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Does the plan balance levels of Superior and Michigan-Huron?

This is the second of nine Study Board decision criteria used in the evaluation of Lake Superior regulation plans. The notion of balancing lake levels was provided as an objective in the Supplementary Orders of Approval for Lake Superior regulation; a balancing formula is included in the current plan, Plan 1977A. In this plan, Lake Superior levels are balanced with Lake Michigan-Huron levels when each are an equal number of standard deviations above or an equal number below the long-term monthly mean for each. In Plan 1977A, there are fixed values for 12 monthly means and 12 monthly standard deviations for each lake based on the simulated lake levels expected with the 1955 Modification of the Rule of 1949. Over time, there will be an equal number of Lake Superior levels within one standard deviation of the mean as there are Lake Michigan-Huron levels within one standard deviation of its mean. But when the distance from the mean is tested in any one month, the lakes are almost unavoidably out of balance as recognized by the existing Orders, which attempt to address this issue. For example, if Lake Superior is currently near average levels and Lake Michigan-Huron is well above average, then the existing Orders encourage a smaller release than would otherwise be made. This would raise levels on Lake Superior and lower them on Lake Michigan-Huron, bringing them closer to balance in the coming months. However, because of the huge volumes in both lakes, relative to the flow between the lakes, it would take many months to adjust the lake levels so that they were above or below their average levels by a proportional amount. This Study Board criteria was applied in the SVM to compare two candidate plans. The SVM provided a pass/fail grade for this criterion. A passing grade means that the sub-criteria for assessing the degree to which the two lakes are balanced, for a particular plan and NBS, met thresholds similar to scores under the 1977A plan. The first sub-metric was the range of imbalance, a measure of magnitude – how far out of balance the lakes typically were. The other sub-metric is a frequency bias, which is a measure of which of lakes Superior or Michigan-Huron is more likely to be more out of balance than the other.
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