Elevation‐dependent changes in reference evapotranspiration due to climate change

The Food and Agriculture Organizations' (FAO) Penman–Monteith reference evapotranspiration (ET 0 ) is a crucial index in the research of water and energy balance. Temporal and spatial variations in ET 0 from 1981–2017 were investigated in the Hengduan Mountains, China . The results showed a change point around the year 2000 in ET 0 series. ET 0 decreased and increased significantly by +3.200 mm/year ( p  < 0.01) from 1981–2000 and by +4.109 mm/year ( p  < 0.01) from 2001–2017, respectively. The contribution analysis shows that the positive significant contribution of air temperature (TA) was offset by negative effects of decreases in downward shortwave radiation (R s ) and wind speed (WS) and an increase in actual vapour pressure (e a ), causing the decrease in ET 0 from 1981 to 2000. WS was the largest contributing factor for the decrease in ET 0 from 1981 to 2000 during spring, winter and annually, while R s and e a were the largest negative contributors in summer and autumn, respectively. An increase in TA was responsible for the increase in ET 0 in all seasons except winter and the annual scale in 2001–2017. The sensitivity analysis shows that ET 0 was most sensitive to TA, and WS was the least sensitive variable. The trends of ET 0 increased with elevation; we denote this as the elevation‐dependence of ET 0 changes. The elevation‐dependence was also noted for the trends of WS and e a , with higher elevations showing larger changes in WS and lower changes in e a . Besides, the sensitivities of TA, R s and e a decreased with elevation, while that of WS increased slightly with elevation. A comprehensive investigation into the trends of climatic drivers and their sensitivities revealed complex trends of the contributions of climatic variables on ET 0 with elevation, with no uniform trend existed in seasons. The results will contribute to our understanding of the response of ET 0 to climate change in a mountainous area, and provide a guideline for the water resources management under climate change.