Biological factors dominate the interannual variability of evapotranspiration in an irrigated cropland in the North China Plain

Understanding the interannual variability (IAV) and investigating the physical and biological controls on the IAV of evapotranspiration (ET) is fundamental for better simulating the hydrological processes in cropland where ET is the dominant component of water cycle. However, this topic has not been fully studied, although the number of long-term field observations is growing. By using a combination of long-term field observations (10 years) and agro-hydrological modelling, this study attempted to examine the IAV of ET and the physical and biological controls on the IAV of ET in an irrigated cropland in the North China Plain. This study reveals that the IAV of ET was only 7% in the selected dry subhumid area, which was much smaller than that of water supply (i.e., the sum of precipitation and irrigation). Moreover, the water supply was not the primary controlling factor that influenced the IAV of annual ET. Biological factors, including the leaf area index and bulk stomatal conductance, were found to be the dominant contributors to the MV of annual ET. Irrigation was an essential water source for crop growth, particularly for winter wheat growth, whereas its contribution to the MV of ET was smaller than that of precipitation because of the sufficient amount of irrigation. This study demonstrates that the variability of biological factors should be adequately represented in eco-hydrological models to accurately simulate the MV of annual ET.