Variability and sensitivity analyses of spring wheat evapotranspiration measurements in Northwest China

The variability and sensitivity of crop evapotranspiration (ET) measurements at field scale are still poorly understood in the irrigated farmland of arid region in Northwest China. The spatial and temporal dynamics and sensitivity of field ET are fundamental for the scaling up and validation of ET estimates from remote sensing data. In the study, we analysed the dynamics, impact factors and sensitivity of spring wheat ET during the growing season in Northwest China. Results indicated that there was a significant effect of first irrigation event on the spatial and temporary variability of ET. At the tillering-shooting stage (before the first irrigation event), spatial variability of ET was the lowest and gradually increased with crop growth. In some experimental plots, spring wheat ET had a significantly higher temporal stability than other plots except for the tillering-shooting stage. The sample sites with higher temporal stability could be used for long-term monitoring samples and for up scaling of ET measurements. In comparison with LAI and ET0, surface soil moisture change ∆θ0–20 cm was the most sensitive variable of ET measurements, which could be used as the auxiliary variable to improve the ET accuracy. With the soil moisture measurements, the relative error of ET was 9.4 % with only half the number of ET sampling data.