Reservoir Remains Nearly Full Despite Dry July

Photo of clouds over Fall River
  • The month of July was a little warmer than average but very dry.
  • Natural flow and diversion were both around 90% of average for most of the month.
  • Moisture from May and June rain has kept total diversion for the season about 100,000 ac-ft below average.
  • As of August 3, Island Park Reservoir is still 82% full, compared with an average of 70% full at this time of year.

Above-average Temperatures Continue through July

Temperature over the month of July was 2 degrees F above average in the Henry’s Fork watershed, continuing the pattern of above-average temperatures. Mean April-June temperature was also 2 degrees above average. Cool nights throughout July offset very warm afternoon highs to keep daily means close to average. As of August 2, Pocatello had set a new record of 30 consecutive days with high temperature at or above 90 degrees. According to the National Weather Service, the current heat wave in southeastern Idaho is record-setting in persistence and duration but not extremes. 

Graph of temperature deviation from average

Graph above sows temperature relative to average. Note consistently above-average temperature since early July.

Spring Rain Keeps Irrigation Demand Low, Despite Dry July

After abundant rain in May and June, July was very dry. Precipitation for the month was about one-third of average across the Henry’s Fork watershed. Only the Teton subwatershed received anything close to average. However, July is always one of our driest months, so despite low precipitation, total water-year precipitation remains at 107% of average 3 days into August. Water-year precipitation was 110% of average at the beginning of July. Even if the remaining two months in the water year are dry, water-year total precipitation will end up at or above average. Half of the stations in the watershed have already received their average water-year total precipitation: Pine Creek Pass, Lewis Lake Divide, Black Bear, Ashton, Rexburg, and Alta. The other sites are each only 2-3 inches short of their average water-year totals.

Table below summarizes climate across the Henry's Fork watershed for the month of July.

Climate table for July 2018

Despite the dry weather in July, the positive effects of abundant rain in May and June have persisted well into the summer. Back at the end of June, moisture availability in the agricultural areas (defined as evapotranspiration minus precipitation, a measure of irrigation need) was about 5 inches greater than average.

Graph of moisture deficit/surplus

Graph above shows accumulated moisture deficit over the water year. On July 1, moisture deficit was 5 inches lower (availability was 5 inches higher) than average.

When this 5-inch advantage is applied to the 250,000 acres of irrigated land in the watershed, the result is a theoretical reduction in irrigation need of about 104,000 acre-feet, compared with average. As of August 2, total irrigation diversion so far this season was 531,382 acre-feet, compared with an average of 637,102 acre-feet on August 2, a difference of 105,720 acre-feet, consistent with the five extra inches of moisture available for crop growth prior to July 1. So, the May and June rain continues to benefit both irrigators and streamflow, despite 5 weeks of dry weather.

Graph of irrigation diversion in Henry's Fork watershed

Graph above shows irrigation diversion so far this year. Note periods of very low diversion during May and June and consistently below-average diversion over the second half of July.

Streamflow Below Average, but Reservoir in Great Shape

Due to early snowmelt and 5 weeks of very dry weather, watershed-total natural streamflow has been around 90% of average since early July. Fall River has led the subwatersheds at about 115% of average streamflow. Natural flow in the upper Henry's Fork has been around 80-85% of average since early July, and Teton River flow has generally been less than 80% of average over the past few weeks.

Graph of natural streamflow in Henry's Fork watershed

Graph above shows natural streamflow in the Henry's Fork watershed so far this water year. Flow has been around 90% of average since late June.

Despite generally below-average streamflow in the upper Henry's Fork watershed (above Ashton), natural streamflow gain in the river between Henry's Lake and Island Park has been steadily increasing since early July and is now right at to average, compared with 74% of average at this time last year. This steady increase in streamflow gain indicates recovery of Big Springs and other groundwater springs from the 2013-2016 drought.

Graph of streamflow gain between Henry's Lake and Island Park

Graph above shows natural gain to the river between Henry's Lake and Island Park Reservoir so far this water year.

As a result of low irrigation demand, outflow from Island Park Reservoir has remained below average all summer and has not exceeded 1,150 cfs since late June.

Graph of outflow from Island Park Reservoir

Graph above shows outflow from Island Park Reservoir.

The combination of below-average outflow and good inflow has kept reservoir volume high. As of August 3, the reservoir is still 82% full, compared with an average of 70% full for the date. The reservoir currently contains 17,232 acre-feet more water than average for this time of year. All other things being equal, this is equivalent to an additional 87 cfs of winter flow below Island Park Dam.

Graph of Island Park Reservoir volume

Graph above shows volume in Island Park Reservoir.

What will August Bring?

Short and medium-term weather forecasts call for continued warm, dry weather. However, irrigation demand is well past its peak, and Fall River will continue to meet much of the lower-watershed irrigation demand for the next month. Outflow from Island Park Reservoir will likely stay between 600 and 800 cfs for the remainder of irrigation season, and the reservoir will end the water year well above average. Winter outflow is certain to be above average for the second year in a row.