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

Stoy, Paul C.; El-Madany, Tarek S.; Fisher, Joshua B.; Gentine, Pierre; Gerken, Tobias; Good, Stephen P.; Klosterhalfen, Anne; Liu, Shuguang; Miralles, Diego G.; Perez-Priego, Oscar; Rigden, Angela J.; Skaggs, Todd H.; Wohlfahrt, Georg; Anderson, Ray G.; Coenders-Gerrits, A. Miriam J.; Jung, Martin; Maes, Wouter H.; Mammarella, Ivan; Mauder, Matthias; Migliavacca, Mirco; Nelson, Jacob A.; Poyatos, Rafael; Reichstein, Markus; Scott, Russell L.; Wolf, Sebastian

Evaporation (inline-formulaE) and transpiration (inline-formulaT) respond differently to ongoing changes in climate, atmospheric composition, and land use. It is difficult to partition ecosystem-scale evapotranspiration (ET) measurements into inline-formulaE and inline-formulaT, which makes it difficult to validate satellite data and land surface models. Here, we review current progress in partitioning inline-formulaE and inline-formulaT and provide a prospectus for how to improve theory and observations going forward. Recent advancements in analytical techniques create new opportunities for partitioning inline-formulaE and inline-formulaT at the ecosystem scale, but their assumptions have yet to be fully tested. For example, many approaches to partition inline-formulaE and inline-formulaT rely on the notion that plant canopy conductance and ecosystem water use efficiency exhibit optimal responses to atmospheric vapor pressure deficit (inline-formulaD). We use observations from 240 eddy covariance flux towers to demonstrate that optimal ecosystem response to inline-formulaD 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 inline-formulaT during ideal transpiring conditions, which has been challenged by observational studies. We demonstrate that inline-formulaT 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 inline-formulaE and inline-formulaT, 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 inline-formulaT 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.

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Stoy, Paul C. / El-Madany, Tarek S. / Fisher, Joshua B. / et al: Reviews and syntheses: Turning the challenges of partitioning ecosystem evaporation and transpiration into opportunities. 2019. Copernicus Publications.

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