Hydrological and thermal co-benefit analysis of bioretention systems using a modified urban tethys-chloris model

Urban overheating and pluvial flooding are two critical challenges intensified by rapid urbanization and climate change. Bioretention and other urban green infrastructure (UGI) present a promising solution by enhancing infiltration and evapotranspiration processes. However, using a unified modeling framework to quantify both hydrological and thermal co-benefits of UGI remain limited. This study addresses this gap by modifying the Urban Tethys-Chloris (UT&C) model to incorporate detailed representations of bioretention systems within urban canyon environments. The enhanced model was validated using open monitoring data from a bioretention site in Toronto and reasonably reproduced overall evapotranspiration behavior, soil-moisture response, and event-scale outflow occurrence, supporting its use for scenario analysis. Scenario experiments compare bioretention-integrated and conventional canyons with identical surface landscape composition under the climates and urban geometries of Hong Kong and Toronto. Results indicate that bioretention reduces stormwater runoff by 47% (Hong Kong) and 73% (Toronto) and provides measurable surface cooling, lowering vegetated surface temperatures by 0.45–0.59°Ϲ and 0.88–1.1°Ϲ during peak summer hours, respectively. Improvements in pedestrian-level thermal comfort are modest. Sensitivity analysis shows subsurface design strongly affects runoff reduction but has limited influence on thermal regulation, while surface geometry affects both in a coupled manner; the tested width-to-depth configuration can create thermal-hydrological trade-offs in Hong Kong. Overall, benefits are evident but context-dependent, and the proposed framework supports site-specific, goal-oriented UGI design and evaluation.