Evaluating runoff retention and associated flood mitigation services of urban blue-green infrastructure in a mega urban centre

Urban blue-green infrastructure (BGI) plays a significant role in regulating stormwater management and mitigating urban flooding. However, the effectiveness of urban BGI in mitigating surface runoff remains insufficiently explored, particularly in densely populated, flood-prone cities. This study presents an integrated geospatial framework to evaluate the runoff retention capacity of urban BGI and their associated flood mitigation services in Kolkata City, India. The Integrated Valuation of Ecosystem Services and Trade-Offs (InVEST)–Urban Flood Risk Mitigation (UFRM) model was applied to estimate the potential runoff generation and retention under two rainfall scenarios in Kolkata City. The model incorporated land use and land cover types, hydrological soil groups, and rainfall inputs to simulate spatial runoff patterns and runoff retention potential across different urban micro-watersheds. Flood mitigation services provided by BGI were quantified in terms of runoff retention capacity and flood volume generation. The results show that urban BGI provides substantial runoff retention capacity, with tree-covered lands and herbaceous wetlands retaining 45.00%–60.00% more runoff than impervious surfaces. Spatial analysis indicates that regions with permeable surfaces and dense vegetation cover enhance infiltration and water absorption, thereby demonstrating the hydrological effectiveness of BGI. The InVEST–UFRM results were validated with Synthetic Aperture Radar-derived flood data and machine learning-based models, including Random Forest (RF) model and eXtreme Gradient Boosting (XGBoost) model, with the area under curve scores (XGBoost model: 0.73; RF model: 0.70) confirming the model’s ability to predict flood risk. This integrated framework provides a practical tool for assessing the flood mitigation potential of BGI and offers practical insights for urban planners and policy-makers seeking to improve urban flood management and support more resilient urban landscapes.