image: Conceived in the 8th century B.C., pioneering work represented by Dalton and Penman and atmospheric turbulence theory founded the contemporary theory of evaporation. view more
Credit: ©Science China Press
Prof. Yuanbo Liu (Nanjing Institute of Geography and Limnology, the Chinese Academy of Sciences) invited eight research groups internationally active in studying land surface fluxes. In the meeting held at Nanjing during 25-25 April 2019, they reviewed progresses and discussed problems in estimating terrestrial evapotranspiration. "We were confused with the existing theories of evaporation for long," Yuanbo says.
Yuanbo and his colleagues sought to identify the mainstreams from the mile-stone works (see Figure 1). The team compared their theoretical origin, assumptions, and restrictions. They attempted to locate the main bottlenecks in contemporary theory and proposed potential solutions to these challenges.
Given global demands for large-scale understanding, evaporation study shifted from homogeneous to heterogeneous surfaces. Worldwide evaporation observation scrutinized unclear processes, such as turbulence intermittency, hysteresis effects, and interactions between different surfaces. “The findings pose challenges to the conventional theory of turbulence similarity and evaporation mechanism," Prof. Hongsheng Zhang (School of Physics, Peking University) says.
The authors classified various measurement or estimation approaches into three categories, in the context of boundary layer meteorology, hydrometeorology, and terrestrial hydrology. The classification offers an alternative viewpoint to specify the evaporation as an interface phase transition. The atmosphere serves as the upper boundary, and the soil or vegetation surface constitutes the lower boundary. “It favors us to grasp the basic similarities and differences among the approaches that based on different principles,” Yuanbo says.
Analogue to electric resistance, the conventional theory uses surface resistance or conductance to describe the efficiency of water vapor into the atmosphere from the surface. “However, the resistance is an interfacial coefficient, not a constant property of the evaporative process. It changes with surface condition, with which makes it difficult to develop rigorous, physically-based, scaling functions for heterogeneous surfaces," Prof. Guoyu Qiu (Peking University Shenzhen Graduate School) says.
The present estimation of terrestrial evaporation contains large uncertainty, calling for new way of thinking. Favorably, global collaboration on on-site instrumental observation and remote sensing monitoring has offered multi-scale datasets. This should promote to synthesize the common mechanism of the diverse processes, and enlighten further efforts on new theory, across spatial and temporal scales.
See the article:
Yuanbo LIU, Guoyu QIU, Hongsheng ZHANG, Yonghui YANG, Yinsheng ZHANG, Quan WANG, Wenzhi ZHAO, Li JIA, Xibin JI, Yujiu XIONG, Chunhua YAN, Ning MA, Shumin HAN, Yifan CUI. 2022. Shifting from homogeneous to heterogeneous surfaces in estimating terrestrial evapotranspiration: Review and perspectives. SCIENCE CHINA Earth Sciences, 65(2): 197-214, https://doi.org/10.1007/s11430-020-9834-y
Shifting from homogeneous to heterogeneous surfaces in estimating terrestrial evapotranspiration: Review and perspectives
https://engine.scichina.com/doi/10.1007/s11430-020-9834-y
Journal
Science China Earth Sciences