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<br />-43- <br /> <br /> <br />001203 <br /> <br />Usually all the surface outflow is measured, at the outlet of the <br />lake or reservoir. However, only major components of surface inflow may <br />be measured, at a gaging station or stations above the lake. Inflow from <br />the intervening ungaged area may be substantial and can only be estimated. <br />Unmeasured ground-water inflow or outflow mayor may not be significant; <br />it is diffic~lt to evaluate (Langbein, Haines, and CUller, 1951). It is <br />also difficult to determine the change in storage accurately, First, it <br />is necessary to conduct detailed, pre-reservoir surveys or subsequent <br />"hydrographic" surveys to provide reliable area and capacity curves. In <br />many instances, there is the added complication of evaluating bank' storage, <br />For example, Langbein (1954) estimated that, owing to bank storage, the <br />effective capacity of Lake Mead is 12 percent greater than that indicated <br />by the accepted area-capacity curves. It is also difficult to obtain an <br />accurate mean lake level since wind effects can so distort the lake <br />profile that a single gage reading may be significantly in error. (See <br />separate chapter, this handbook, on "Reservoirs and lakes.") <br /> <br />Another item of the water budget is the precipitation on the lake <br />surface, Ordinarily, such precipitation is measured by a s~ngle gage or <br />a network of gages along the shore. For large lakes. it is questionable <br />whether measurements at shore stations provide a true value of precipi- <br />tation over all the Lake (Kresge, Blust, and Ropes, 1963). <br /> <br />In summary, water-budget values of evaporation generally can be con- <br />sidered more reliable, percentagewise, over intervals of a month or longer <br />than over intervals of a day or less. Measurements of daily inflow and <br />outflow may embody both random and certain systematic errors that tend to <br />balance over longer periods of time (see following chapter of this <br />handbook on "Streamflow"). The effect of errors in other measured budget <br />items tends to diminish likewise. Budget items that 'are unmeasured <br />commonly can be estimated fairly closely over all of a year-long water <br />cycle, but not for periods of a month or less, The difficulty of ' obtain- <br />ing reliable daily values by the water-budget method is demonstrated by <br />the considerable effort expended in selecting Lake Hefner for critical <br />water-loss investigations (U. S, Geol. Survey, 1954). <br />