Can Blue-Green Infrastructure Used For Stormwater Management Mitigate Urban Heat?

Due to their multifunctionality, blue-green infrastructure (BGI) such as bioretention cells and green roofs are increasingly adopted to manage stormwater and mitigate urban heat. Despite their multifunctional potential, current studies simulating BGI benefits tend to focus on a single objective, often overlooking how the proposed designs would perform across multiple functions. As a result, the heat mitigation potential of stormwater-focused BGI is not yet well understood.The goal of this study is to assess the impact that BGI primarily used for stormwater management, such as bioretention cells, porous pavements, and green roofs have on 2 m air temperature during the hottest hours of the day. To do so, we employ a microclimate model (Urban Tethys-Chloris, UT&C) to simulate over 20 BGI scenarios in three street canyon types—urban, residential, and industrial - in a town near Zurich, Switzerland. We also explore how properties affecting the stormwater management (e.g., variations in coverage, vegetation types, and soil properties) can alter canyon temperatures. Using measurements collected during the summer of 2024, the model was calibrated and validated (RMSE of 2.2°C and r2 of 0.84).Results show that BGI elements replacing impervious surfaces on the ground provide the greatest cooling effects (0.4 to 1°C of cooling). For example, bioretention cells replacing impervious surfaces achieved a temperature reduction of up to 1°C in urban street canyons. Porous pavements, though without vegetation, also contribute to cooling by allowing stormwater infiltration and direct evaporation, reducing temperatures by an average of 0.4°C. In contrast, replacing existing vegetation with bioretention cells slightly increased temperatures, likely due to soil properties that improve stormwater infiltration, resulting in drier topsoil layers and reduced evaporative cooling. Green roofs had negligible impact on 2m air temperature, likely because their cooling effect did not extend far enough to influence the street canyon. Sensitivity analysis demonstrated that dense vegetation, characterized by high albedo, a large leaf area index, and high evapotranspiration capacity, notably lowers temperatures compared to sparse vegetation with low albedo and limited evapotranspiration. Future work will assess how these results change under different scenarios, including with other types of BGI related to stormwater management, irrigation schemes, and in a future, more extreme climate. Overall, this work offers a deeper understanding of multifunctional BGI designs, highlighting potential trade-offs between stormwater management and heat reduction. By addressing these complexities, it supports a more holistic integration of BGI benefits in urban planning strategies.