Calculated Risks of Impounding Reservoir Yield

A system of droughts risks is developed which governs the yield of an impounding reservoir. By using a frequency curve of annual events the probability that an event (drought in this case) of selected magnitude will not occur in any one year can be determined. The corresponding probability that this event will not occur during any design period of n years is the nth power of the one-year probability. This is based on the law of multiplication of probabilities. By considering various magnitudes and various design periods a system of governing probabilities or risks is calculated. This system of risks allows the engineer to exercise his judgment as to the severity of these drought risks as they might affect municipal pumpage or yield.