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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2019-85
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2019-85
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Reviews and syntheses 12 Mar 2019

Reviews and syntheses | 12 Mar 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

Reviews and syntheses: Turning the challenges of partitioning ecosystem evaporation and transpiration into opportunities

Paul C. Stoy1, Tarek El-Madany2, Joshua B. Fisher3,4, Pierre Gentine5, Tobias Gerken1,6, Stephen P. Good7, Shuguang Liu8, Diego G. Miralles9, Oscar Perez-Priego2,10, Todd H. Skaggs11, Georg Wohlfahrt12, Ray G. Anderson11, Martin Jung2, Wouter H. Maes9, Ivan Mammarella13, Matthias Mauder14, Mirco Migliavacca2, Jacob A. Nelson2, Rafael Poyatos15,16, Markus Reichstein2, Russell L. Scott17, and Sebastian Wolf18 Paul C. Stoy et al.
  • 1Department of Land Resources and Environmental Sciences, Montana State University, Bozeman, MT, USA
  • 2Max Planck Institute for Biogeochemistry, Hans Knöll Straße 10, Jena D-07745, Germany
  • 3Jet Propulsion Laboratory, California Instituteof Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
  • 4Joint Institute for Regional Earth System Science and Engineering, University of California at Los Angeles,Los Angeles, CA 90095, USA
  • 5Department of Earth and Environmental Engineering, Columbia University, New York, 10027, USA
  • 6The Pennsylvania State University, Department of Meteorology and Atmospheric Science, 503 Walker Building, University Park, PA, USA
  • 7Department of Biological & Ecological Engineering, Oregon State University, Corvallis, Oregon, USA
  • 8National Engineering Laboratory for Applied Technology of Forestry and Ecology in South China, Central South University of Forestry and Technology, Changsha, China
  • 9Laboratory of Hydrology and Water Management, Ghent University, Coupure Links 653, 9000 Gent, Belgium
  • 10Department of Biological Sciences, Macquarie University, North Ryde, NSW 2109, Australia
  • 11U.S. Salinity Laboratory, USDA-ARS, Riverside, CA, USA
  • 12Institut für Ökologie, Universität Innsbruck, Sternwartestr. 15, 6020 Innsbruck, Austria
  • 13Institute for Atmospheric and Earth System Research / Physics, Faculty of Science, FI-00014 University of Helsinki, Finland
  • 14Karlsruhe Institute of Technology, Institute of Meteorology and Climate Research-Atmospheric Environmental Research, Garmisch-Partenkirchen, Germany
  • 15CREAF, E08193 Bellaterra (Cerdanyola del Vallès), Catalonia, Spain
  • 16Laboratory of Plant Ecology, Faculty of Bioscience Engineering, Ghent University, Coupure links 653, 9000 Ghent, Belgium
  • 17Southwest Watershed Research Center, USDA Agricultural Research Service, Tucson, AZ, USA
  • 18Department of Environmental Systems Science, ETH Zurich, Zurich, Switzerland

Abstract. Evaporation (E) and transpiration (T) respond differently to ongoing changes in climate, atmospheric composition, and land use. Our ability to partition evapotranspiration (ET) into E and T is limited at the ecosystem scale, which renders the validation of satellite data and land surface models incomplete. Here, we review current progress in partitioning E and T, and provide a prospectus for how to improve theory and observations going forward. Recent advancements in analytical techniques provide additional opportunities for partitioning E and T at the ecosystem scale, but their assumptions have yet to be fully tested. Many approaches to partition E and T rely on the notion that plant canopy conductance and ecosystem water use efficiency (EWUE) exhibit optimal responses to atmospheric vapor pressure deficit (D). We use observations from 240 eddy covariance flux towers to demonstrate that optimal ecosystem response to D is a reasonable assumption, in agreement with recent studies, but the conditions under which this assumption holds require further analysis. Another critical assumption for many ET partitioning approaches is that ET can be approximated as T during ideal transpiring conditions, which has been challenged by observational studies. We demonstrate that T frequently exceeds 95 % of ET from some ecosystems, but other ecosystems do not appear to reach this value, which suggests that this assumption is ecosystem-dependent with implications for partitioning. It is important to further improve approaches for partitioning E and T, yet few multi-method comparisons have been undertaken to date. Advances in our understanding of carbon-water coupling at the stomatal, leaf, and canopy level open new perspectives on how to quantify T via its strong coupling with photosynthesis. Photosynthesis can be constrained at the ecosystem and global scales with emerging data sources including solar-induced fluorescence, carbonyl sulfide flux measurements, thermography, and more. Such comparisons would improve our mechanistic understanding of ecosystem water flux and provide the observations necessary to validate remote sensing algorithms and land surface models to understand the changing global water cycle.

Paul C. Stoy et al.
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Status: open (until 23 Apr 2019)
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Short summary
Key findings are the near optimal response of T to atmospheric water vapor pressure deficite accros methods and scales. Additionally, the notation that T/ET intermittently approaches 1 which is a basis for many partitioning methods does not hold for certain methods and ecosystems. To better constrain estimates of E and T from combined ET measurements we propose a combination of independent measurement techniques to better constrain E and T at ecosystem scale.
Key findings are the near optimal response of T to atmospheric water vapor pressure deficite...
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