Understanding the impacts of historic climate variability and climate change on lakes in the Great Lakes region

Lakes and wetlands are prevalent in the Great Lakes Region of the United States. They play an important role in regional and local hydroclimatology due to the large differences in albedo, heat capacity, roughness and energy exchange in comparison to that of the land surface. Lakes and wetlands affect the regional water cycle by providing additional storage for surface runoff within a watershed. Water leaves the watershed more slowly, which leads to an increase in evaporation and baseflow. Lakes influence the energy cycle by changing the partitioning of energy cycle components (i.e. latent heat, sensible heat, and net radiation) due to differences in albedo, heat capacity, and roughness with vegetative surface. Climate variability and climate change affect the regional scale water and energy cycle by modifying climatic variables such as precipitation, air temperature, solar radiation, and wind speed. These variables influence the seasonal dynamics of lakes including heat storage during the ice-free season and ice cover in the winter. Changes in lake ice and heat storage in turn feedback to local and regional climate making them a potentially important part of the climate system in regions with lots of small lakes (i.e. Great Lakes Region). The overarching goal of this study is to understand the impacts of historic climate variability and climate change on lakes and wetlands in the Great Lakes Region of the United States. To study hydroclimatic impacts, an integrated approach combining in-situ and remotely sensed observations with land surface modeling is adopted. The Variable infiltration capacity (VIC) model with a physically based lake algorithm was used to study the long-term (1916-2007) impacts of historic climate variability and climate change. The VIC model was calibrated and evaluated against daily streamflow, energy fluxes, inundation area, lake water temperature, and lake ice freeze-up and break-up dates. A method for developing depth-area relationships for the lakes in the study domain is derived based on observed bathymetric data for lakes in Michigan and applied to lakes in Minnesota and Wisconsin. Results suggest that the presence of lakes and wetlands considerably affect the regional water and energy balance in the three states. Domain average annual evapotranspiration increases by 5% while domain average annual total runoff is reduced by 11.7% when lakes and wetlands are included into model simulations. The domain average latent heat is increased by 6% while the domain averaged sensible heat is reduced by 5%. The presence of lakes and wetlands reduces the regional average Bowen ratio by 13%. Analysis of historic climatic extremes suggests that wetter and cooler cold season conditions are the most favorable for the recharge of lakes and wetlands during the spring resulting in large degrees of inundation. The inundation of lakes and wetlands during spring is significantly (correlation = 0.64) related to spring snowmelt suggesting that reduced snow storage and melt lead to lower inundation of lakes and wetlands. The onset of the lake stratified season is significantly (correlation = -0.65) related to cold season air temperature, while the length of the stratified season is significantly related to the earlier onset of the stratified season (correlation = -0.80) and the heat storage in lakes (correlation = -0.30). Longer ice-free seasons result in later ice freeze-up in lakes due to increased heat storage. The timing of lake ice freeze-up significantly (correlation = -0.50) affects the cumulative ice thickness of lakes. Larger cumulative ice thickness leads to later onset of the stratified season and a reduction of thermal capacity during the stratified season. These results explained the feedback of lake ice cover on the interseasonal dynamics of lakes. Results also suggested that ice freeze-up has shifted later while ice break-up has shifted earlier in lakes during the period of 1916-2007. Earlier ice break-up and later ice freeze-up in lakes resulted in longer ice-free seasons which were influenced by increase in air temperature during the cold season and large scale climatic oscillations.