Effects of changing climate on reference crop evapotranspiration over 1961–2013 in Xinjiang, China

To know the importance of different climate variables on reference crop evapotranspiration (ET_o), a step-by-step sensitivity analysis of ET_o to single, two, and multi-climate variables (C) was conducted. ET_o in north, south, and entire Xinjiang Province, China, over 1961–2013 was estimated using the Penman-Monteith equation. Trends in the involved six Cs (i.e., minimum temperature—T_min, average temperature—T_ave, maximum temperature—T_max, wind speed at 2 m—U₂, sunshine hour—n, and relative humidity—RH) were detected by the modified Mann-Kendall test. Nineteen scenarios of changed Cs were preset to obtain recalculated ET_o values considering the actual trend in each C and the Pearson’s correlation relationship between ET_o and Cs. The results showed that ET_o was mostly sensitive to T_max, U₂, and n. Sensitivity of ET_o to the two overlapped changes of T_min and T_max caused larger increases in ET_o than T_max and T_ave, T_ave and T_max, T_max and (−n), T_max and RH, T_max and (−U₂), and T_min and T_ave, but the overlapped changes (−U₂) and (−n) caused larger decreases in ET_o than the other two C scenarios. The simultaneously increased T_max, T_min, T_ave, and RH plus decreased U₂ and n had caused the actual decreases in ET_o in Xinjiang. In general, the effects of decreased U₂ and n on decreasing ET_o compensated the effects of increased T_max on decreasing ET_o in Xinjiang.