“If you want to know something … go measure it.” That’s the advice I was once given by a colleague. That advice, although often not as straightforward as it sounds, has proven to be valuable in helping understand how important evaporation is from the Great Lakes. Surprising as it may sound, year-long, continuous direct measurements of evaporation from the Great Lakes were not made until a project funded by the federal governments of Canada and the United States through the IJC made it possible.
Maintaining the meteorological instruments at Spectacle Reef Lighthouse in Lake Huron, September 2013. Credit: Pakorn Petchprayoon.
Part of the mandate of the International Upper Great Lakes Study was to help understand the role that evaporation plays in lake water levels. Mathematical models had and are still being used to estimate evaporation. But the accuracy of those models could not be properly judged without direct measurements to refine and improve those models.
So, in June 2008, Stannard Rock Lighthouse (far offshore in Lake Superior) was the first of five sites instrumented with state-of-the-art meteorological equipment that directly measures evaporation and several other variables.
The same equipment was later installed at Granite Island, closer to shore on Lake Superior. With continued IJC support, sites on Lake Huron (Spectacle Reef Lighthouse), Lake Michigan (White Shoal Lighthouse), and Lake Erie (Long Point Lighthouse) have since been fitted with the same meteorological instruments.
The location and tall, stable structures of these lighthouses makes them an ideal venue for this mission. In addition to their continued use as navigation aides, they now serve as important research platforms. Our access and use of these facilities has been made possible by the continued support of the U.S. Coast Guard, Michigan’s State Historic Preservation Office, the Great Lakes Lighthouse Keepers Association, and several dedicated volunteers.
A map showing the locations of the current evaporation observation network.
This network of five offshore measurement locations operate simultaneously year-round, even through bitter winters, and have provided some fascinating results.
For example, nearly all of the evaporation occurs during the winter months. During the summer, energy from the sun is used to heat the water (which is colder than the air at that time), and this energy is released six months later in the winter (when the water is warmer than the air). The lakes provide the water and heat source that fuels lake-effect snow.
This year, during long and cold outbreaks of polar air (the “Polar Vortex”), northern Lake Michigan has been ice-covered at the same time that northern Lake Huron was ice-free. Our measurements show very high evaporation and heat loss from ice-free Lake Huron. At the same time, the mostly ice-covered Lake Michigan has had nearly no evaporation or heat loss. These measurements are helping to untangle the complex relationships between ice cover and evaporation from the Great Lakes.
Spectacle Reef Lighthouse in northern Lake Huron surrounded by open water, January 2014. Credit: Dick Moehl.
This network of sites continues to improve our understanding of lake evaporation and the factors that control it. These observations have helped calibrate several remote-sensing observations to help us better understand spatial variability, and mathematical models that help us make to better predictions of changes in water levels through changes in evaporation.
Peter Blanken is with the Department of Geography at the University of Colorado.