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<br />001212 <br /> <br />VII. 5 <br /> <br />The positive step that minimizes wind-tide errors is to operate <br />stage stations both up-wind and down-wind for the common directions of <br />wind movement. Even with a single station, but with fluctuations of <br />stage charted continuously, periods of wind effect commonly can be <br />identified and eliminated from consideration. Given only once- or twice- <br />a-day staff-gage readings from a single stage station, some of the erroneous <br />values may be isolated (1) if they deviate abnormally from the mean value <br />of stage after the record has been "smoothed" by moving-average technique; <br />Or (2) by comparing inflow measured at discharge stations upstream with <br />that computed from the seeming daily changes in reservoir contents. <br /> <br />Slope owing to through-flow <br /> <br />In numerous reservoirs and lakes, especially those that are shallow, <br />there is an appreciable water-surface gradient from head to outlet during <br />periods of large through-flow, as during each yearly snoWlRelt freshet. For <br />such reservoirs and periods, computations of aggregate contents or of change <br />in contents may be considerably in error if based on stage at a single <br />station and on a flat-pool capacity table. Accuracy may be improved by <br />developing a "faJllily" of capacity tables for various gradients, according <br />to observed or computed backwater profiles. <br /> <br />A striking example of such gradients is from a power reservoir in <br />Wisconsin Where, for several hours on one particular day, the station <br />near the principal outlet indicated a stage too low for flow over a remote <br />uncontrolled wasteway. Actually, however, there was an overpour of 0.7 foot. <br />Water-surface gradients existed both along and across the reservoir, due <br />jointly to through-flow and to wind tide. <br /> <br />As in the case of wind tides, errors from slope owing to through,-flow <br />are best minimized by installing additional stage stations. <br /> <br />Inflow <br /> <br />In appraising the hydrologic effects of a reservoir or a regulated <br />lake, commonly it is desirable to know the aggregate discharge of inf10wing <br />streams,. In most cases only part of this inflow is measured at stream <br />gaging stations so that COll!Inonly an effective inflow i,s computed from <br />measured outflow and concurrent change in reservoir contents, adjusted for <br />evaporation from the reservoir surface and for precipitation on that surface. <br />Generally this practice indicates somewhat too much inflow over the year <br />because it excludes two reservoir-budget items previoUSly outlined: <br />(1) evaporation usually is greater than the natural evapotranspiration that <br />it replaces; from month to month, the ratio between evaporation and evapo- <br />transpiration varies; (2) the reservoir modifies the direction of ground- <br />water movement and, in effect, commonly diverts some water from the reservoir <br />reach to the river below the dam. An additional item also should be considered <br />here: (3) Beneath the reservoir the ground is saturated perennially whereas <br />naturally the ground storage was reCharged and depleted alternately; the effect <br />of the reservoir is to shift some water from one season to another. <br /> <br />