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

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, data gaps commonly result from adverse weather conditions and equipment malfunctions. This study utilizes the full-factorial method to address the ET gaps at 265 sites across multiple global flux networks. At the site scale, the gap-filled ET data exhibit temporal variations and dynamic fluctuation trends that closely mirror those of the measured ET data at adjacent time points. The gap-filled ET data were compared with two widely used ET products: the Land component of the Fifth Generation of European Reanalysis (ERA5-Land) and the Global Land Evaporation Amsterdam Model (GLEAM). The analysis revealed that, across 265 sites, over 85% of the sites demonstrated strong consistency between the gap-filled ET data and the two ET products. Specifically, the mean absolute error (MAE) was 0.63 mm/d, and the root mean square error (RMSE) was 0.85 mm/d. At 34 sites with notable discrepancies, the gap-filled ET data more closely aligned with the measured ET data in terms of both seasonal variation and numerical range, compared to the ET products. Overall, the gap-filled ET data from 265 sites (representing 1510 site-years) exhibited high accuracy and reliability. 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 0.92, 21.48 to 28.98 W/m², and 28.41 to 38.01 W/m², respectively. The average energy balance closure rate is 0.82, indicating a relatively high degree of closure in the energy balance. These 265 sites, featuring high-quality filled ET data, can be utilized for ET model validation, ET product verification, water demand assessment, and other related tasks.