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

Authors: STOY, PAUL - Montana State University; EL-MADANY, TAREK - Max Planck Institute For Biogeochemistry; FISHER, JOSHUA - Jet Propulsion Laboratory; GENTINE, PIERRE - Columbia University - New York; GERKEN, TOBIAS - Montana State University; GOOD, STEPHEN - Oregon State University; KLOSTERHALFEN, ANNE - Agrosphere Institute; LIU, SHUGUANG - Central South University Of Forestry And Technology; MIRALLES, DIEGO - Ghent University; PEREZ-PRIEGO, OSCAR - Max Planck Institute For Biogeochemistry; RIGDEN, ANGELA - Harvard University; Skaggs, Todd; WOHLFAHRT, GEORG - Harvard University; Anderson, Raymond - Ray; COENDERS-GERRITS, A.MIRIAM - Delft University Of Technology; JUNG, MARTIN - Max Planck Institute For Biogeochemistry; MAES, WOUTER - Ghent University; MAMMARELLA, IVAN - University Of Helsinki; MAUDER, MATTHIAS - Karlsruhe Institute Of Technology; MIGLIAVACCA, MICRO - Max Planck Institute For Biogeochemistry; NELSON, JACOB - Max Planck Institute For Biogeochemistry; POYATOS, RAFAEL - Centre For Ecological Research And Forestry Applications (CREAF); REICHSTEIN, MARKUS - Max Planck Institute For Biogeochemistry; Scott, Russell - Russ; WOLF, SEBASTIAN - Swiss Federal Institute Of Technology Zurich

Submitted to: Biogeosciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/21/2019
Publication Date: 10/1/2019
Citation: Stoy, P.C., El-Madany, T.S., Fisher, J.B., Gentine, P., Gerken, T., Good, S.P., Klosterhalfen, A., Liu, S., Miralles, D.G., Perez-Priego, O., Rigden, A.J., Skaggs, T.H., Wohlfahrt, G., Anderson, R.G., Coenders-Gerrits, A.M.J., Jung, M., Maes, W.H., Mammarella, I., Mauder, M., Migliavacca, M., Nelson, J.A., Poyatos, R., Reichstein, M., Scott, R.L., Wolf, S. 2019. Review & syntheses: Turning the challenges of partitioning ecosystem evaporation and transpiration into opportunities. Biogeosciences. 16(19):3747-3775. https://doi.org/10.5194/bg-16-3747-2019.
DOI: https://doi.org/10.5194/bg-16-3747-2019

Interpretive Summary: 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, develop new syntheses of existing data and analyses, and provide a prospectus for how to improve theory and observations going forward. The research will be of interest to scientists and stakeholders concerned with the impacts of climate variability on the functioning of different global ecosystems.

Technical Abstract: Evaporation (E) and transpiration (T) respond differently to ongoing changes in climate, atmospheric composition, and land use. It is difficult to partition ecosystem-scale evapotranspiration (ET) measurements into E and T, which makes it difficult to validate satellite data and land surface models. 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 create new opportunities for partitioning E and T at the ecosystem scale, but their assumptions have yet to be fully tested. For example, many approaches to partition E and T rely on the notion that plant canopy conductance and ecosystem water use efficiency 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 more analysis is necessary to determine the conditions for which this assumption holds. Another critical assumption for many 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 can exceed 95'% of ET from certain 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 fluxes and provide the observations necessary to validate remote sensing algorithms and land surface models to understand the changing global water cycle.