How Variations in Photosynthetically Active Radiation Affect Vegetation Carbon–Water Coupling Processes: A Study Based on the Vegetation Microclimate Process (VMcP) Model

Vegetation physiological processes are critical regulators of terrestrial carbon–water cycles and local microclimate dynamics, with photosynthetically active radiation (PAR, 400–700 nm) serving as a primary driving force. However, most vegetation–climate process models simplify the fraction of PAR in global solar radiation as a constant 50%, potentially introducing diurnal simulation biases that propagate into cumulative annual errors in vegetation carbon–water coupling estimates. To address this limitation, we first evaluated the performance of three empirical models for simulating the dynamic PAR fraction and integrated the most accurate model into the Vegetation Microclimate Process (VMcP) model, and further used typical meteorological year (TMY) data of Beijing, Shanghai and Shenzhen as input to compare the differences in vegetation carbon–water processes before and after the improvement. The results show that the diurnal variation range of PAR fraction in global solar radiation is between 39% and 58%. The existing models that neglect the dynamic changes in PAR may overestimate vegetation transpiration cooling and photosynthetic carbon sequestration by 2.3% and 3.5%, respectively. Meanwhile, Shenzhen (64.3 W/m2; 1.59 g/m2·d), characterized by favorable light and thermal conditions, is more prone to large errors compared with Shanghai (47.6 W/m2; 1.21 g/m2·d) and Beijing (39.5 W/m2; 0.93 g/m2·d). This study provides a novel tool for the accurate assessment of vegetation-mediated microclimate improvement, and offers a new perspective for nature-based climate solutions.