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<br />Water Resources; or by measuring it using a USGS <br />topographic map (readily obtainable from many com- <br />mercial cources). Of course, ideally, there is a USGS <br />gaging station located on your stream from which you <br />can get this flow data directly. <br /> <br />Once you have the head and average annual flow for <br />your site, it's hydropower potential can then be calcu- <br />lated using the following formula: <br /> <br />kW = QHE <br />11.8 <br /> <br />where: <br />Q= <br />H= <br />E= <br /> <br />1.0 to 1.5 times average annual flow <br />available head in feet <br />overall efficiency = 0.8 (A reasonable <br />first approximation <br />installed capacity in kilowatts <br /> <br />kW = <br /> <br />The average annual energy produced can be estimated <br />as follows: <br /> <br />kWh = 8760 QHE pf <br />11.8 <br /> <br />where: <br />kWh = average annual generation in <br />kilowatt-hours, with Q, H, and E de- <br />fined as before <br />pf = annual plant factor <br /> <br />Estimating annual plant factor is a complex process, <br />as it depends on the size of turbine-generator unit(s) <br />installed, on the statistical variation of streamflow at <br />the site throughout the year, and on the regulatory <br />effects of upstream reservoirs, if any. If an existing <br />dam or reservoir is involved, the decreed use of the <br />stored water must be determined. The historic release <br /> <br />22 <br />