GOSHEN COUNTY – It’s no secret, this has been a milder than average winter.
Warmer temperatures and below-average snowfall amounts may be welcome in some quarters, but, for the agriculture community, it can spell problems for the coming irrigation season. According to snowpack reports from the Natural Resource Conservation Service, the areas which feed the Platte River Valley are currently anywhere from 70 to slightly more than 80 percent of normal snow pack for this time in the year.
But all the news is not grim. While there isn’t as much snow as there was last year at this time, the reservoirs along the North Platte still contain an abundance of water. According to the report, dated Jan. 1, Upper and Lower North Platte reservoirs were showing 147 percent and 128 percent above average storage, respectively, mostly carryover from the 2017 irrigation season.
“I wouldn’t say that’s extraordinary,” said Mahonri Williams, chief of the Resource Management Division for the Wyoming area office of the Bureau of Reclamation in Casper. “Our carryover last year was very good. It’s comparable to other high in-flow years we’ve had.”
As of the latest reports, Seminoe Reservoir – the furthest-upstream reservoir in the system – is about 80 percent full, Williams said. Pathfinder Reservoir is 77 percent full and Glendo Reservoir is at about 53 percent of normal capacity, he said.
While the numbers at Glendo may seem low, the reservoir is purposefully allowed to drain at the end of each irrigation season to leave capacity for upstream water releases to feed electrical generating stations along the river as part of normal annual operations. The reservoir at Glendo slowly refills with that discharge water over the course of the winter, Williams said.
So far this winter, snowfall has been well below average, according to the NRCS report, just 64 percent of average on the Lower North Platte River Basin and 87 percent for the Upper North Platte River Basin for the period of October through December. That compares to 97 percent last year.
A greater amount of precipitation in January 2017 pushed those snowpack amounts to well above the median amount. That trend continued through March, which accounted for the abundance of water last year and the strong carryover for this year, Williams said.
The lack of precipitation this year is due to a La Nina weather pattern currently sitting over the entire region, said Rob Cox, science and operations officer for the National Weather Service in Cheyenne.
“We were in more of a neutral weather pattern last year,” Cox said. “This year, it’s more of a La Nina, which tends to favor a dryer, windier pattern.”
With a La Nina pattern, the jet stream tends to move further to the north, taking with it the moisture which would otherwise accumulate as snow, he said. This particular weather pattern change is definitely out of the ordinary.
Under normal conditions, La Nina conditions would be preceded by El Nino conditions, Cox said. The two are characterized by cooler surface water temperatures in the central and eastern Pacific Ocean near the equator, in the case of La Nina, compared to warmer temperatures in the same area during an El Nino period. Both conditions can, and do, have strong impacts on global weather and climate.
“This year, we weren’t anticipating to be in a La Nina pattern,” Cox said. “It’s kind of difficult to predict. Usually these patterns are very seasonal, tending to happen from the fall through the spring time frame.”
Despite the below average precipitation and snowpack currently available this year, the final tale for the 2018 irrigation season has yet to be written, Williams said. The Bureau of Reclamation has yet to begin collating data for its annual runoff forecast, the report which quantifies how much water can be expected to enter the North Platte River system this spring.
And there’s still a good bit of winter left before the snowmelt begins.
“The precipitation we get in the spring, in terms of its impact on the runoff, is very important,” he said. “Rain in the lower elevations and more snow in the mountains plays a large roll in terms of how much runoff we get.
“Even though things are below median right now, we still have a lot of winter to go to accumulate snowpack,” Williams said. “We also have the added benefit of very good carryover in our reservoirs. That’s the bright spot in terms of looking ahead to the water supply for our irrigators.”