Assessing the effect of climate change on reference evapotranspiration in China

Reference evapotranspiration (ET (0) ) is a key parameter in hydrological and meteorological studies. In this study, the FAO Penman-Monteith equation was used to estimate ET (0) , and the change in ET (0) was investigated in China from 1960 to 2011. The results show that a change point around the year 1993 was detected for the annual ET (0) series by the Cramer's test. For the national average, annual ET (0) decreased significantly (P < 0.001) by -14.35 mm/decade from 1960 to 1992, while ET (0) increased significantly (P < 0.05) by 22.40 mm/decade from 1993 to 2011. A differential equation method was used to attribute the change in ET (0) to climate variables. The attribution results indicate that ET (0) was most sensitive to change in vapor pressure, followed by solar radiation, air temperature and wind speed. However, the effective impact of change in climate variable on ET (0) was the product of the sensitivity and the change rate of climate variable. During 1960-1992, the decrease in solar radiation was the main reason of the decrease in ET (0) in humid region, while decrease in wind speed was the dominant factor of decreases in ET (0) in arid region and semi-arid/semi-humid region of China. Decrease in solar radiation and/or wind speed offset the effect of increasing air temperature on ET (0) , and together led to the decrease in ET (0) from 1960 to 1992. Since 1993, the rapidly increasing air temperature was the dominant factor to the change in ET (0) in all the three regions of China, which led to the increase in ET (0) . Furthermore, the future change in ET (0) was calculated under IPCC SRES A1B and B1 scenarios with projections from three GCMs. The results showed that increasing air temperature would dominate the change in ET (0) and ET (0) would increase by 2.13-10.77, 4.42-16.21 and 8.67-21.27 % during 2020s, 2050s and 2080s compared with the average annual ET (0) during 1960-1990, respectively. The increases in ET (0) would lead to the increase in agriculture water consumption in the 21st century and may aggravate the water shortage in China