Quantifying non-additive synergy in Blue–Green–grey Infrastructure for adaptive urban water resilience: A case study of Zixing City, China

Study region This study was conducted in the central urban area and northern ecological zone of Zixing City, Hunan Province, China. This medium-sized mountainous urban catchment is affected by rapid urbanization, wetland degradation, impervious surface expansion, and fragmented hydrological connectivity, leading to reduced stormwater regulation capacity and increased water-quality risks. Study focus This study examined whether integrated Blue-Green-Grey Infrastructure (BGGI) can generate non-additive synergistic effects under different rainfall conditions and clarified the underlying mechanisms. A coupled SWMM-based hydrological and water-quality simulation and graph-theoretic network analysis framework was used to evaluate constructed wetlands (CWs), low-impact development (LID), and hydrological connectivity enhancement (HYCE). Eight scenarios, including baseline, single-measure, dual-measure, and fully integrated configurations, were compared under three 120-min design rainfall events. Rather than treating integrated performance as a general combination effect, this study quantified non-additive synergy against a multiplicative expectation derived from independent single-measure effects. Dummy variable regression, two-way ANOVA, graph-theoretic analysis, and sensitivity tests were used to identify rainfall-dependent functional roles, assess the mediating role of structural connectivity, and test robustness. New hydrological insights for the region The fully integrated scenario outperformed all single- and dual-measure scenarios in both runoff regulation and pollutant reduction. Compared with the theoretical combined expectation, integrated BGGI showed positive non-additive synergistic effects across all rainfall scenarios and indicators, with absolute synergistic gains of 8.41–56.16 %age points and relative synergistic gains of 16.73 %–183.71 %. Peak-flow reduction showed stable synergistic gains across rainfall intensities, whereas TN and TP achieved higher relative gains under low-intensity rainfall. These results indicate that integrated BGGI performance is not a simple additive outcome of individual measures, but arises from functional complementarity among LID-based source control, HYCE-mediated runoff routing and hydraulic redistribution, and CW-based downstream retention and purification. Sensitivity tests confirmed that the main conclusions were not dominated by any single scenario parameter. This study provides a mechanism-oriented framework for distinguishing non-additive synergy from general combination effects and supports rainfall-adaptive, network-coordinated BGGI planning.