Seasonal and interannual variations in evapotranspiration and energy budget over a rainfed maize field in the Chinese Loess Plateau

Evapotranspiration (ET) is an important part of surface hydrological cycle and energy exchange process. Under the background of frequent drought and water resource shortage, it is of great significance to study the seasonal and interannual variations in ET and energy budget of farmland ecosystem in arid regions. This will help to reveal the process of crop ET and water consumption in farmland ecosystems and its response to drought conditions and environmental factors. Based on continuous eddy-covariance observation data, this study analyzed the seasonal and interannual variations in ET and energy budget of one maize farmland ecosystem in the semi-arid region of Chinese Loess Plateau and its response to environmental changes during 2019-2020. Results showed that ET and meteorological factors presented obvious seasonal and interannual changes during the study period. The annual total ET was 339.3 mm/yr and 386.5 mm/yr during the drought year 2019 and the normal year 2020, respectively. It was 16.4% and 30.4% lower than the annual total precipitation (P) in the same period. The ET/P ratio for two years was 0.84 and 0.70, respectively. While the latent heat flux and sensible heat flux showed obvious seasonal and interannual variation trends, it also reflected that the latent heat flux dominated the net radiation energy budget in the growing season and the sensible heat flux is the main consumption component of the net radiation energy budget in the non-growing season. During 2020, due to better moisture conditions, the Bowen ratio was smaller and sensible heat exchange was more moderate, making the air more stable. The volatility of albedo in 2020 was significantly greater than that in 2019, which was closely related to the frequent precipitation in the normal year. The results of the path analysis model showed that soil water content (SWC) had stronger impacts on the ET variation during the drought year (2019) at the scales of entire year, growing season and non-growing season. Meanwhile, leaf area index (LAI) had more significant impacts on the ET variation during the hydrologically normal year (2020) when the water supply was much more sufficient. We also found that there was a strong coupling relationship between the atmosphere and vegetation during the study period (decoupling factor Ω varied between 0 and 0.5), indicating that the ecosystem ET is mainly controlled by canopy conductance(g_s) and vapor pressure deficit (VPD). Moreover, g_s decreased with the increase of VPD, and VPD played a stronger role in controlling g_s during 2020 which was with better water supply condition.