Seasonal variation and controlling factors of evapotranspiration over dry semi-humid cropland in Guanzhong Plain, China

Abstract The Guanzhong Plain is a critical food production area in the Yellow River Basin that frequently suffers from water shortages. In this study, long-term (June 2013 to June 2018) water and energy fluxes were observed, and path analysis was conducted over an irrigated winter wheat (Triticum aestivum L.) / summer maize (Zea mays L.) rotation field to identify the controlling factors of evapotranspiration (ET). Total ET for each crop year ranged from 627 to 775 mm, with an average growing season ET of 398 mm for wheat and 310 mm for maize. There is significant seasonal variation in both ET and surface conductance (Gs). Daily ET varied from 0.0 to 6.0 mm d–1 for wheat and 0.0 to 6.7 mm d–1 for maize. The peak daily values of Gs were 29.5 mm s–1 for wheat and 19.5 mm s–1 for maize. The direct and indirect effects of environmental and biological factors—net radiation (Rn), surface conductance (Gs), saturation vapor pressure deficit (VPD), leaf area index (LAI), air temperature (Tair), and volumetric soil water content (VWC)—on ET were calculated using the path analysis method. Rn was determined to be the primary controlling factor of ET for both the summer maize and winter wheat growing seasons. Also, Gs was found to be another controlling factor that has more controlling power in the summer maize growing season than in the winter wheat season. VPD had a significant positive and direct effect on ET for both of the crop seasons, while it had a significant negative and indirect effect on ET through Gs in the summer maize season. VWC and Tair only directly affected the wheat ET. In addition, VWC had two significant paths that can indirectly affect ET through LAI and Gs. The revealed seasonal patterns and controlling factors of evapotranspiration in this agroecosystem provide a theoretical basis for optimizing water resources management of the Yellow River.