TRAVERSE CITY, Mich. (AP) — Rising and falling water levels in the Great Lakes are influenced by evaporation, rain and snowfall in ways that aren't fully understood and are becoming less predictable as the climate warms, according to scientific reports made public Tuesday.
This winter's bitterly cold weather is forming ice on areas of the lakes that will reduce evaporation by blocking water vapor from rising into the atmosphere and keeping water temperatures cool well into summer. That should help water levels continue a rebound that began in 2013 following an unusually long slump across most of the lakes dating from the late 1990s, said John Lenters, senior scientist at LimnoTech, an environmental consulting firm in Ann Arbor.
But recent studies show that evaporation and ice cover regulate lake levels in more complex ways than previously believed, according to a new paper published by the Great Lakes Integrated Sciences and Assessments Center, a joint U.S.-Canadian research team led by Lenters. The team is developing a network to measure evaporation rates continuously, providing data that will improve water level forecasts — a helpful tool for the cargo shipping and tourism industries and others whose livelihoods are affected by the inland seas' ups and downs.
Meanwhile, another paper being released this week says the Great Lakes have ebbed and flowed on a fairly consistent 10-year cycle for much of the past century, although the steep decline that began in 1998 suggests the pattern may have been broken. That study, led by Carl Watras of the Wisconsin Department of Natural Resources, says the fluctuations have been influenced by atmospheric trends hatched as far away as the northern Pacific Ocean.
Taken together, the studies confirm that climate is by far the biggest player in determining water levels, Watras said. Human actions make little difference, despite noisy debates over whether structures should be built to regulate flows between lakes or whether communities outside the watershed should be permitted to draw from them, he said.
"The two big factors are precipitation and evaporation," said Watras, who is also a research fellow with the University of Wisconsin's Center for Limnology, whose paper is being published this week in the journal Geophysical Research Letters. "The question looking ahead is how they will behave as the climate evolves. Will we get more of both? Or will one begin to dominate?"
Rainfall and runoff from melting snow have helped compensate for water lost to evaporation over the years. But winter ice cover has shrunk dramatically in recent decades, removing an important restraint on evaporation.
Unless precipitation keeps pace, "it's pretty darn clear that rising regional temperatures will increase evaporation and ... the lake levels are going to decline," said Don Scavia, director of the Graham Sustainability Institute at the University of Michigan, which oversees the Lenters-led research project.
Great Lakes levels fluctuate seasonally, rising during wet springs and dropping in the fall. They also go through longer-term high and low periods. Although most of the lakes are hundreds of feet deep far from shore, changes in water levels make a big difference in shallower areas such as shorelines and busy shipping channels.
Lenters' team kicked off the first coordinated effort to calculate year-round evaporation rates on the lakes in 1998, establishing five stations across the region where measurements are taken.
Their studies have shown that even though ice cover during cold winters limits evaporation, the benefit is partially offset by heavy evaporation in the weeks before the ice forms as the water rapidly cools, the paper says. Yet heavy ice can keep the water colder for longer than usual and delay the onset of the high-evaporation season.
Such nuances illustrate the importance of expanding the evaporation monitoring network, said Drew Gonewold, a hydrologist with the National Oceanic and Atmospheric Administration's Great Lakes Environmental Research Laboratory. It provides data for six-month lake level forecasts produced by the U.S. Army Corps of Engineers.
"In order to understand what's driving water levels and correctly respond when water levels get low, we need to know really well the relative impact of precipitation and evaporation," Gronewold said.
For the Wisconsin study, Watras and colleagues used data from inland lakes as well as the Great Lakes to link water level changes to atmospheric patterns similar to the jet stream that originate over the Pacific. Those air flows regulate the influx of moist air from the Gulf of Mexico into the Great Lakes region, influencing rainfall during warm-weather months that can replenish water lost to evaporation.
The study showed that shifts in the air patterns correlated with ups and downs in the Great Lakes that happened over roughly decade-long periods until the most recent drop-off that began in 1998 and may only now be ending, Watras said.
It's too soon to say whether the roughly 10-year pattern has ended, or whether the recent low-water period will prove to be a "hiccup" that temporarily interrupted it, he said. But climate change may be ushering in a time of lengthier and more pronounced rises and falls.
"Something's happening now that has not happened in the past, as far as we know," he said.
Follow John Flesher on Twitter at https://twitter.com/JohnFlesher