How Has the Recent Climate Change Affected the Spatiotemporal Variation of Reference Evapotranspiration in a Climate Transitional Zone of Eastern China?

Reference evapotranspiration (ET0) is essential for agricultural production and crop water management. The recent climate change affecting the spatiotemporal variation of ET0 in eastern China continues to still be less understood. For this purpose, the latest observed data from 77 meteorological stations in Anhui province were utilized to determine the spatiotemporal variations of ET0 by the use of the Penman-Monteith FAO 56 (PMF-56) model. Furthermore, the Theil-Sen estimator and the Mann-Kendall (M-K) test were adopted to analyze the trends of ET0 and meteorological factors. Moreover, the differential method was employed to explore the sensitivity of ET0 to meteorological factors and the contributions of meteorological factors to ET0 trends. Results show that the ET0 decreased significantly before 1990, and then increased slowly. The ET0 is commonly higher in the north and lower in the south. ET0 is most sensitive to relative humidity (RH), except in summer. However, in summer, net radiation (R-n) is the most sensitive factor. During 1961-1990, R-n was the leading factor annually, during the growing season and summer, while wind speed (u(2)) played a leading role in others. All meteorological factors provide negative contributions to ET0 trends, which ultimately lead to decreasing ET0 trends. During 1991-2019, the leading factor of ET0 trends changed to the mean temperature (T-a) annually, during the growing season, spring and summer, and then to R-n in others. Overall, the negative contributions from u(2) and R-n cannot offset the positive contributions from T-a and RH, which ultimately lead to slow upward ET0 trends. The dramatic drop in the amount of u(2) that contributes to the changes in ET0 in Region III is also worth noting.