Study corrects misconceptions of Great Lakes evaporation
The recent Arctic blast that gripped much of the nation will likely contribute to a healthy rise in Great Lakes water levels in 2014. 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 by the Great Lakes Integrated Sciences and Assessments Center – 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.
"Understanding how lake levels are changing is very important to our region," said Thomas Dietz, MSU professor of environmental science and policy and co-director of GLISA. "This affects shipping, recreation and infrastructure on the lake shore."
The new-found 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.
"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."
By piecing together the results from several studies, Lenters and his colleagues showed that years with high Great Lakes ice cover require a large amount of heat loss from the lakes in the preceding autumn and early winter to cool the water enough to form ice.
So what does all this mean for Great Lakes water levels in 2014?
Although the recent cold spell has led to high evaporation rates this winter, the extensive ice cover is likely to stick around longer into the spring than is typical. That may lead to cooler summer water temperatures and a later start to the 2014 Great Lakes evaporation season.
"A new understanding of the impacts of climate variability on Great Lakes evaporation is emerging as a result of this GLISA-funded project," said Don Scavia, director of U-M’s Graham Sustainability Institute and GLISA co-director. "In light of these new findings, continued long-term monitoring of Great Lakes evaporation and related hydrological processes is paramount for understanding and predicting the future impacts of climate variability and change on Great Lakes water levels.”