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<br />Ou1214, <br /> <br />VII.? <br /> <br />Outflow <br /> <br />Records of reservoir outflow are derived in many ways. Those <br />derived from a standard measuring device in the channel below and <br />separate from the dam--such as a flume, weir, or rated gaging section-~ <br />are to be preferred because they are subject to fewer undetected errors, <br />Outflow records based solely on calibration of the various passages <br />through or over the dam--conduits, valves, turbine draft tubes, and <br />spillway--are subject to both random Cind systematic errors. Major <br />obstruction of outflow structures by floating drift usually is noted <br />in the operator's log so that the outflow record can be adjusted <br />accordingly; transient or partial obstruction may not be detected. <br />Owing to wear, backlash in gate operating machinery may increase, <br />with resulting uncertainty as to actual gage opening, Wear of valve <br />seats, especially of small valves, may not be detected. Manufacturer's <br />coefficients for hydro-electric turbines and their draft tubes may not <br />be as accurate as is desirable for an outflow record. All such means <br />for measuring outflow at the dam should be re-calibrated frequently, to <br />minimize systematic errors. <br /> <br />Owing to their hydraulic characteristics, and because commonly they <br />function infrequently, spillways are difficult to calibrate in place. <br />Thus, records for this part of the outflow may be derived only from hydraulic <br />characteristics of the spillway as designed. Even so, such outflow records <br />may be quite as accurate as those from a downstream measuring weir when that <br />weir is "drowned," or nearly so, For outflow over an ungated spillway remote <br />from the dam, the only records available may be those estimated from the <br />designer's hydraulic characteristics. Such records are of course acceptable <br />if tempered by knowledge of their inherent potential errors. <br /> <br />Loss by evaporation <br /> <br />Although another chapter of' this handbook deals specifically with <br />limitations in records of evaporation, it is appropriate at this point <br />to cover some aspects that apply to reservoirs and lakes. <br /> <br />Loss from a reservoir by evaporation commonly is derived from the <br />record of an adjacent land pan by applying a suitable coefficient; generally <br />this procedure is sufficiently reliable for yearly values. However, for <br />periods of a month or less, the ratio between pan evaporation and reservoir <br />evaporation may vary considerCibly, because water temperature in the reservoir <br />may lag seasonally behind ambient air temperature. <br /> <br />