POCATELLO – Eastern Idaho’s snowpack and reservoir storage has improved over the winter, but high temperatures over the coming months could drive up demand for that water.
The 2024-2025 winter brought close-to-normal and above-normal precipitation to Idaho Falls and Pocatello, respectively, which has resulted in high reservoir levels and a snowpack forecast to give close-to-average runoff. However, the National Weather Service predicts the region will see higher-than-average temperatures and below-average precipitation over the spring and summer, which would cause a higher demand for storage water.
And if a water shortage is to occur this upcoming growing season, last year’s long-term mitigation agreement significantly changes how surface water and groundwater users would respond.
“If they don’t have enough water in those reservoirs to make up for the water they’re not getting out of the sky, then that’s where the issues start,” said Sherrie Hebert, observation program lead of the NWS Pocatello office.
What does eastern Idaho’s water storage look like ahead of the growing season?
The three-month average temperatures from December to February for both Idaho Falls and Pocatello were not out of the ordinary, despite having their second- and third-hottest December on record, respectively. Temperatures cooled off enough in January and February to bring the overall average temperature within a normal range.
The two cities have also seen typical and above-average precipitation levels. The accumulated precipitation over the winter has allowed eastern Idaho to build up its water storage ahead of the growing season.
According to provisional data from the U.S. Bureau of Reclamation, the entire Upper Snake River reservoir system is at 82% of capacity. Although the Jackson Lake is 76% full and the Palisades Reservoir is 72% full, the American Falls Reservoir is 93% full.
How does the snowpack look?
Craig Chandler, the water master for Water District 1, said the winter’s precipitation has improved the snowpack.
“The snowpack has certainly improved from the levels that were we were at early winter,” Chandler said.
The snowpack of the Snake Basin above Palisades is at 114% of median snow water equivalent. The snow water equivalent represents how much liquid water would result if the snowpack melted instantaneously, and because the figure is the percentage of the median, 100% represents the median snow water equivalent.
As for how much runoff this growing season will add to river flows, Chandler said, “It’s a little bit more complicated than just looking at the snowpack.”
“There are a number of agencies out there that look at all this data, and … they come up with a runoff forecast … where they look at all these various pieces of data and then come up with a prediction for the amount of runoff that’s going to come from the snowpack,” Chandler said.
For reference, base flows have stayed at around 89% of average this winter, which Chandler described as a “little low.” The runoff forecast from the Bureau of Reclamation for the beginning of March was 94% of average runoff.
“Now, that’s probably still a good number. It may have kicked up a little bit from there with the snows that we got over the last couple weeks, but we’re probably sitting somewhere in that 95% to 100% of average range for the forecasted runoff,” Chandler said.
Will it be enough?
While reservoir levels and the snowpack have improved over the winter, summer temperatures and precipitation levels play a significant role in how far that water supply will stretch.
“If we have a dry, hot summer, then irrigation needs (are) going to increase, and then that puts more demand on the reservoirs, but if we have good precipitation this year, and not a really hot summer, then that’s not going to put a lot of stress on them,” Hebert said.
Chandler affirmed this, saying, “If we have higher temperatures, that’s going to lead to an increase in irrigation demand. Farmers are going to need to put more water on their crops to help them grow.”
This would mean higher storage water use from surface water users, depleting reservoir levels quicker.
Thanks to last year’s agreement, the 2024 Stipulated Mitigation Plan, groundwater users with junior water rights have “safe harbor” from water curtailments as long as they follow the mitigation plan. But Chandler said that depending on temperature and precipitation levels, groundwater users may have to acquire more storage water to deliver to senior surface water users.
“One of the details of the agreement is an obligation to deliver storage water to the surface water coalition. And so if there’s a higher storage use, that may lead to a scenario where a larger volume of storage water needs to be supplied to keep the senior surface water irrigators whole,” Chandler said.
Hebert said the three-month outlook for April, May and June forecasts temperatures rising to around 35% above normal, with precipitation levels dropping to around 40% to 50% below normal. Looking further into July, August and September, the temperatures are forecast to around 60% to 70% above normal temperatures, with around 40% below normal precipitation.
“So that’s not looking too good for our farmers,” Hebert said.
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