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Low Water Levels at Old Mission Point Lighthouse, Lake Michigan, July, 2000.
Lakes Michigan and Huron fell to record low levels, the Army Corps of Engineers announced today. Despite recent rains, water levels continue to fall. High air temperatures, as much as 20 degrees above normal, increased evaporation rates far above normal, offsetting the positive effects of the beneficial rains. Warmer than normal water covered by less ice than normal also contributed to the high evaporation rates. Moreover, soils parched by months of drought sucked up much of the rain that fell on land before it could flow into the lakes.
Jeff Masters explains the story of the high evaporation rates for the middle of winter.
The record low water levels on Lake Michigan and Lake Huron in January occurred despite the fact that precipitation over their watershed was 61% above average during the month. However, precipitation over the past 12 months was only 91% of average, and runoff into the lakes depends upon precipitation over longer than a 1-month period. Furthermore, evaporation over these lakes was much higher than average during January, making the net water supplied to the lakes (runoff into the lakes, plus precipitation over the lakes, minus evaporation from the lakes) only 63% of average. What caused the increased evaporation? Well, very warm water temperatures, for one. According to NOAA's Great Lakes Environmental Research Laboratory (GLERL), water temperatures over Lake Michigan are currently about 2.5°F above average, and are about 1°F above average over Lake Huron. These warm water temperatures are the lingering effects of the extraordinary warmth of 2012, which was the warmest year on record over much of the Great Lakes region. Also increasing the evaporation from the lakes during January was the presence of less ice cover than average, which exposed more open water to the air.
He also explained that the lakes were lowered 10 to 16 inches fifty years ago by dredging of the St. Clair river, the outlet of Lakes Huron and Michigan. Today's water levels cannot be compared to water levels taken before the dredging.
Predicting future lake levels is challenging because rainfall has been increasing for the past 100 years while temperatures have been rising. Climate models predict these trends to continue. Will increasing evaporation outpace the effects of more rainfall? That's what appears to be happening now. The deep Great Lakes have all been warming rapidly.
Winter air temperatures over the lower Great Lakes increased by about 2.7°F (1.5°C) from 1973 - 2010, and by 4 - 5°F (2.3 - 2.7°C) over the northern Lakes, including Lake Superior. Lake Superior's summer surface water temperature warmed 4.5°F (2.5°C) over the period 1979 - 2006 (Austin and Colman 2007). During the same period, Lake Michigan warmed by about 3.3°F (1.7°C), Lake Huron by 4.3°F (2.4°C),
Increasing evaporation rates tied to warming water and air temperatures are lowering lake levels.