Attribution analyses of reference evapotranspiration changes in China incorporating surface resistance change response to elevated CO2

Reference evapotranspiration (ET0) is a key component of the water cycle. In this study, a modified Penman-Monteith method incorporating the CO2 effect on surface resistance was used to estimate past change in ET0 in China for 1961-2019 and to project future changes for 2040-2099. A partial-differential equation was used to attribute the changes in ET0. Results indicated the following. (1) A shift point in the annual ET0 series occurred in 1993, in which annual ET0 in China had been decreasing and then increased significantly by 18.93 and 11.19 mm.decade(-1) before and after 1993, respectively, with ET0 having changed most drastically within the temperate continental climate zone. (2) Wind speed and solar radiation were the major dominant factors responsible for the change in ET0 during 1961-1992, and their contribution rates were 48.30 and 38.92%, respectively. During 1993-2019, increasing maximum air temperature was the dominant factor contributing to increasing ET0, with a contribution rate of 81.71%, yet wind speed was the dominant factor affecting ET0 changes at 45% of the stations in China. (3) In future projections, maximum air temperature is expected to be the dominant factor influencing the increase in ET0, the values of which were projected at 1.49 and 16.05 mm.decade(-1) during 2040-2099 under RCP4.5 and 8.5 scenarios, respectively, with projections from five GCMs. The increasing surface resistance response to elevated CO2 was found to be an important contributor to the decrease in ET0. Particularly for the RCP8.5 scenario, the increase in surface resistance was found to lead to the decrease in ET0 by 0.68 mm.year(-1) (-42.37%). This suggests that historical and future tendencies towards aridity in China may be considerably weaker and less extensive than previously assumed.