According to geometry dimensions and installation characteristic of the 20 cm pan, the evaporation model of 20 cm pan was built by modifying the radiation and thermal storage items in Penman formula. Following, the model was calibrated by the continuous 14-day hourly measurements which were from Gulang Heterogeneous Underlying Surface Layer Experiment (GHUSLE). The analysis results showed that the model could simulate the diurnal variation of pan evaporation. In addition, the results indicated that day performance of the model was good with a root-mean-square error of 0.72 mm·d-1 and a mean relative error ratio of 6.7%. The simulation analysis showed that the contributions of radiative and aerodynamic components in the 20 cm pan evaporation process accounts for about 1/3 and 2/3 of the total pan evaporation, respectively. Sensitivity tests of four kinds of convention meteorological factors on pan evaporation showed that the pan evaporation were most sensitive to the wind speed, followed by air relative humidity, solar radiation and air temperature, and the corresponding rates of changes were 0.602, -0.590, 0.528 and 0.370, respectively.
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