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13 <br />facility. Reservoir storage and releases from a dam are related to <br />each other, as shown in the following equation (see also Fig. 1-2) : <br />y <br />AS = Qi - Q9 - Qn - W - E, (1-1) <br />where AS = change in storage volume of the reservoir over a fixed <br />time interval, <br />Qi = reservoir inflow, over the same interval, <br />Qg = generating releases passing through the turbines, <br />Qn = non-generating releases (e.g., spillage, leakage, or <br />instream releases), <br />W = consumptive withdrawal from the reservoir, and <br />E = net evaporative losses from the reservoir. <br />As described previously, store-and-release facilities rely on <br />+ available storage to operate in a peaking mode (OS ? 0; Eq. 1-1). <br />Hydroelectric projects can also be operated in a run-of-river mode. <br />As defined in this report, run-of-river facilities are those projects <br />that are operated with no change in reservoir storage (AS = 0; Eq. <br />1-1). In reality, a gradation of project types exists in which the <br />amount of peaking or seasonal storage is constrained by the acceptable <br />water elevation changes within the reservoir (Hildebrand 1980, Szluha <br />et al. 1981). <br />Another important aspect related to the design of hydroelectric <br />projects is the location of the powerhouse. In the simplest case, the <br />powerhouse is located at the base of the dam, and the distance between <br />the dam and the confluence of the tailrace with the river is minimal <br />(L = 0; Fig. 1-2). Many small hydropower projects, however, <br />especially those in New England (U.S. Department of Energy 1981), <br />utilize long penstocks or canals to gain additional head. For <br />example, about one-half of all the New England sites under review by <br />FERC since 1979 would require diversions greater than 150 m (Knapp