Gap-Filling for Daily Evapotranspiration Observations with full-factorial method at Global Flux Sites

Abstract. Evapotranspiration (ET) plays a crucial role in the regional water-energy cycle, illustrating intricate interactions among climate, vegetation and soil. Eddy covariance (EC) technology is a primary method for measuring ET. However, incomplete data due to adverse weather conditions and equipment malfunctions are common. This study utilizes the full-factorial method to address the ET gaps at 339 sites across multiple global flux networks, and the filled ET data are compared with three ET products: the Land component of the Fifth Generation of European Reanalysis (EAR5-Land), Global Land Evaporation Amsterdam Model (GLEAM), and Breathing Earth System Simulator (BESS). Results indicate high consistency between filled ET data and three ET products at 264 out of 339 sites. The absolute average mean error (|MAE|) and root mean system error (|RMSE|) are 0.32 mm/d and 0.92 mm/d, respectively. Among the remaining 75 sites, 49 sites exhibit better agreement between filled ET and measured ET data than ET products in terms of seasonal variations and numerical ranges. Further verification is needed for the reliability of filled ET data at the other 26 sites due to a scarcity of measured ET data. Overall, the gap-filled ET data from 313 sites (2210 site-years) demonstrate high-quality. These sites exhibit a strong correlation between available energy and turbulent fluxes, with R², MAE, and RMSE for different surface types ranging from 0.84 to 0.94, 21.49 to 28.67 W/m², and 28.37 to 36.91 W/m², respectively. And the average energy balance closure rate is 0.73, indicating a relatively high degree of closure in the energy balance. These 313 sites with high-quality filled ET data can be utilized for ET model validation, ET product verification, water demand assessment, and other related tasks. The filled ET dataset can be publicly accessed at https://doi.org/10.57760/sciencedb.11651 (Wang & Jiang, 2024).