It’s not just the snowpack
A new study showing the link between warmer spring temperatures and decreased river flows could spell more challenges for water managers trying to stretch supplies from major western rivers.
The research suggests that warmer-than-average spring temperatures may have a bigger effect on upper Colorado River flows than expected.
“Forecasts of stream flow are largely based on precipitation,” said University of Arizona professor Connie Woodhouse. “What we’re seeing since the 1980s is that temperature plays a larger role in stream flow and in exacerbating drought, said Wodehouse.
Most of the streamflow in the upper Colorado comes from mountain snowpack, and that in itself is a problem, with a distinct long-term trend of declining spring snowpack across the entire northern hemisphere. The study shows that temperatures during the runoff season (March to July) can have a significant impact on the amount of water that ends up in the river.
The findings are based on an analysis of temperature records, cool-season precipitation and stream flow for the years 1906 to 2012. The researchers specifically wanted to know how winter snowfall, March-July temperatures and November soil moisture levels influence annual Colorado River flows at Lees Ferry, Arizona.
Using the streamflow data, the researchers identified six droughts that occurred in the Upper Colorado River Basin from 1906 to 2012. A drought was defined by consecutive years with below-average streamflow punctuated by no more than one year of normal or above-average flow. The drought periods were: 1931-1940, 1950-1956, 1959-1969, 1972-1977, 1988-1996 and 2000-2012.
“In certain years temperature became a very strong influence. It was a bit of a surprise.” Woodhouse said. “If we have a warmer spring, we anticipate that the river flows will be less relative to the amount of snowpack.”
The researchers found that winter precipitation and average runoff-season temperatures varied from drought to drought.
“The 1950s was the driest period, but also the coolest,” Woodhouse said. “In contrast, the most recent drought of 2000 to 2012 was the warmest, but only moderately dry.”
If the temperatures during the runoff season – March to July — were cooler than average, streamflow was higher than expected on the basis of winter precipitation alone, the team found. However, when runoff-season temperatures were above average, streamflow was less than expected.
During and since the 1980s, average Upper Colorado River Basin temperatures during the runoff season have been increasing.
“If we have a warmer spring, we can anticipate that the flows will be less relative to the amount of snowpack,” Woodhouse said. “What we’re seeing is not just the future – it’s actually now. That’s not something I say lightly.”
The team’s paper, “Increasing Influence of Air Temperature on Upper Colorado River Streamflow,” is scheduled for online publication in Geophysical Research Letters today at http://onlinelibrary.wiley.com/doi/10.1002/2015GL067613/full. Woodhouse’s co-authors are Gregory Pederson of the U.S. Geological Survey in Bozeman, Montana; Kiyomi Morino of the UA Laboratory of Tree-Ring Research; Stephanie McAfee of the University of Nevada, Reno; and Gregory McCabe of the USGS in Denver. The U.S. Department of the Interior’s Southwest Climate Science Center funded the research.