## 1 Introduction  
Urban agglomerations--characterized by high building density and strong inter-city connectivity--are particularly vulnerable to urban flooding due to extensive impervious surfaces and intensified land use, which significantly increase stormwater runoff volumes [1, 2]. Uncoordinated development has historically resulted in the proliferation of impervious surfaces, exacerbating stormwater runoff and impairing natural hydrological processes. These disruptions significantly affect the social, economic, and ecological stability of urban environments [3, 4, 5]. To address these challenges, various decentralizedstormwater management approaches have been adopted globally. Notable examples include the United States' Low Impact Development (LID) practices [6]--which emphasize site-level techniques to mimic natural runoff patterns--and Japan's Water Balancing Hydrological System. In recent years, these concepts have evolved into the broader framework of Green Stormwater Infrastructure (GSI), which builds on LID principles while incorporating additional ecological and social co-benefits such as habitat creation, temperature regulation, and enhanced urban amenity. In China, the \"Sponge City\" initiative integrates green, gray, and blue infrastructure for sustainable flood mitigation, with cities like Guangzhou and Shantou serving as demonstration areas for implementing GSI [7; 8]. GSI refers to nature-based infrastructure solutions--such as green roofs, rain gardens, permeable pavements, and vegetated swales--that facilitate the retention, infiltration, and filtration of stormwater at the source. Beyond hydrological benefits, GSI contributes to biodiversity conservation, urban heat island mitigation, and improved public health and livability [9]. However, several implementation barriers persist, including spatial constraints, cost limitations, variability in soil permeability, and public resistance [10; 11]. A critical barrier to effective GSI deployment is the spatial mismatch between supply and demand. In this study, GSI supply is defined as the environmental and infrastructural capacity of a district to deliver stormwater management services--captured by metrics such as green coverage, permeable surfaces, and waterway density. In contrast, GSI demand refers to the socio-hydrological need for such services, determined by factors including flood risk exposure, population vulnerability, and urban development intensity [12; 13]. Equitable alignment between supply and demand is essential to ensure efficient infrastructure performance and social resilience. Yet a universally accepted framework to quantify and spatially evaluate this balance remains lacking, especially at sub-city or district scales. The Greater Bay Area (GBA) of China--a megalopolis comprising 11 cities and 642 administrative districts--is a globally significant economic zone that faces acute hydrological stress (Figure 1). Geographically situated in a subtropical monsoon climate, the GBA is frequently subjected to extreme precipitation, typhoons, and seasonal storms. These meteorological extremes, coupled with rapid urbanization and extensive land development, have significantly reduced the region's natural absorption capacity. Large swathes of impervious surfaces--especially in urban cores such as Guangzhou, Shenzhen, and Hong Kong--prevent effective infiltration, leading to high surface runoff volumes and increased urban flood vulnerability. Historical events such as Typhoon Mangkhut in 2018 and the intense rainstorms of 2022 have highlighted these vulnerabilities through widespread inundations and infrastructure disruptions. At the same time, the region exhibits several favorable conditions for the implementation of GSI, including policy support under China's \"Sponge City\" initiative, an expanding urban ecological network in peripheral zones, and ongoing investment in integrated water management.