Long‐Range Reservoir Outflow Temperature Planning

A method for long‐range reservoir operation planning to meet an outflow temperature goal is described. The outflow temperature of TVA's Norris Reservoir is characteristic of the seasonally modified temperature regime produced by deep and large storage reservoirs. Outflow temperatures stay around 10° C from the beginning of the year until about mid‐July. Then they rise to their annual peak of about 18° to 22° C by the end of October and decline to about 10° C by the end of the year. For downstream water uses it may be beneficial to keep the maximum outflow temperature in fall at or below a target temperature. This goal can be accomplished by limiting the total amount of discharge after the spring filling of the reservoir. The limit on discharge is expressed by the cold water index, which is the ratio of total discharge during the planning period to the reservoir volume at the start of the period. The practical use of the concept is illustrated by modifying historic operations which did not meet a prescribed outflow temperature goal. The effect of such operational modifications on other reservoir uses, especially on power generation, is also described.