below the mean is 8,000 acre-feet per year (AFY). Flow available for human use will be lower than the lower mark (8,000 AFY) in only 1 out of every 40 years over a long period of time.

Now let us consider demand. The demand numbers in our illustration are conveniently chosen to match some of the numbers in the description of supply, above. Any other numbers could be assumed, but they would make the illustration harder to follow. Assume that current drought-year demand (labeled DE in Figure D-1)2 is at the lower tick mark. Then the community served by this water system will experience a water shortage only 1 year out of 40. As defined above, this is a reliability level of 97.5 percent.

Suppose drought-year demand is projected to grow by 2,000 AF over the next decade.3 As drought-year demand grows, reliability will decrease in the sense that the likelihood of a water shortage will increase from 1 in 40 to 1 in 2. That is, the reliability level would fall from

FIGURE D-1. Normal distribution of annual hydrologic flows. SOURCE: Cooley et al. (2006).


We define drought-year demand as the demand that would exist when flow is at a point chosen by the planner on the horizontal axis of Figure D-1—in this case, demand when flow is at the lower tick mark. Note that drought-year demand will often be higher than average-year demand because outdoor water use will increase when rainfall is below average or temperature is above average.


A water demand projection is based on many factors, such as projected growth in popuzlation and employment in the service area.

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