Blue-Green Factor Mapping in QGIS for Climate-Resilient Urban Water Management

Rapid urbanisation and climate change continue to increase impervious surface cover in cities, intensifying stormwater runoff, flooding and pollution while reducing natural infiltration. Blue-green infrastructure (BGI) integrates vegetated and water-based elements into urban planning to restore hydrological function and build climate resilience. This study examines the Blue-Green Factor (BGF), an area-weighted indicator of blue-green performance developed in Norway in 2014, situates it within the international lineage of performance-based green area indicators (Berlin's Biotope Area Factor and the Malmö, Stockholm, Helsinki, Seattle and Oslo variants), and evaluates its transition from a spreadsheet-based, property-level tool to a Geographic Information System (GIS)-based workflow in QGIS. The study combines three explicitly separated components: (A) a narrative review of the literature on sustainable urban drainage, nature-based solutions, green area indicators and GIS-based planning; (B) a reproducible demonstration of the BGF-QGIS workflow on the openly documented reference locality of the tool (Ås, Norway), reporting the calculation procedure, the default weighting factors and the interpretation of the resulting score of 0.871; and (C) a document analysis of Bratislava's greenery governance and regulation to assess transferability. The results show that GIS-based BGF mapping improves spatial visualisation, multi-layer data integration and scalability relative to the original approach. A BGF score is a dimensionless weighted ratio that can exceed 1 and is meaningful only relative to thresholds defined within the same scheme (for example, Oslo's residential minimum of 0.7–0.8), while comparative and cost-effectiveness evidence cautions that BGF-type weightings can favour low-cost, lower-performing measures. In Bratislava, the quantity-based green-area coefficient is being complemented by the Ecoindex developed by the Metropolitan Institute of Bratislava, which creates a clear policy niche for BGF-type spatial assessment. The study concludes that BGF-QGIS offers a transparent screening tool for climate-resilient urban water management, provided that weights are locally calibrated, the composition of each score is reported, and the workflow is applied to Bratislava localities in future work.