Idaho reservoirs will need more runoff in 2019 than they did this year for irrigation water supplies to be adequate. The goal is within reach but could be challenged by warmer- and dryer-than-normal conditions expected for winter and spring.
“We will need more runoff this year to have adequate supply for the 2019 season,” said Ron Abramovich, water supply specialist with the USDA Natural Resources Conservation Service in Boise. He spoke at the Idaho Water Supply Outlook conference in Boise Nov. 8.
A year ago, water carried over in reservoirs from the 2017 irrigation season, thanks to the heavy preceding winter, was plentiful and “made for easier planting decisions.” Minimal runoff was needed to ensure adequate irrigation supplies.
The Boise River Basin in 2019 will need 64 percent of average runoff to have a marginally adequate supply for irrigation, Abramovich said. A year earlier, the basin needed 51 percent. Water stored in the basin’s three reservoirs is at 101 percent of the long-term average compared to 136 percent a year ago.
Water volume in Owyhee Reservoir in southeastern Oregon is 84 percent of average compared to 159 percent a year ago. Runoff this year was about 34 percent of average. The Owyhee River Basin in 2019 will need 44 percent of average runoff to have a marginally adequate supply for irrigation, he said.
The Upper Snake River Basin in southeast Idaho and Wyoming in 2019 will need runoff that’s 69 percent of average to produce adequate water supplies for irrigation, Abramovich said. Runoff was 127 percent of average this past year. Current reservoir levels are 124 percent of normal.
The snow line usually is about 500 feet higher during El Nino years, he said.
Troy Lindquist, senior hydrologist with the National Weather Service in Boise, said a majority of models predict an El Nino to develop over the last quarter of 2018. He said there is a 70 to 75 percent chance — some scientists peg an even higher likelihood — that a weak El Nino will develop.
An El Nino weather pattern is associated with warmer sea-surface temperatures in the central and eastern Pacific Ocean, a southward shift of the Pacific jet stream and increased potential for warmer temperatures in the northern half of the U.S.