## 3 Results
### Coupling and Coordination of GSI Systems  
Figure 4 illustrates the spatial distribution of the CD across the GBA. The average CD value is 0.68, indicating a moderate level of coupling between GSI supply and demand systems. However, significant regional disparities persist. The highest CD value, 0.99, is observed in Zhonglutan Town in Baiyun District, Guangzhou, reflecting near-optimal integration between supply capacity and demand intensity. In contrast, Baokou Town in Huidong County, Huizhou, records the lowest CD (0.00047), revealing a critical decoupling likely associated with misaligned infrastructure deployment. Figure 3: Classified spatial distribution of GSI supply and demand across the GBA. (**a**) GSI supply levels. (**b**) GSI demand levels. 254 districts are categorized as having \"favorable coupling\", while 213 are identified as exhibiting \"primary coupling\". These two groups constitute the majority of the region and are concentrated in economically advanced and administratively proactive areas, including Guangzhou, Shenzhen, Foshan, Dongguan, Hong Kong, and Macao. The elevated CD values in these cities can be attributed to a combination of integrated green infrastructure systems, strategic spatial planning, and supportive policy frameworks [46]. By contrast, lower levels of coupling are predominantly found in Huizhou, Zhaoqing, and Jiangmen. Although these areas possess substantial natural ecological resources, the lack of structured urban stormwater management systems hampers their capacity to respond effectively to flood risk. The underutilization of natural assets in these regions underscores the need for enhanced institutional capacity and policy intervention to improve GSI coordination [47]. The CCD, which integrates both internal coupling and external systemic alignment, reveals further dimensions of spatial disparity. While several districts with high CD also demonstrate high CCD--reflecting consistent alignment across ecological, infrastructural, and socio-economic domains--other districts remain weakly coordinated despite favorable environmental conditions. High CCD values are mainly concentrated in the Pearl River Delta, where cities exhibit strong governance, active investment in ecological infrastructure, and synchronized planning with social services. Kowloon City, Hong Kong, ranks highest with a CCD value of 0.65, falling within the \"favorable coupling coordination\" category. The district's high population density, strong economic performance, and advanced green infrastructure contribute to its efficient and sustainable GSI system. Conversely, Baokou Town again registers the lowest CCD (0.008), placing it in the \"extreme decoupling co  
Figure 4: Spatial patterns of GSI coupling and coordination in the GBA. (**a**) District-level classification of CD (Coupling Degree); (**b**) proportional distribution of CD levels; (**c**) district-level classification of CCD (Coupled Coordination Degree); and (**d**) proportional distribution of CCD levels. ordination\" category. Despite a rich endowment of green space and water bodies, low population density and limited development result in minimal demand for GSI services, leading to inefficiency and underutilization of infrastructure. Mild to severe decoupling coordination is prevalent in peripheral areas such as Zhaoqing, Huizhou, and Jiangmen. These locations, although ecologically endowed, face systemic weaknesses in aligning GSI deployment with socio-economic needs. To further delineate spatial mismatches, a quadrant analysis was conducted by combining CD and CCD classifications. This approach categorizes districts into four types based on their coordination profiles: (I) Coupled and Coordinated, (II) Not Coupled but Coordinated, (III) Not Coupled and Not Coordinated, and (IV) Coupled but Not Coordinated (see Figure 5). Figure 5: Combined classification of GSI coupling and coordination across districts.