Calibration of a theoretically derived relationship between pan evaporation and evapotranspiration Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/nk322h189

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  • Theoretical derivations to describe pan evaporation, E[subscript p], and evapotranspiration, ET[subscript r], were developed based on a flow equation expressed as the product of the driving force for vapor between the evaporative surface and a plane of reference times a conductivity coefficient of vapor through air. Under the assumptions of this study, the driving force for both vapor flows was the same. A theoretical relationship expressing the evapotranspiration as the product of pan evaporation times the ratio of their conductivity coefficients was obtained. A data set containing 5800 daily observations for Coshocton, Ohio, Davis, California, and Kimberly, Idaho, was available for testing the theoretical models. The data included pan evaporation, standard meteorological observations, and lysimeter measured evapotranspiration. The theoretical models for E[subscript p] and ET[subscript r] were compared with statistically developed models for the same flows. Using half of the data set for each location, three variables of wind run, day-period temperature and day-period relative humidity and twenty-four transformations and interactions were included in a stepwise analysis. The statistical models obtained included the triple interaction of wind run, temperature and the logarithm of the relative humidity, that was theoretically derived, plus a temperature-based correction factor. To improve the calibration of the models, correction for autocorrelation of the error term and data filtering with a 5-day moving average were performed. A verification of the ET[subscript r]/E[subscript p] relationships was conducted with the second half of the data set. A double-mass curve was plotted for each location between estimated and measured evapotranspiration values. The pan evaporation-based model overestimated ET[subscript r] for Coshocton and underestimated for Davis and Kimberly. An adjusting factor was obtained from the slope of the double mass-curves and the final function estimated the overall average of evapotranspiration with less than 10 percent error by use of a 5-day moving average of pan evaporation measurements.
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