Influences of removing linear and nonlinear trends from climatic variables on temporal variations of annual reference crop evapotranspiration in Xinjiang, China

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Reference crop evapotranspiration (ETo) is a key parameter in field irrigation scheduling, drought assessment and climate change research. ETo uses key prescribed (or fixed or reference) land surface parameters for crops. The linear and nonlinear trends in different climatic variables (CVs) affect ETo change. This research aims to reveal how ETo responds after the related CVs were linearly and nonlinearly detrended over 1961–2013 in Xinjiang, China. The ETo-related CVs included minimum (Tmin), average (Tave), and maximum air temperatures (Tmax), wind speed at 2m (U2), relative humidity (RH) and sunshine hour (n). ETo was calculated using the Penman-Monteith equation. A total of 29 ETo scenarios, including the original scenario, 14 scenarios in Group I (ETo was recalculated after removing linear trends from single or more CVs) and 14 scenarios in Group II (ETo was recalculated after removing nonlinear trends from the CVs), were generated. The influence of U2 was stronger than influences of the other CVs on ETo for both Groups I and II either in northern, southern or the entirety of Xinjiang. The weak influences of increased Tmin, Tave and Tmax on increasing ETo were masked by the strong effects of decreased U2 &n and increased RH on decreasing ETo. The effects of the trends in CVs, especially U2, on changing ETo were clearly shown. Without the general decreases of U2, ETo would have increased in the past 53years. Due to the non-monotone variations of the CVs and ETo, the results of nonlinearly detrending CVs on changing ETo in Group II should be more plausible than the results of linearly detrending CVs in Group I. The decreasing ETo led to a general relief in drought, which was indicated by the recalculated aridity index. Therefore, there would be a slightly lower risk of water utilization in Xinjiang, China.