Reservoir Reliability and Firm Yield
How are reliability and firm or safe yield related? To illustrate these terms, reliability is defined as the assurance that a reservoir will meet normal water demands at a selected level of risk. Firm or safe yield is defined as an outflow rate that can be continuously provided by a reservoir over a historic period of record, or is sometimes defined as the yield that can be provided in all but one or two years over a historic period of record. Firm yield analysis is outlined in Reservoirs, The Sequel.
Firm or safe yield is calculated as a reservoir is designed. Reliability is generally not calculated as a reservoir is designed.
Figure 1 is a plot of reservoir storage volume for a reservoir over the period 1928 to 1998 with uniform water deliveries. The reservoir never ‘fails’.
Although the reservoir never fail to meet demand, there are some very low reservoir levels, e.g. the volume in the reservoir in water year 1941 is low because the reservoir failed to fill. Rainfall in September that year prevented failure to meet demands. The reservoir reached very low reservoir levels in w. y. 1945 and in 1988 as well.
Figure 2 shows the reservoir volumes that would be expected to occur over the next seven months with the watershed conditions and reservoir storage as they were on July 1, 1941. Weather year 1993, plotted in red, is one of the weather years that will cause a failure to meet outflow demand.
Figure 2 Reservoir volumes in weather year 1993 are plotted in red
Figure 3 plots the data as in Figure 2 as exceedance probabilities. The risk that the reservoir will reach zero storage is 17 percent. If the reservoir meets essential M&I or irrigation demands will would be necessary in July to reduce water deliveries and conserve water.
Reservoir reliability vs. firm or safe yield is confusing to non-specialists and many water users. Why would a reservoir that never fails to meet demands historically need to restrict water deliveries? This depends on assessment of reliability or risk – firm or safe yield, despite these names, says nothing about risk in any one year. The rainfall in September 1941 that prevented reservoir failure that year was highly unlikely – but a firm or safe yield calculation is unaware to the risk of continuing normal water deliveries.
The number of times water delivery restrictions will occur historically depends on the reliability that the reservoir operators wish to maintain, or level of risk that they select.