Lake level response to seasonal climatic variability in the Lake Michigan-Huron system from 1920 to 1995

Recent research indicates a shift in the timing and range of the seasonal lake-level cycle of Lake Michigan-Huron since 1860. The largest changes occur during the winter-spring transition. The objectives of this study are to (1) quantitatively assess seasonal variations in precipitation, runoff, and evaporation, (2) evaluate long-term trends in seasonal water supply to Lake Michigan-Huron, and (3) understand how variations in net basin supply at a seasonal timescale contributed to development of extreme lake levels recorded in Lake Michigan-Huron between 1920 and 1995. Total water input, which is the sum of overlake precipitation and runoff, has increased during autumn and winter seasons at rates of 14.6 and 9.3 mm/decade, respectively. A rise in autumn total input reflects an increase in overlake precipitation (6.3 mm/decade) and runoff (8.3 mm/decade). The increase in winter input was driven by runoff (7.5 mm/decade) at the expense of decreasing spring runoff (7.8 mm/decade). This shift to an increasing dominance of winter runoff at the apparent expense of spring runoff is most pronounced post 1965 and suggests a hydrologic response to a warming climate. Extreme lake levels principally reflect either anomalies occurring in one or more seasons or a directional shift over many years, but often with seasonal bias. Seasonal changes in overlake precipitation, runoff, and overlake evaporation lead to the formation and persistence of extreme lake levels. The effects of seasonal water supply anomalies depend on the coincidence to long-term decadal-scale variation in lake level.