Interface-Scale Synergy of Blue–Green Infrastructure Across Contrasting Urban Fabrics: Multi-Year Landsat Evidence and Hydrological Scenario Modelling in Beijing

High-density urban renewal requires blue–green infrastructure (BGI) strategies that address surface warming and stormwater runoff under land constraints. This study examined how BGI area and interface configuration regulate water–thermal performance across three purposively selected 100 ha urban fabrics in Beijing. Multi-year Landsat observations and matched 30 m land-cover data quantified radiometric land-surface-temperature (LST) contrasts and distance gradients, while a transparent Python event-runoff model evaluated relative hydrological scenarios using wet-day rainfall-depth quantiles, storage, impervious-area capture, and terrain-informed connectivity. The evidence streams were compared using the scenario cooling–storage index (SCSI), descriptive half-response area (A50), and interface–area substitution ratio (SAR). Stable surface-cooling contrasts occurred in Xicheng and Daxing; Chaoyang supported only exploratory thermal comparison because mapped BGI was sparse. Hydrological outputs represented configuration-dependent simulated runoff-volume responses, not calibrated predictions. In equal-weight 100 ha analyses, bootstrap intervals supported SAR above one across 1–10 ha in Xicheng and 1–3 ha in Daxing, while nested 50 ha windows and thermal weights of 0.2–0.8 shifted transition ranges. Corridor A50 was not identifiable within the tested 1–10 ha range in Daxing, so corridor saturation remains unresolved. Interface compensation was therefore morphology-, scale-, and weighting-dependent. The framework enables conditional comparison of BGI configurations while preserving the distinction between observed thermal and scenario-based hydrological evidence.