## 3 Results  
### Supply and Demand Levels of GSI  
Figure 3 presents the spatial distribution of GSI supply and demand indicators across the GBA. The GSI supply indicators are conceptually divided into two categories: natural ecological services (e.g., vegetation cover and waterway networks) and artificial ecological services (e.g., parks and permeable pavements). This distinction facilitates a more nuanced interpretation of how different landscape components contribute to stormwater management. The spatial patterns of these indicators reveal substantial heterogeneity among the urban districts (Figure S1). In general, peripheral cities such as Zhaoqing and Huizhou demonstrate stronger capacities for natural ecological services, largely due to the presence of preserved green space and water bodies. Conversely, core cities like Guangzhou, Shenzhen, and Hong Kong feature more developed artificial infrastructure, including urban green parks and engineered drainage systems, reflective of their economic advancement (Figure S1a-e). These observations differ somewhat from an earlier study, which categorized natural GSI based on ecological land types (e.g., forests and wetlands), and artificial GSI based on constructed features such as green roofs and bioswales [39].  
Demand-side indicators exhibit similar spatial clustering. High GSI demand is concentrated in the southeast and central urban zones of the GBA, including Guangzhou, Shenzhen, Hong Kong, and Dongguan. These areas face elevated flood risk due to high impervious surface ratios, aging drainage systems, and dense populations (Figure S1f-p). Districts rich in cultural heritage assets, such as historic architecture and protected landmarks, also exhibit heightened vulnerability and consequently a stronger demand for GSI interventions [27; 45].  
Figure 3 depicts the classified spatial distribution of GSI supply and demand across the region. Only nine districts exhibit a high level of GSI supply, all located in Hong Kong. In contrast, the majority of districts--particularly in Guangzhou (74%), Jiangmen (74%), and Shenzhen (46%)--fall into low or moderate supply categories. Demand patterns follow a similar trend; while 163 districts register high demand, 479 are categorized as having low to moderate demand. Notably, high-demand districts are concentrated in Guangzhou (56%), Shenzhen (31%), Foshan (41%), and Macao (50%), whereas low-demand zones are prominent in Huizhou (74%), Jiangmen (78%), Zhaoqing (89%), and Zhuhai (54%). Figure 3 further highlights the extent of spatial mismatches. In locations where supply exceeds demand, resources may be underutilized, pointing to inefficiencies in planning and investment. Conversely, districts with high demand but insufficient supply--such as urban cores--face compounded flood risk and infrastructure strain. This imbalance underscores the need for targeted GSI expansion in high-demand areas, particularly those with insufficient green coverage and vulnerable populations.  
Across the GBA, the highest observed GSI supply score (0.53) is in Kowloon, Hong Kong, reflecting its proactive planning and strong institutional support. The lowest (0.076) is found in Da'ao, Jiangmen, likely due to limited technical capacity and lack of investment. For demand, the peak value (0.60) is recorded in Guangta Street, Yuexiu District, Guangzhou, a densely populated area with scarce green space. The lowest demand (0.002) is found in Baokou Town, Huizhou, which benefits from abundant vegetation and natural drainage capacity.