Variation Characteristics of Pan Evaporation and the Influencing Factors in the Yongding River Basin during 1958–2018

Based on the daily observation data of 14 meteorological stations in the Yongding River basin from 1958 to 2018, the variation characteristics of pan evaporation over 60 years were analyzed using climate tendency rate and Mann–Kendall trend test. Moreover, the main influencing factors were identified by complete correlation coefficient and multivariate regression analysis. Results showed that under the background of global warming, the temperature of the Yongding River basin increased at the rate of 0.29°C/10 a in 60 years, while pan evaporation decreased significantly at the rate of −48.88 mm/10a (standardized statistics Z = −4.5). Evidently, an “evaporation paradox” phenomenon exists. There are significant differences in the spatial and temporal distribution of evaporation in the basin. Evaporation decreases significantly in spring and summer, accounting for 35% and 37% of the annual evaporation, respectively. The evaporation is unevenly distributed over an area, and the downtrend of the downstream of the Yongding River basin is more significant than that in the upper regions of the mountain area (Tianzhen, Yuxian, and other areas). Net radiation, average wind speed, and air saturation difference have a strong correlation with the decrease in evaporation. Compared with the base period of 1958–1979, the decrease in the average wind speed and net radiation during 1980–2018 contributed to 77% and 66% decrease in evaporation, respectively, and the contribution rate of the air saturation difference is −41%. The decrease in net radiation and average wind speed is the main factor leading to the decrease of evaporation. The air pollution problem and change in the underlying surface of the Yongding River basin cause the net radiation and average wind speed to decrease significantly, which may lead to serious water imbalance in the lower plains of the basin.