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UBC Theses and Dissertations

Interactions between the land surface and the near-surface atmosphere : implications for evaporative demand and evapotranspiration under a changing climate Kim, Yeonuk


Terrestrial evaporation (also referred to as evapotranspiration) is a central variable controlling water, energy, and carbon cycles. However, our ability to estimate evapotranspiration is hampered by the uncertainties in parameterizations of land surface conditions in currently available models. Although the near-surface atmosphere both drives evapotranspiration and is also changed by evapotranspiration (i.e., land-atmosphere feedback), some evapotranspiration models cannot properly address this feedback process. This thesis addresses this critical knowledge gap by introducing a series of alternative theories that consider land-atmosphere interactions, predict potential evaporation, and estimate actual evapotranspiration. Overall, the goal of this thesis is to provide improved tools for use in hydrology and water resources management along with a detailed analysis derived from the application of these tools over multiple spatial and temporal scales. Chapter 1 provides a general background to the research topic. Chapter 2 introduces a novel physical expression of evapotranspiration that shows how land-atmosphere interactions control evapotranspiration. The proposed physical expression is evaluated using field observations and a global-scale evapotranspiration product. Chapter 3 focuses on climatic controls on evapotranspiration, which is also known as potential evaporation. In this chapter, a scientific debate regarding the trend of the potential evaporation is addressed by extending the land-atmosphere coupling theory introduced in Chapter 2. To this end, field-scale observations, watershed-scale observations, and climate simulations are used to assess model performance and to evaluate trends of potential evaporation. Chapter 4 derives a physically-based evapotranspiration model that requires only readily obtainable meteorological information. This new approach is evaluated using field-scale observations around the globe. Finally, Chapter 5 provides the overall conclusions from the research detailed in the preceding chapters. Overall, this thesis improves the estimation of evapotranspiration by minimizing empirical parameters required for modeling purposes, and by advancing our understanding of how land-atmosphere interactions control terrestrial evapotranspiration. In particular, the results suggest that the near-surface atmosphere should be understood as a reflection of the land surface dryness instead of as an atmospheric demand for soil moisture that is independent of surface conditions. This perspective underlies the models presented in this thesis, and by extension will facilitate more accurate predictions of evapotranspiration.

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