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<br />0'01 ~n- <br />\ l.l;J1. <br /> <br />and Mead in advance. Any surplus releases that occur are due to monthly <br />reservoir operations like flood control, or the contents of Lakes Powell <br />or Mead exceeding their maximum capacities. <br /> <br />If a surplus strategy is used, the annual surplus releases from Lakes <br />Powell and Mead and most of the distribution of these releases are com- <br />puted at the beginning of each water year (October). <br /> <br />The annual surplus release is divided into three categories. Category I <br />is surplus water for CAP and MWD and consists of as much surplus as is <br />available up to their maximum capacities. Category II is surplus water <br />for Mexico and consists of any remaining surplus up to 200,000 acre- <br />feet. Category III is surplus used for power generation operational <br />flexibility and consists of any remaining surplus. Category III surplus <br />water flows to Mexico. <br /> <br />Category I annual surplus is distributed to CAP and MWD on a monthly <br />basis, proportioned to each aqueduct by the ratio of the monthly capa- <br />city available in the aqueduct to the annual capacity available in the <br />aqueduct under a normal demand schedule. Category II annual surplus to <br />Mexico is distributed on a monthly basis by proportioning the annual <br />amount according to the normal Mexican demand schedule. Category III <br />annual surplus to Mexico (for power generation operational flexibility) <br />in months October through March is distributed according to a generally <br />observed historical Mead release pattern. In months April through <br />September, no Category III surplus releases are scheduled. <br /> <br />Categories I and II surplus releases are committed to be delivered <br />throughout the water year regardless of the actual runoff. Half of <br />Category III and Lake Powell surplus releases are committed to be deliv- <br />ered throughout the first 6 months of the water year. The remaining <br />half of Category III and Lake Powell surplus releases are not committed <br />in order to provide some operational flexibility in case the actual <br />runoff is not as great as the inflow used in the surplus release com- <br />putations. All surplus releases and deliveries that have been committed <br />by the surplus strategy are added into the normal releases and deliv- <br />eries stored in the model and are thereby processed as the normal <br />releases and deliveries are processed. <br /> <br />Equations used to determine annual surplus releases are mass balance <br />equations that take the predicted change in reservoir storage from the <br />beginning to end of the water year, add assumed inflows, and subtract <br />uses or minimum required releases. For computing the surplus release <br />from Mead, the inflow is the natural inflow that would occur at the nor- <br />mal probability level equal to the assurance level specified for the <br />surplus strategy. For computing the surplus release from Powell, the <br />inflow is the average natural inflow. Powell storage is checked against <br />602(a) required storage using a 90 percentile normal probability natural <br />inflow. (This value is typically used although a different percentile <br />could be specified in the input data.) <br /> <br />A detailed description of the surplus strate9Y is given in the Bureau of <br />Reclamation report "Colorado River Alternative Operating Strategies for <br />Distributing Surplus Water and Avoiding Spills." <br /> <br />41 <br />