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A number of correlations of precipitation and the levels of Lakes Michigan and Huron indicated a computable relationship and also an apparent lag between precipitation and its effect. When land-area precipitation only was considered, a correlation of lake level with four-year precipitation data gave a correlation coefficient of 0.69. A correlation of precipitation and the difference in annual mean levels indicated a correlation coefficient of 0.68.

In order to include the effect of additional variables, a correlation of water-year precipitation on a two-year basis and net basin supply was made. A correlation coefficient of 0.79 was obtained, and a lag between precipitation and its effect was again indicated. Monthly precipitation values were correlated with seasonal changes in level, and correlation coefficients of 0.85 and 0.81 were obtained with antecedent precipitation and the seasonal rise and antecedent precipitation and the seasonal fall in levels, respectively. Finally, it is pointed out that geological conditions in the Michigan-Huron basin provide a physical reason for the indicated lag between precipitation and its effect.