Simple Method for Estimating Pan Coefficients: Conversion of Pan Evaporation to Reference Evapotranspiration

Reference evapotranspiration rates are often estimated by using pan evaporation data because pan data are widely available. Conversion of pan evaporation estimates to reference evapotranspiration relies on the determination of the pan coefficient ( Kp ). A simple linear equation has been suggested for estimating Kp on the basis of the energy balance, and it is expressed as a linear combination of wind speed and surface temperature at the reference height. In this study, the new equation for estimating Kp was calibrated by using a single-criteria multiple-parameter optimization technique, and cross-validation was performed to test the equation by using data from 10 meteorological stations in Korea. Because lysimeter data were not available, the Penman-Monteith equation was used as a reference in this study to calibrate and validate the suggested equation. The results showed that the averaged coefficient of determination ( r2 ) was 0.92, the NSC was 0.93, and the RMSE was 8.99 mm. In general, the suggested equation provided reasonably accurate estimates of the reference evapotranspiration in the Korean Peninsula. The suggested equation presented here is advantageous because only two readily available pieces of climate data are required to estimate Kp .