Study on Variability of Wheat Evapotranspiration in Irrigation Farmland Areas of Northwest China

Evapotranspiration (ET) is the process that returns water from the earth surface to the atmosphere and therefore completes the hydrologic cycle. The collection and analysis of crop ET at different spatial and temporal scales has received much attention. Studies showed that the coefficients of variation(CV) for wheat ET at different growing stages is large with CV ranging from 30% to 50% even for "apparent" homogeneous fields of experimental farmland. During the stage of tillering-shooting, shooting-heading and heading-milk, the CV of accumulative ET decreased with the increasing of measurement time scales. The spatial variation analysis of ET based on the semivariance function showed that ET data obeyed a normal distribution and optimal spherical models. At the original tillering-shooting stage, the spatial structure of accumulative ET was the strongest, and then turned to sharply reduce at the shooting-heading stage. The temporal stability of ET was also important for determining the mean ET. The significant correlation of ET that estimated from different growing stages showed temporal stability. The temporal stability analysis showed that wheat ET had a higher temporal stability after the stage of tillering-shooting, which was often used to determine sample sites for the field mean ET fluxes.