Variations and Sensitivity Analysis of Reference Evapotranspiration During 2010-2019 in the Zhangye Farmland Oasis, Northwest China

Reference evapotranspiration (ET 0 ) is an important parameter for agricultural water management in the arid Zhangye farmland oasis. However, the ET 0 variations in this oasis over the last decade and meteorological forcings of these variations are unknown. This study investigated the ET 0 variations during 2010-2019 in this oasis using the FAO-56 Penman-Monteith (PM) and Hargreaves equations. Results showed that the ET 0 features daily and monthly variations with peak values in mid-July and an annual cycle. Although the estimated ET 0 series based on the two equations have high correlations in the time domain, the Hargreaves equation always underestimates the ET 0 compared to the PM equation. The yearly ET 0 showed statistically significant increasing trends (90% significance level) during 2010-2019, while statistically significant increasing trends in monthly ET 0 are found only in March and November. Increasing trends reflected in monthly and yearly ET 0 are mainly attributed to the increasing maximum temperature and sunshine duration and decreasing relative humidity. Sensitivity analysis demonstrated that the meteorological factor to which the ET 0 is most sensitive varies with time scale and equation. Moreover, regression equations used to correct the underestimation associated with the Hargreaves equation for estimating ET 0 in the Zhangye farmland oasis also were constructed.