Have recent storms helped water supply?

Two weeks ago, I projected that springtime runoff in the upper Henry’s Fork watershed would likely be the lowest in at least 35 years. Since then, a number of spring storm fronts have passed through the area. Have they helped?

The short answer is no. Although temperatures have been cooler over the past two weeks and some of the storms have produced a little rain and snow, the primary effect of these springtime fronts has been strong winds, especially in valley areas where agricultural fields are bare. This has just resulted in a lot of blowing dust and even lower soil moisture than we had a few weeks ago. The small amount of precipitation that has fallen in the valleys has not been sufficient even to keep up with evapotranspiration. As a result, irrigation demand is already as high as it usually is in June. Water managers who have been working in the upper Snake River basin for decades have never seen irrigation diversion rates this high in the first half of April.

As of today’s water rights accounting, total streamflow in the upper Snake River is 8,610 cfs, but irrigation diversion is at 11,738 cfs. This means that about 6,200 acre-feet of reservoir storage is already being used to meet demand. In fact, the contents of the reservoir system attained their maximum about a week ago, and total storage has been declining since then. On average, maximum reservoir system content is attained around June 20. Having said that, one reason that storage water is being used right now is because the cooler weather has slowed snowmelt and dropped inflows, while irrigation demand has continued to increase. Once the cold front that brought us a few inches of snow last night passes to our east, temperatures will rebound and are projected to be above average from the upcoming weekend through the end of the month. Warmer temperatures will increase snowmelt, so at some point later in the spring, streamflow should exceed demand again, allowing at least a little more water to be added to reservoir storage.

The Island Park Drought Management Planning Committee met on April 3 to determine springtime operations at Henry’s Lake and Island Park Reservoir. My forecast of record low streamflow was consistent with those of the state and federal agencies, and we agreed to keep outflow from Island Park much lower than it usually is this time of year in order to fill the reservoir during the month of April. This will allow us to increase outflow to match inflow during May and June.

Outflow from Henry’s Lake was increased from about 20 cfs to 55 cfs on March 28, as the lake reached 100% full. However, the cooler weather has decreased inflow to Henry’s Lake, and its level has dropped slightly over the past two weeks. A return to warmer weather and higher snowmelt will increase inflows again within a few days, and Henry’s Lake will remain essentially full until irrigation water is needed later in the summer. Unfortunately, it does not look like the Henry’s Lake water rights will fill, so some of the physical water in Henry’s Lake will belong to users in the Magic Valley and need to be delivered to them later in the summer. Currently, inflow to Henry’s Lake is around 60 cfs, around the 25th percentile over the last 35 water years and only 70% of the mean for this time of year. I project springtime inflow to Henry’s Lake to stay well below average all spring and summer.

Outflow from Island Park Reservoir was increased from around 280 cfs to about 350 cfs on April 3 in anticipation of a little more snowmelt and to help increase power generation efficiency at the Island Park hydroelectric power plant. Despite the increase in outflow, the reservoir has continued to fill steadily over the past 10 days and should fill in the next week. Inflow from the watershed between Henry’s Lake and Island Park Reservoir remains at around 500 cfs, compared to a long-term mean of 570 cfs. Once Island Park fills, outflow will be set to match inflow, which should average 500-550 cfs during May before declining to around 450 cfs in June. So, early-season flows in the river between Island Park and Ashton will very likely be much lower than average for that time of year.

During most years, release of Island Park storage to meet irrigation needs does not occur until late June or early July, but we expect releases to begin in early- to mid-June this year. Although it is too soon to tell how much water will be needed and when, it is likely that streamflows will increase sometime in June, providing average- to slightly above-average streamflows for the Harriman Ranch opener. By late June, it is likely that river flows will be much higher than average, as irrigation demand continues to increase.

At this time, although we are certain that Island Park Reservoir will physically fill with water, we are not certain that Fremont-Madison Irrigation District’s (FMID) water rights to the storage in Island Park will fully fill. Currently, FMID’s storage rights in Island Park stand at 103,388 acre-feet, about 77% of the full right. Because the reservoir system as a whole is currently being drafted, rather than filled, no water is accruing to any storage rights, including FMID’s. If snowmelt accelerates enough later in the spring so that supply exceeds demand again, FMID’s storage right will resume accrual, but full fill of its right will require supply to exceed demand for about an entire month. At this point, that seems unlikely. The result of this is that, some of the water in Island Park will belong to users in the Magic Valley, and this water will need to be delivered there at some point during the summer, resulting in high streamflows in the Henry’s Fork downstream of Island Park Dam.

How much more snowmelt we will get is dependent both on temperature and on how much snow remains in the mountains. In the Henry’s Fork watershed, the latter is pretty dismal. Snow-water-equivalent at the sites upstream of Island Park reservoir ranges from 16% to 72% of average. Water-year-to-date precipitation averages around 65% of average in the Henry’s Fork watershed.  Snow-water-equivalent and precipitation figures at other sites around the upper Snake basin are slightly better but still well below average.

By almost all measures, the current drought is as bad as anything we have seen in recent decades, prompting the Fremont County Commission, at FMID’s request, to declare a drought emergency already this spring. HFF will be monitoring the situation closely and will remain involved in management of Island Park Reservoir throughout the spring and summer, through the Drought Management Planning Committee. I will provide updates as frequently as possible, especially when large flow changes are scheduled.