% of average at its peak in April to 82% of average on June 1.
• Lack of snow means that streamflow will drop rapidly once the current wet weather ends.

May was Warm and Wet

Over the whole month of May, watershed-mean temperature was 4 degrees F warmer than average. Precipitation was above average at all stations except Crab Creek, and the valley areas received as much as twice average precipitation for the month. Watershed-total precipitation remains at 108% of average. Moisture availability in the agricultural regions is over 3 inches above average for this time of year.

Watershed temperature relative to average. Note how much colder the middle of May was in 2017 versus 2018.

Climate summary table for May 2018.

Net moisture availability in agricultural areas of Henry’s Fork watershed.

Very Little Snowpack Remains on June 1

Because of warm temperatures all month, snowmelt was well above average. In mid-April, snow-water-equivalent (SWE) was 117% of average. By June 1, this above-average snowpack dwindled to only 82% of average. The following graph and table illustrate just how much less snow remained on the ground on June 1 2017 versus that in 2018. May of 2017 was very cold, and snowpack was higher to begin with at all stations except White Elephant and Island Park, which accumulated more SWE this year than in 2017. However, watershed-wide SWE on June 1, 2017 was 62% of the 2017 peak and 144% of average for the date. Compare that to June 1, 2017 data: SWE is down to 30% of this year’s peak, compared with an average of 43% for the date. The most notable differences between 2017 and 2018 are at White Elephant and Grassy Lake, where essentially no snow was left on June 1, versus 8.5 and 13.2 inches, respectively, in 2017. Across the whole watershed, last year’s June-1 SWE was 175% of this year’s value. This lack of snow means that once we get out of the current rainy pattern, streamflow will drop rapidly. At this point, only continued rain will keep streamflow up.

Graph of watershed-average SWE for 2018, compared with average and with 2017.

June-1 SWE summary for 2017, 2018, and average.

May Rain Keeps Streamflow Up and Irrigation Demand Down

Watershed-wide natural streamflow peaked for the season on Thursday, May 24, following over one inch of rain across the whole watershed. Late-May rain also decreased need for irrigation water, and diversion dropped to 70% of average for the date by the end of the month.

Graph of watershed-total natural streamflow.

Graph of total diversion in Henry’s Fork watershed.

Henry’s Lake Full; Island Park Reservoir 99% Full

Henry’s Lake filled rapidly during May due to snowmelt early in the month and rain at the end of the month. Island Park Reservoir was a little over full pool prior to the May 25 rain event, but outflow was increased to catch runoff from that event. The reservoir dropped to 98% full but climbed back to 99% full after outflow reductions on May 29. The reservoir will reach full pool again during the first week of June and remain full until irrigation delivery is needed, probably during the last week of June.

Graph of Henry’s Lake volume.

Graph of Island Park Reservoir volume.

The graph below compares regulated flow (actual flow) at Island Park Dam with natural flow (flow that would be in the river without dams). When natural flow is higher than regulated flow, the difference is stored in Henry’s Lake and/or Island Park Reservoir. These are the green areas. When natural flow is lower, water is being released from one or both reservoirs. These are the red areas. Note that the shape of the regulated hydrograph generally followed the shape of the natural hydrograph during May but allowed release of a little water prior to the peak of snowmelt early in the month and at the peak of the rain event at the end of the month. In between, water was stored, as both reservoirs filled.

Graph of natural flow versus regulated flow at Island Park.

Outlook for June: Warm and Dry

As of June 1, forecasts call for warm, dry weather for the whole month. If this forecast is correct, irrigation demand will increase and natural streamflow will decrease. At some point late in the month, this will require delivery of storage water from Island Park Reservoir to meet irrigation demand. I expect delivery to be close to average for most of the summer.