Spatial and Temporal Variation in Reference Evapotranspiration and Its Climatic Drivers in Northeast China

Reference evapotranspiration (ET0) is an important component of the global water cycle, and its long-term change directly influences the regional water supply and demand balance. Under the background of global change, investigating spatiotemporal trends in ET0 and its response to climate change is of great importance for the conservation and rational utilization of water resources. Based on daily climate data from 91 meteorological stations during 1960–2017 in Northeast China, this study calculated ET0 using the Penman-Monteith method and analyzed its spatiotemporal change trends and primary driving factors. The results show the following: (1) During 1960–2017, the annual ET0 in Northeast China showed a nonsignificant upward trend at a rate of 1.45 mm/10a. A mutation point of ET0 was detected in 1993. From 1960 to 1993, ET0 experienced a significant decrease (p < 0.1), while annual air temperature showed a significant upward trend (p < 0.01), which indicated the appearance of an evaporation paradox. This was because the remarkable drop in wind speed and sunshine duration played a great role in the reduction of ET0. From 1994 to 2017, the evaporation paradox disappeared. (2) ET0 trend in Northeast China was significantly and positively related to altitude. In the lower altitude regions (<500 m), ET0 generally decreased, while in the higher altitude areas (>500 m), ET0 displayed an upward trend. (3) Based on the results of multiple regression analysis, relative humidity was the primary driving factor for ET0 trends in Northeast China during 1960–2017. At diverse altitudes, the primary climatic factors influencing ET0 were different. In high-altitude areas (>500 m), the change in ET0 was mainly influenced by relative humidity, while wind speed was the primary driving factor at low altitudes (<500 m).