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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/hess-2018-553
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/hess-2018-553
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 09 Nov 2018

Research article | 09 Nov 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Hydrology and Earth System Sciences (HESS).

When does vapor pressure deficit drive or reduce evapotranspiration?

Adam Massmann1, Pierre Gentine1, and Changjie Lin1,2 Adam Massmann et al.
  • 1Department of Earth and Environmental Engineering, Columbia University, New York, NY 10027
  • 2State Key Laboratory of Hydroscience and Engineering, Department of Hydraulic Engineering, Tsinghua University, Beijing, CN 100084

Abstract. Increasing vapor pressure deficit (VPD) increases atmospheric demand for water, and vapor pressure deficit is expected to rise with increasing greenhouse gases. While increased evapotranspiration (ET) in response to increased atmospheric demand seems intuitive, plants are capable of reducing ET in response to increased VPD by closing their stomata, in an effort to conserve water. Here we examine which effect dominates response to increasing VPD: atmospheric demand and increases in ET, or plant physiological response (stomata closure) and decreases in ET. We use Penman-Monteith, combined with semi-empirical optimal stomatal regulation theory and underlying water use efficiency, to develop a theoretical framework for understanding how ET responds to increases in VPD.

The theory suggests that for most environmental conditions and plant types, plant physiological response dominates and ET decreases with increasing VPD. Plants that are evolved or bred to prioritize primary production over water conservation (e.g. crops) exhibit a higher likelihood of atmospheric demand-driven response (ET increasing). However for forest, grass, savannah, and shrub plant types, ET more frequently decreases than increases with rising VPD. This work serves as an example of the utility of our simplified framework for disentangling land-atmosphere feedbacks, including the characterization of ET response in an atmospherically drier, enriched CO2 world.

Adam Massmann et al.
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Adam Massmann et al.
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Short summary
Plants can sense increasing dryness in the air and close up the pores on their leaves, preventing water loss. However, drier air also naturally demands more water from the land surface. Here we develop a simplified theory for when land surface water loss increases (atmospheric demand dominates) or decreases (plant response dominates) in response to increased dryness in the air. This theory provides intuition for how ecosystems regulate water in response to changes in atmospheric dryness.
Plants can sense increasing dryness in the air and close up the pores on their leaves,...
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