Abstract
The opening and closing of plant stomata regulates the global water, carbon and energy cycles. Biophysical feedbacks on climate are highly dependent on transpiration, which is mediated by vegetation phenology and plant responses to stress conditions. Here, we explore the potential of satellite observations of solar-induced chlorophyll fluorescence (SIF)—normalized by photosynthetically-active radiation (PAR)—to diagnose the ratio of transpiration to potential evaporation (‘transpiration efficiency’, τ). This potential is validated at 25 eddy-covariance sites from seven biomes worldwide. The skill of the state-of-the-art land surface models (LSMs) from the eartH2Observe project to estimate τ is also contrasted against eddy-covariance data. Despite its relatively coarse (0.5°) resolution, SIF/PAR estimates, based on data from the Global Ozone Monitoring Experiment 2 (GOME-2) and the Clouds and Earth’s Radiant Energy System (CERES), correlate to the in situ τ significantly (average inter-site correlation of 0.59), with higher correlations during growing seasons (0.64) compared to decaying periods (0.53). In addition, the skill to diagnose the variability of in situ τ demonstrated by all LSMs is on average lower, indicating the potential of SIF data to constrain the formulations of transpiration in global models via, e.g., data assimilation. Overall, SIF/PAR estimates successfully capture the effect of phenological changes and environmental stress on natural ecosystem transpiration, adequately reflecting the timing of this variability without complex parameterizations.
Highlights
Plant transpiration dominates the global flux of terrestrial evaporation and is a central element of the hydrologic cycle over land [1,2]
When correlations are grouped by IGBP biome type at the flux tower scale, the highest correlations between solar-induced chlorophyll fluorescence (SIF)/photosynthetically-active radiation (PAR) and the in situ τ data are found at densely forested sites (Figure S3)—some exceptions are further explored in Section 4, including the US-Me2 evergreen needleleaf forest site
We present a novel diagnostic of transpiration efficiency, understood here as the ability of the land surface to meet the atmospheric demand for water vapor
Summary
Plant transpiration dominates the global flux of terrestrial evaporation and is a central element of the hydrologic cycle over land [1,2]. The biophysical and biochemical feedbacks on climate depend on vegetation phenology and plant physiological response to environmental conditions and nutrient cycling [7,8,9,10] These responses vary among plant species and make the modelling of transpiration challenging [11,12,13]. Perhaps unsurprisingly, current LSMs still constrain transpiration based on empirical relationships between stomatal conductance and environmental variables such as soil moisture, temperature and vapor pressure deficit [18,19,20] These formulations are supported by limited observational evidence at global scales, and their assumptions affect the modelled heat and drought response of ecosystem transpiration, which remains uncertain in current models [7,21,22]. Two-source energy balance models utilize remotely sensed surface temperatures to constrain transpiration, but often still rely on Penman–Monteith, Priestley and Taylor, or aerodynamic formulations [31], or solve for latent heat as the residual term in the energy balance [32,33]
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