Modes of Variability in the Great Lakes Water Balance

The Laurentian Great Lakes support hundreds of species of fish, birds, and aquatic plants as well as economic activity for millions of people living in the basin. However, the ecological, economic, and recreational value of the lakes is highly dependent on water level. A rapidly changing climate makes predicting trends in the main level-independent components of the water balance both more important and more difficult than ever. The decade from 2010 to 2020 saw both record lows and highs in Great Lakes water levels, defying the supposition that a warmer climate would lead to strictly decreasing lake volume. The reality is more complicated; not only are water levels constantly changing, but the manner in which they fluctuate is shifting as well.

Increasing variability indicates the water level or component being measured is exhibiting greater and more rapid shifts between highs and lows. Here, we evaluate historical precipitation, evaporation, and runoff for each of the Great Lakes, with an emphasis on understanding variability over different time horizons. We find that modes of variability have changed, particularly for evaporation, suggesting it as a major driver in the recent fluctuations between extreme high and low water levels. Evaporation variability on a twenty year time scale has increased by an average of 14% since 1989, while precipitation variability has increased by an average of 19%. The cumulative effect of increasingly variable components is manifested as an average increase of 26% in water level variability for this period. However, change in water level variability is not uniform across the lakes. Over the past 70 years, level variability on Superior has increased by 165% while Ontario has experienced a 70% decrease, likely due to the control of the Moses-Saunders Dam.

Recognizing the effects of control structures on level variability is necessary to craft more informed regulatory policies that consider the impacts of variability on both shoreline ecosystems and coastal communities. Moreover, understanding level variability is crucial to building a more complete picture of the impacts of climate change on the Great Lakes region, as well as for other freshwater systems as scientists continue accumulating evidence of an increasing trend in lake surface water temperatures around the world.