Title: From Runoff to Resilience: Exploring Multifunctional Nature-Based Solutions for Sustainable Urban Stormwater Management

Climate change and rapid urbanization intensify stormwater-related challenges in cities, particularly in compact urban environments. Traditional grey infrastructure often fails to address these risks in a flexible and adaptive manner. Nature-based solutions (NbS) offer a multifunctional and resilient complement to traditional grey infrastructure. This study presents a Social-Ecological-Technological Systems (SETS)-framed methodology for evaluating urban risks, feasibility, and the multifunctional performance of NbS at the city scale. Using a GIS-based framework, we assess NbS feasibility across social, ecological, and technological domains, simulate stormwater retention under different scenarios, and quantify co-benefits including heat mitigation, water storage, water quality, habitat provisioning, and recreation. Applied to Barcelona, the study finds that implementing NbS (including green roofs, rain gardens, urban parks, and permeable pavements) over 160 hectares in Scenario 1 (S1), aligned with the city’s greening strategy, and 2,498 hectares in Scenario 2 (S2), which maximizes NbS feasibility, could reduce city-scale flood volume by up to 4.6% for T1 events, increase water storage capacity by 43%, improve habitat quality by 36%, and reduce the proportion of the population underserved by urban nature by nearly 50%, compared to the current land use and land cover (S0). While the reduction in runoff volume is moderate, especially for high-intensity storm events, our findings highlight the substantial additional value of NbS through the provisioning of co-benefits and risk reduction for vulnerable urban communities. Although the assumptions and simplifications of the numerical models used in this study may influence the results, our findings underscore the importance of integrating NbS not only as technical solutions for stormwater management but also as strategic tools for enhancing urban resilience, equity, and climate adaptation, while unlocking their transformative potential to reconfigure urban systems towards more sustainable and inclusive futures. • A SETS-based GIS framework evaluates NbS needs, feasibility, and performance • Feasibility analysis shows potential to expand green area up to ninefold • NbS assessed for runoff mitigation and five socio-ecological co-benefits • In Barcelona, runoff reduction is modest, but co-benefits show strong value • The proposed framework is transferable to other urban contexts