Emipirical estimates of evapotranspiration from eddy covariance: challenges and opportunities

Global freshwater is becoming an increasingly valuable resource, both due to increased human use as well as due to ecological importance in a changing climate. Understanding the hydrological cycles which govern water availability requires broad scale estimates of terrestrial evaporation, or evapotranspiration, which incorporate the complex signals of plant water use via transpiration. In this regard, evapotranspiration estimated from eddy covariance has proven a valuable resource in understanding ecosystem scale water fluxes at sites around the world, and recent advances in methods for directly estimating transpiration from eddy covariance data provide the opportunity to understand the influence plants have on water cycles. However, linking these ecosystem scale estimates to global scale processes requires a model to act as an intermediary, such as the empirical models used in the FLUXCOM products which train machine learning models on eddy covariance data linked with remote sensing data.

Here we look at the next generation of global terrestrial water flux estimates from FLUXCOM, including both the total evapotranspiration and the individual components of transpiration and abiotic evaporation. We benchmark these new estimates against previous FLUXCOM products, as well as compare to the state-of-the-art evapotranspiration estimates from process based models and remote sensing products. The high spatial and temporal scale allows for a close look at how the transpiration to evapotranspiration ratio varies both in space and time. We also outline estimate uncertainties from potential measurement biases to feature selection, and discuss the next steps for high quality empirical water flux estimates.