Abstract
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.
Highlights
Some 70 000 km3 of water leaves terrestrial ecosystems and enters the atmosphere through evapotranspiration (ET) every year (Jung et al, 2019; Oki and Kanae, 2006)
Along with technological and data limitations, we argue that a fundamental challenge in modeling ET at the global scale is difficulty measuring transpiration (T ) through plant stomata and evaporation (E) from non-stomatal surfaces at the ecosystem scale (Fisher et al, 2017; McCabe et al, 2017)
T / ET is sensitive to ecosystem characteristics, namely the leaf area index (LAI) (Berkelhammer et al, 2016; Fatichi and Pappas, 2017; Wang et al, 2014; Wei et al, 2015), especially on sub-annual timescales (Li et al, 2019; Scott and Biederman, 2017), noting that LAI is related to P at longer timescales
Summary
Some 70 000 km of water leaves terrestrial ecosystems and enters the atmosphere through evapotranspiration (ET) every year (Jung et al, 2019; Oki and Kanae, 2006). LSMs struggle to simulate the magnitude and/or seasonality of ET at the ecosystem scale (Fig. 1), suggesting fundamental gaps in our understanding of the terrestrial water cycle. These issues need to be resolved to effectively manage water resources as climate continues to change (Dolman et al, 2014; Fisher et al, 2017). Interest in partitioning E and T from ecosystem ET measurements has grown in recent years (Anderson et al, 2017b), and many new measurements and modeling approaches seek to do so but often rely on assumptions that need further testing. We finish with an outlook of how carefully designed ecosystem-scale experiments can constrain models of E and T to improve our understanding going forward
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