Impact of diffuse radiation on evapotranspiration and its coupling to carbon fluxes at global FLUXNET sites

Abstract

Diffuse radiation is widely known for its higher light use efficiency in ecosystem carbon uptake than direct radiation, which inevitably alters ecosystem evapotranspiration (ET), as plant carbon uptake and water use are closely coupled through leaf stomata. However, although the impact of diffuse radiation on ecosystem carbon uptake has been extensively explored, its impact on ecosystem ET remains unclear on the global scale and across different plant functional types (PFTs). In this study, we explored the impacts of diffuse radiation on ecosystem ET and its coupling to net ecosystem exchange (NEE) based on long-term eddy-covariance observations and the derived diffuse radiation fraction (Kd) at 201 FLUXNET stations. We found that the increase in diffuse radiation results in a net enhancement of ET when Kd was below 0.42-0.48 for most PFTs. Diffuse radiation was more effective in promoting ET than direct radiation because the diffuse fertilization effect (DFE) on NEE tightly regulates ET through stomatal coupling. On average, the efficiency of diffuse radiation (Kd > 0.8) was 1.51 (±0.80) times higher than that of direct radiation (Kd < 0.2) in promoting ET when the PAR level was less than 300 W m−2, which was lower than the ratio of 2.40 (±0.93) for NEE under the same conditions, because the simultaneous reductions in vapor pressure deficit (VPD) in part offset the enhancement of ET. Consequently, the increase in ET results in a higher evaporative fraction under shaded conditions with more diffuse radiation. We emphasized the importance of considering the DFE on ecosystem ET in assessing aerosol-induced perturbations in the water cycle under the current and future climate.