Great Lakes evaporation study dispels misconceptions, points to need for expanded monitoring program

University of Michigan News: The recent Arctic blast that gripped much of the nation will likely contribute to a healthy rise in Great Lakes water levels in 2014, new research shows. But the processes responsible for that welcome outcome are not as simple and straightforward as you might think.

Yes, extreme winter cold increases ice cover on the Great Lakes, which in turn reduces evaporation by preventing water vapor from escaping into the air. But this simplistic view of winter ice as a mere "cap" on Great Lakes evaporation is giving way to a more nuanced conception, one that considers the complex interplay among evaporation, ice cover and water temperature at different times of year.

In a report released today by the Great Lakes Integrated Sciences and Assessments Center (GLISA)—a federally funded collaboration between the University of Michigan and Michigan State University — a team of American and Canadian scientists notes that while ice cover affects evaporation, the reverse is true as well: evaporation rates in the autumn help determine the extent of winter ice cover.

High evaporation rates in the fall can nearly offset water-level gains that result from extensive winter ice cover, complicating efforts to forecast Great Lakes water levels, which have declined in most of the lakes since the late 1990s, rebounding somewhat during a wet 2013.

The newfound appreciation for evaporation's varied roles reveals gaps in our current understanding of fundamental environmental processes and highlights the need for sustained funding for the project's Great Lakes evaporation monitoring network, said John Lenters, the study's lead investigator and a senior scientist at Ann Arbor-based LimnoTech, an environmental consulting firm.

The binational group's network of five stations is one of the few sources of direct, year-round observations of Great Lakes evaporation. "It's our hope that we will soon have the funding and infrastructure in place to maintain—and even expand—the network well into the future," Lenters said. "This will be extremely important for improving Great Lakes water-level forecasting and for understanding the long-term impacts of climate change."...

A weather station on Granite Island, Lake Superior. This weather station is part of a five-site network to measure year-round Great Lakes evaporation. In addition to evaporation, the weather station measures air temperature, relative humidity, barometric pressure, carbon dioxide, wind speed and direction, precipitation, solar radiation, and water temperature. Image credit: John Lenters