Impact of diffuse radiation on evapotranspiration and its coupling to carbon fluxes at global FLUXNET sites
Graphical abstract
This study presents the impacts of diffuse radiation on ecosystem evapotranspiration (ET) and its coupling to net ecosystem exchange (NEE) using a long-term eddy-covariance observations and the derived diffuse radiation fraction (Kd) at 201 FLUXNET sties. We find that diffuse radiation is more effective in promoting ET than direct radiation, as the diffuse fertilization effect (DFE) on NEE tightly regulates the ET through stomatal coupling. The increase of ET leads to a higher evaporative fraction under the shaded conditions with more diffuse radiation. We emphasize the importance of considering the DFE on ecosystem ET in assessing the aerosol-induced perturbations in water cycle under the current and future climate.
Introduction
Solar radiation is one of the major factors driving terrestrial evapotranspiration (ET). Over 60% of the solar radiation received by the land surface is used for ET (Kiehl and Trenberth, 1997). Most of this energy was allocated to plant transpiration for the majority of ecosystems (Schlesinger and Jasechko, 2014; Berkelhammer et al., 2016; Fatichi and Pappas, 2017). Compared to direct radiation, diffuse radiation is more efficient in improving plant physiological processes such as photosynthesis (Gu et al., 2002; Mercado et al., 2009; Cheng et al., 2015; Yue and Unger, 2017; Wang et al., 2018; Zhou et al., 2021b). It is well known that photosynthesis and transpiration are tightly coupled through the regulation of leaf stomata (Jarvis and McNaughton, 1986; Medlyn et al., 2011; McElrone et al., 2013). This coupling indicates that not only the quantity of radiation but also its composition can affect terrestrial ET.
The high efficiency of diffuse radiation in promoting plant photosynthesis is mainly because diffuse beams can penetrate into the canopy and illuminate shaded leaves, therefore improving the light use efficiency of the whole canopy (usually called the diffuse fertilization effect, DFE) (Gu et al., 2003; Mercado et al., 2009; Yue and Unger, 2017; Wang et al., 2018). Consequently, changes in diffuse radiation can alter ET, as stomatal conductance is enhanced following increases in photosynthesis by DFE (Knohl and Baldocchi, 2008). To date, the impact of DFE on ecosystem carbon fluxes has been widely explored (Gu et al., 1999; Niyogi et al., 2004; Jing et al., 2010; Cirino et al., 2014), but its impacts on ET are much less assessed (Wang et al., 2021; Zhou et al., 2021a).
ET is mainly determined by the quantity of input energy. Previous studies have shown that ecosystem ET significantly decreases due to reductions in total radiation by aerosols and/or clouds (Ramanathan et al., 2001; Rocha et al., 2004; Steiner et al., 2013; Liu et al., 2014). However, physiological modeling shows that transpiration can be enhanced by the increase in diffuse radiation because stomatal conductance is stimulated by increased photosynthesis (Steiner and Chameides, 2005; Knohl and Baldocchi, 2008). Such enhancement of diffuse radiation on ET was also estimated on the global scale but occurred only at low aerosol loading conditions with a small diffuse fraction (Kd, the fraction of diffuse to total radiation) (Lu et al., 2017), because covarying factors such as reductions in total radiation and vapor pressure deficit (VPD) can jointly suppress ET (Wang et al., 2021). Although it has been recognized that diffuse radiation plays an important role in ecosystem water flux (Zhang et al., 2021; Zhou et al., 2021a), the observed evidence is still lacking at the global scale, and key processes remain unclear.
Under high Kd conditions, ET is observed to decrease because latent heat flux (LE) is decreased to balance the reductions in the incident radiation (Murthy et al., 2014; Latha et al., 2018). Although both LE and sensible heat (H) fluxes decrease simultaneously, the reduction rate of LE is significantly smaller than that of H, leading to an increase in the evaporative fraction (EF, calculated as LE/(LE+H)) (Wang et al., 2008; Liu et al., 2014). Such enhancement of EF may be attributed to the increase in canopy transpiration, which buffers the reductions in ET (Lu et al., 2017; Sarangi et al., 2021). However, the actual impacts of diffuse radiation on EF remain less quantified due to the lack of observations, especially on the global scale.
In this study, we quantify the responses of ET to diffuse radiation on the global scale based on long-term observations of carbon and water fluxes at 201 FLUXNET sites and take advantage of a gap-filled dataset of diffuse fraction (Kd) for the same field network (Zhou et al., 2021c). To reveal the underlying mechanisms, we explored the impacts of diffuse radiation on the coupling of water and carbon fluxes by comparing the responses of NEE (net ecosystem exchange) and ET to diffuse radiation for different plant functional types (PFTs). We further examined the contributions of DFE to energy partitioning through changes in EF.
Section snippets
FLUXNET data
Site-level water and CO2 flux data were used from FLUXNET (http://fluxnet.fluxdata.org/), which is a worldwide network measuring ecosystem fluxes by the eddy covariance method from 1991 at half-hourly or hourly time intervals at more than 200 sites (Baldocchi et al., 2001; Sun et al., 2006). The network also provides simultaneous measurements of environmental variables such as air temperature (K), relative humidity (%), incoming shortwave radiation (SW, W m−2), photosynthetically active
Spatial patterns of ET and Kd
The ET at the FLUXNET sites showed a downward trend as the latitude increases (Fig. 1a). This tendency was consistent with the pattern of incoming shortwave radiation, which decreases from low to high latitudes. In contrast, Kd was relatively higher at middle and high latitudes and lower in tropical and subtropical areas (Fig. 1b). However, the spatial correlation coefficient between ET and Kd was only -0.17 (p<0.05), with the largest correlation of -0.26 (p<0.001) in the mid-high latitudes
Discussion
Terrestrial ET is mainly controlled by the physical energy input from incident solar radiation, as well as the physiological regulation of plant stomata (Sarangi et al., 2021).
As concluded in Fig. 7, ET should present a similar response to diffuse and direct radiation when only physical control of energy input was considered (Fig. 7a) because the two types of solar radiation are almost consistent in their energy attributes at a certain level. However, since diffuse radiation distributes light
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The research in this paper is funded by Jiangsu Science Fund for Distinguished Young Scholars (grant no. BK20200040). This work used eddy covariance data acquired and shared by the FLUXNET community, including the following networks: AmeriFlux, AfriFlux, AsiaFlux, CarboAfrica, CarboEuropeIP, CarboItaly, CarboMont, ChinaFlux, Fluxnet-Canada, GreenGrass, ICOS, KoFlux, LBA, NECC, OzFlux-TERN, TCOS-Siberia and USCCC. The ERA-Interim reanalysis data were provided by ECMWF and processed by LSCE. The
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