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<br />onZ3S <br /> <br />VII.l <br /> <br />LIMITATIONS IN HYDROLOGIC DATA AS APPLIED TO <br /> <br />STUDIES .oF WATER CONTROL AND WATER MANAGEMENT: <br />PART 2 <br /> <br />SOURCES OF ERROR IN HYDROLOGIC RECORDS.-Continued <br /> <br />VII. RESERVOIRS AND LAKES <br /> <br />Some general oonsiderations <br /> <br />For regulating the flow of a stream and so making that stream <br />more usable, man oommonly resorts to storage in a reservoir. or, by <br />suitable works, manipulates the storage in a natural lake. Hence, <br />a majority of river-management studies require that the hydrologio <br />effects of suoh storage be appraised. The basic prinoiple is silllple: <br />inflow equals the ooncurrent algebraio sum of outflow and change in <br />storage. With this statement of principle, however, si.n1plioity ends. <br /> <br />A oomprehensive'reservoir budget must oonsider items of inflow <br />and outflow other than flow in the river ohannel above and below the <br />reservoir. For example, the reservoir oreates a new, ..and possiblY a <br />major item of outflow--evaporation from its water surfaoe. But this <br />is offset in part by evapotranspiration, from land and native vegetation, <br />that has been extinguished within the area that the reservoir inundates. <br />The net effect may be diffioul t to esti.n1ate aocurately, in patt because <br />"before and after" regimens of .air temperature and humidity may differ, <br />perhaps substantially. In the case of a regulated natural lake, this <br />nel( item of outflow commonly is negligibly small, unless the regulation <br />substantially enlarges mean water_surface area. <br /> <br />Another somewhat intangible item involves ohange in configuration <br />of the water table, also in direction and amount of ground...water movement.. <br />In the usUlll situation,. the reservoir is on a reach of strellll1 that . <br />naturally gained in flow (effluent ground water). Owing tQ the reservoir, <br />the water table would rise beneath and adjllCent to the area inundated,. <br />ground_water gradient would flatten, and effluent seepage into the <br />reservoir would diminish. Immediately downstream from the dam,' however, <br />ground-water gradient probably WOuld steepen and effluent seepage wOUld <br />increase. Usually the two effects would be greatest early in the life <br />of the reservoir and would tend to d:l1ninish after reservoir and stream <br />have come to hydraulic equilibrium. The two have the same algebraic <br />sign in a water budget for the reservoir; if they should be overlooked, <br />the indicated water supply would be too large. <br /> <br />