Causes of variability in monthly Great Lakes water supplies and lake levels

The purpose of this study was to identify those water budget components of the Great Lakes that have most frequently been a major cause of anomalous net basin supplies (NBS) and of rising and falling lake levels at the monthly time scale. Principal component analysis and a simple counting of relative frequencies revealed that on the upper lakes NBS anomalies are most sensitive to over-lake precipitation, but on the lower lakes they are most sensitive to runoff. This shift is due to a downstream increase in the magnitude and variability of runoff. Evaporation variability plays a larger role in the NBS of the upper than the lower lakes and is most important during dry months. During wet months evaporation is not as much suppressed as one might assume from the simple cloud cover/insolation/temperature/evaporation relationship; this is most likely due to an increase in wind speed. High and rising as well as low and falling lake levels are the result of anomalous NBS on all lakes and represent condition beyond the capabilities of lake-level regulations. Changing conditions-low but rising levels or high but falling levels-are the result of anomalous NBS for all of the lakes except Ontario I for which almost all such changes are achieved by regulating the outflow.