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<br />001216 <br /> <br />VII.9 <br /> <br />Even in instances such as the preceding example, however, a reliable <br />translation from pan to reservoir may still be possible if temperature'of <br />water in the pan has been observed, In the example, pan evaporation would <br />have tldminished but water temperature in the pan would have increased, <br />Lake evaporation could have been computed by the method described in the <br />previously cited Research Paper 38, which would have adjusted for transfer <br />of heat through sides and bottom of the pan. <br /> <br />Numerous evaporation records have been taken from pans floating <br />in a reservoir. From casual consideration one might infer such records <br />to be near-perfect indicators of reserVoir evaporation. However, both <br />systematic and random errors result from (1) rocking of the pan so that <br />its outside is alternately wetted and dried, with consequent transfer <br />of heat energy through the panls sides; (2) insplash and outsplash; <br />(3) difference in temperature between water in the pan and that in the <br />reservoir immediately outside, so that heat energy is transferred through <br />sides and bottom of the pan; and (4) differences between the parameters <br />of evaporaticn at the site of the pan and means of the several parameters <br />over all the reservoir. In general, records of evaporation from floating <br />pans must at this time be applied with caution. Because they are much <br />more numerous, and in spite of their serious shortcomings, records from <br />land pans remain the common basis from which reservoir evaporation is <br />derived, <br /> <br />A fully satisfactory method for measuring short-period evaporation <br />from a reservoir, rather than deriving a probable value from an adjacent <br />pan, is yet to be developed. In most situations the water-budget method, <br />which would determine evaporation as the unmeasured residual, holds little <br />promise--ordinarily, none of the budget items can be measured precisely <br />and all the net error appears in the derived value of evaporation. <br />Mass-transfer and energy-budget methods, under development in recent <br />years, are promising in principle but each determination involves extensive <br />instrumentation that may be impracticable. <br /> <br />