Tuolumne River, CA
Project: Turlock Irrigation District
Reservoir reliability is the assurance that a reservoir will meet normal water demands at a selected level of risk. Firm (or safe) yield is an outflow rate that can be continuously provided by a reservoir over a historical period of record or may be the yield that can be provided in all but one or two years over a historical period of record.
Reservoirs, The Sequel outlines firm yield analysis.
Firm or safe yield is calculated as a reservoir is designed. Reliability is generally not calculated as a reservoir is designed.
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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 fails to meet demand, some deficient reservoir levels may be present, such as the volume of the reservoir in the 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 water years 1945 and 1988.
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. The weather year 1993, plotted in red, is one of the weather years that will cause a failure to meet outflow demand.
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 be necessary for 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 the 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 of 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.