The spatiotemporal variation of reference evapotranspiration and the contribution of its climatic factors in the Loess Plateau, China

Reference evapotranspiration (ET0) is an important parameter of agricultural activity and hydrometeorological studies. In this study, we used the Food and Agriculture Organization Penman-Monteith equation to evaluate ET0, and to investigate the change point of ET0 in the Loess Plateau region from 1960 to 2013. The results showed that a change point was detected at approximately the year 1990 for annual ET0 series from 108 meteorological stations using Cramer's statistical test. The annual ET0 decreased significantly (p < 0.05) by -1.22 mm year -1 from 1960 to 1990, especially during the summer months, which contributed the most to the total annual reduction, while it increased significantly (p < 0.001) by 1.15 mm year -1 from 1991 to 2013, with the spring months contributing the most. The ET0 and its trend in the five integrated management divisions of the Loess Plateau have significant spatial heterogeneity. The highest and lowest ET0 were found in the third and fifth divisions from 1960 to 2013. The ET0 decreased significantly in the first division and increased in the fourth divisions (1960-1990 and 1991-2013, respectively). Using differential equations to quantitatively evaluate the contribution of various factors, the wind speed was mostly responsible for the variability in the ET0 trend from 1960 to 1990, followed by solar radiation and vapor pressure. The positive effect of air temperature on ET0 trend was offset by the other three factors, and the combined effects of the four climatic variables led to the decrease in the ET0 trend. However, the rapidly increasing air temperature became the predominant factor in the change in the ET0 trend after 1990. A spatiotemporal variation of pre-dominant contribution to the ET0 trend was identified. The temperature dominant region changed from the third to the first division for 1960-1990 and 1991-2013. Radiation did not change. Vapor pressure changed from the first to the fourth and the wind speed changed from the forth to the third. This study could contribute to a better understanding of the response of the spatial and temporal variation of ET0 to increased climate change. Additionally, this research also provides scientific support to regional planning and management.