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
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
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
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.