Spatiotemporal Variations in Reference Evapotranspiration and Its Contributing Climatic Variables at Various Spatial Scales across China for 1984-2019

Reference evapotranspiration (ET0) is of great significance in studies of hydrological cycle, agricultural water resources, and hydrometeorology. The present study collected daily meteorological data at 536 meteorological stations in China from 1984 to 2019, calculated daily ET0 using the FAO Penman-Monteith equation, analyzed the spatial distribution and temporal variation characteristics of ET0 and meteorological variables at four different spatial scales (continental, regional, provincial, and local), and discussed the sensitivity of ET0 to the meteorological variables and the contribution rates of the meteorological variables to the ET0 variations. The results showed that ET0 increased at 406 out of the 536 stations (75.7%), with the trends being significant at 65 stations at the 5% significance level, and 147 at the 1% significance level. The slope calculated using Sen's method and linear trend method showed that the annual ET0 at the continental scale increased by approximately 12 mm/decade. Most of the stations showed decreasing trends in relative humidity (H-m), sunshine duration (S-D), and wind speed at 2 m height (U-2) while increasing trends in the maximum air temperature (T-max) and minimum air temperature (T-min). ET0 was most sensitive to H-m (sensitivity coefficient, S-t = -0.66), followed by T-max (S-t = 0.29), S-D (S-t = 0.18), U-2 (S-t = 0.16), and T-min (S-t = 0.07). Most of the stations showed increasing trends in S-t for H-m (56.16%), T-max (72.95%), T-min (87.31%), and U-2 (90.49%), and decreasing trends for S-D (69.78%). The variations in H-m, T-max, and T-min increased the ET0 at most of the stations (82.28%, 98.13%, and 69.03%, respectively). The variations in S-D and U-2 decreased ET0 at most of the stations (66.04% and 56.34%, respectively). Some ET0 characteristics in a few regions can be well described using a single spatial scale. However, most regions exhibited significantly different ET0 characteristics across spatial scales. The results of this project can provide reference for hydrological analysis and agricultural water management under climate change conditions and provide data and information for other hydrology-related applications.