Trends in the Lake Superior Water Budget Since 1948: A Weakening Seasonal Cycle

Abstract

An analysis of Lake Superior water levels from 1948–1999 reveals that the seasonal cycle has decreased in amplitude by 20% (from 40 cm to 32 cm). This change is manifested as a downward trend in summer and autumn lake levels (when levels are typically highest) accompanied by roughly no change in winter and spring lake levels (and an overall 4-cm drop in annual mean levels). The decreased rates of seasonal rise and fall in lake level over the 51-year interval reflect a large decrease in the net monthly influx of water during the late spring (up to –1,360 m 3/s) coupled with a nearly compensating increase in net influx during late autumn (up to +1,100 m 3/s). Analysis of the Lake Superior water budget indicates that these changes are primarily the result of trends in runoff and over-lake precipitation. A systematic decrease in outflow through the St. Marys River is also evident during July-December (in association with the lower lake levels), as well as a moderate shift in the seasonal pattern of lake evaporation (but not the annual mean). The observed water budget trends are primarily related to variations in climate, rather than lake regulation. Land surface effects are also important, as suggested by a 20% increase in annual mean evapotranspiration during the 51-year interval and large changes in monthly storage (e.g., snowmelt, groundwater, etc.). Significant uncertainties are present in the calculated water budget, and it is suggested that a likely source of error is in measured precipitation and (especially) runoff.