Project Yields by a Probability Method

The conventional method of estimating future yields from a reservoir is based on the assumption that the historical sequence of runoff will be repeated. Because such a recurrence is improbable, the better approach is to use some statistical method based on the laws of probability, with modifications for known tendencies of hydrologic phenomena. One simplified method involves (1) the selection of a period and runoff record for use in the study, (2) the use of electronic computers to establish hypothetical runoff sequences and compute-and-plot accumulated deviations from the mean, (3) graphical analyses of the sequences for safe yields, and (4) statistical treatment of the yield values to determine the probabilities of occurrence. This method applied to Glen Canyon Reservoir resulted in a 50% probability that its safe yield, during a 50-yr repayment period, would be at least 7.8 million acre-ft per yr if the reservoir were at minimum power pool at the beginning; it would be at least 8.4 million acre-ft per yr if the reservoir were full at the beginning of repayment.