Great Lakes Precipitation Masks Evaporation

Recent water-level declines in the Lake Michigan–Huron system correlate with an increase in evaporation over the past century and a half.

The extent of falling water levels on North America's Great Lakes may have been hidden by higher precipitation in the past few decades, which could have obscured higher evaporation rates in the region. photo: NASA

The water levels of Lakes Michigan and Huron may be dropping faster than anyone realized. Higher precipitation in the past few decades may have masked the rapidly falling lake levels.

Lakes Michigan and Huron are often considered to be one single system since they are interconnected at the Straits of Mackinac. Dredging of the St. Clair River where the lakes drain into Erie, the next lake in the chain, has been considered as a possible reason for the lowering water levels.

However, the Great Lakes Environmental Research Laboratory (GLERL) and Duke University have been examining data collected from 1860 to 2006. They have found a slight correlation between precipitation and the 11-year sunspot cycle.

They did find that the increased precipitation since 1970 is masking the expected drop due to evaporation. "Evaporation made a dramatic switch in 1977 from less than –1 millimeter per year (mm/yr) to almost 4 mm/yr, according to the team's modeling. 'We see some patterns but not enough [to be] consistent with climate change,'" Cynthia Sellinger of GLERL says, referring to both increased evaporation and decreased precipitation.

Other researches claim that the model used is too simple. The independent U.S.–Canada International Joint Commission (IJC), which advises on policy and management for the Great Lakes, is examining possible reasons behind the lower Michigan–Huron lake levels. IJC expects to report results by spring 2009.

Frank Quinn, formerly a GLERL hydrologist and a current participant in the IJC study says, "my contention is that the current low levels are due to changes in water supply," mainly between the upper lakes and Lake Erie. He also concurs that the relatively simple data analysis in the new research does not capture long-term natural hydrological cycles of the lakes, including glacial rebound. Other human impacts include not only dredging, but such things as gravel mining.

read the full story at Environmental Science and Technology, "Great Lakes evaporate away", Nov 21, 2007
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