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

Abstract

Reference crop evapotranspiration (ET₀) is a key parameter in field irrigation scheduling, drought assessment and climate change research. ET₀ uses key prescribed (or fixed or reference) land surface parameters for crops. The linear and nonlinear trends in different climatic variables (CVs) affect ET₀ change. This research aims to reveal how ET₀ 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 2 m (U₂), relative humidity (RH) and sunshine hour (n). ET₀ was calculated using the Penman-Monteith equation. A total of 29 ET₀ scenarios, including the original scenario, 14 scenarios in Group I (ET₀ was recalculated after removing linear trends from single or more CVs) and 14 scenarios in Group II (ET₀ was recalculated after removing nonlinear trends from the CVs), were generated. The influence of U₂ was stronger than influences of the other CVs on ET₀ 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 ET₀ were masked by the strong effects of decreased U₂ & n and increased RH on decreasing ET₀. The effects of the trends in CVs, especially U₂, on changing ET₀ were clearly shown. Without the general decreases of U₂, ET₀ would have increased in the past 53 years. Due to the non-monotone variations of the CVs and ET₀, the results of nonlinearly detrending CVs on changing ET₀ in Group II should be more plausible than the results of linearly detrending CVs in Group I. The decreasing ET₀ 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.