Modeling the effects of vegetation dynamics on the hydrological performance of a bioretention system

Bioretention systems are widely used to mitigate the adverse effects of urbanization on the hydrological cycle. Vegetation plays a key role in the hydrological performance of bioretention systems; however, few model studies have focused on the effects of vegetation dynamics on performance. In this study, we propose an ecohydrological model of bioretention systems by coupling a vegetation growth model with a hydrological model. The eco-hydrological model was verified using 2-year observed data on vegetation coverage, soil moisture, water depths in the gravel layer, and outflow rates of a bioretention system in Shenzhen, China. Based on the validated model, scenarios were designed to simulate seasonal or interannual changes in vegetation coverage and evaluate their influence on the hydrological performance of the bioretention system. The results indicate that (i) the model is able to explicitly describe vegetation dynamics and hydrological processes of the system, and the Nash Sutcliffe Efficiency of vegetation coverage, soil moisture, water depth in the gravel layer, and outflow rates range from 0.6 to 0.9; (ii) vegetation coverage and evapotranspiration volume are moderately or highly sensitive to most parameters of the vegetation module; the volume of exfiltration into the surrounding soil and outflow are sensitive to three parameters (potential maximum vegetation coverage (fcmax), potential heat units required for maturation (PHU), and accumulated heat units at the beginning of vegetation coverage decline (HUpoint)) in the months with a long antecedent dry period; and (iii) the seasonal and interannual variation in vegetation mainly affects soil moisture and evapotranspiration; in addition, the variations affect exfiltration and outflow for rainfall events with a long antecedent dry period (>9 d). Therefore, an eco-hydrological model that considers the influence of vegetation dynamics can be used to evaluate the long-term hydrological performance of bioretention systems. This study investigated the performance of one species (Canna indica L.). Further studies are necessary to prove the adaptability of the model to bioretention systems with other vegetation types in different climatic zones.