Dynamic analysis of pan evaporation variations in the Huai River Basin, a climate transition zone in eastern China.

Pan evaporation (Epan), which we examine in this study to better understand atmospheric evaporation demand, represents a pivotal indicator of the terrestrial ecosystem and hydrological cycle, particularly in the Huai River Basin (HRB) in eastern China, where high potential risks of drought and flooding are commonly observed. In this study, we examine the spatiotemporal trend patterns of climatic factors and Epan by using the Mann-Kendall test and the Theil-Sen estimator based on a daily meteorological dataset from 89 weather stations during 1965-2013 in the HRB. Furthermore, the PenPan model is employed to estimate Epan at a monthly time scale, and a differential equation method is applied to quantify contributions from four meteorological variables to Epan trends. The results show that Epan significantly decreased (P<0.001) at an average rate of -8.119mm·a-2 at annual time scale in the whole HRB, with approximately 90% of stations occupied. Meanwhile, the generally higher Epan values were detected in the northern HRB. The values of the aerodynamic components in the PenPan model were much greater than those of the radiative components, which were responsible for the variations in the Epan trend. The significantly decreasing wind speed (u2) was the most dominant factor that controlled the decreasing Epan trend at each time scale, followed by the notable decreasing net radiation (Rn) at the annual time scale also in growing season and summer. However, the second dominant factor shifted to the mean temperature (Ta) during the spring and winter and the vapor pressure deficit (vpd) during the autumn. These phenomena demonstrated a positive link between the significance of climate variables and their control over the Epan trend.