Integration of the vegetation phenology module improves ecohydrological simulation by the SWAT-Carbon model

Abstract. Vegetation phenology and hydrological cycles closely interact from leaf and species levels to watershed and global scales. As one of the most sensitive biological indicators of climate change, vegetation phenology is essential to be simulated accurately in hydrological models. Although the Soil and Water Assessment Tool (SWAT) has been widely used for estimating hydrological cycles, its lack of integration with the phenology module has led to substantial uncertainties. In this study, we developed a process-based vegetation phenology module and coupled it with the SWAT-Carbon model to investigate the effects of vegetation dynamics on runoff in the upper reaches of the Jinsha River watershed in China. The modified SWAT-Carbon model showed reasonable performance in phenology simulation, with root mean square error (RMSE) of 9.89 d for the start of season (SOS) and 7.51 d for the end of season (EOS). Simulations of both vegetation dynamics and runoff were also substantially improved compared to the original model. Specifically, the simulation of leaf area index significantly improved with the coefficient of determination (R2) increasing by 0.62, the Nash–Sutcliffe efficiency (NSE) increasing by 2.45, and the absolute percent bias (PBIAS) decreasing by 69.0 % on average. Additionally, daily runoff simulation also showed notable improvement, particularly in June and October, with R2 rising by 0.22 and NSE rising by 0.43 on average. Our findings highlight the importance of integrating vegetation phenology into hydrological models to enhance modeling performance.