Shifting from homogeneous to heterogeneous surfaces in estimating terrestrial evapotranspiration: Review and perspectives

Terrestrial evapotranspiration (ET) is a crucial link between Earth's water cycle and the surface energy budget. Accurate measurement and estimation remain a major challenge in geophysical, biological, and environmental studies. Pioneering work, represented by Dalton and Penman, and the development of theories and experiments on turbulent exchange in the atmospheric boundary layer (ABL), laid the foundation for mainstream methodologies in ET estimation. Since the 1990s, eddy covariance (EC) systems and satellite remote sensing have been widely applied from cold to tropical and from arid to humid regions. They cover water surfaces, wetlands, forests, croplands, grasslands, barelands, and urban areas, offering an exceptional number of reports on diverse ET processes. Surface nocturnal ET, hysteresis between ET and environmental forces, turbulence intermittency, island effects on heterogeneous surfaces, and phase transition between underlying surfaces are examples of reported new phenomena, posing theoretical and practical challenges to mainstream ET methodologies. Additionally, based on non-conventional theories, new methods have emerged, such as maximum entropy production and nonparametric approaches. Furthermore, high-frequency on-site observation and aerospace remote sensing technology in combination form multi-scale observations across plant stomata, leaves, plants, canopies, landscapes, and basins. This promotes an insightful understanding of diverse ET processes and synthesizes the common mechanisms of the processes between and across spatial and temporal scales. All the recent achievements in conception, model, and technology serve as the basis for breaking through the known difficulties in ET estimation. We expect that they will provide a rigorous, reliable scientific basis and experimental support to address theoretical arguments of global significance, such as the water-heat-carbon cycle, and solve practical needs of national importance, including agricultural irrigation and food security, precise management of water resources and eco-environmental protection, and regulation of the urban thermal environment and climate change adaptation.