## 3 Results
### The Spatial Characteristic of SIUS  
#### 3.2.1 The Spatial Distribution of SIUS for 18 Prefecture-Level Cities in Henan  
Henan Province comprises 18 prefecture-level administrative divisions. There are differences in the total amount and sectoral composition of SIUS in prefecture-level cities (See Figure S1). ZZ, the capital city of Henan Province, has the largest GDP, accounting for 21% of the province's GDP. Therefore, its SIUS is the largest, accounting for 15% of the province. In addition to ZZ, the SIUS of XX, NY, ZMD and LY also have a high share of SIUS in the province, with 8.3%, 8%, 7.5% and 7%, respectively. Including ZZ, the top five regions account for 45% of the total stock in the province. In addition, the prefecture-level city with the lowest SIUS is JY, accounting for only 1% of the province, followed by LH, with 8.85 Mt.  
Figure 4: Relationship between per capita SIUS and per capita GDP in Henan Province and other regions [13; 42].  
The spatial distribution pattern of SIUS in prefecture-level cities across Henan Province is depicted in Figure 5. From the spatial distribution pattern, it is evident that regions with high SIUS density correlate strongly with large populations and a high per capita GDP (Figure 6). For steel stock density, the SIUS density in Henan Province is 2723.68 t/km\({}^{2}\). Among all cities, ZZ ranks first in the province in GDP, making it the city with the highest SIUS density, which is 8855.35 t/km\({}^{2}\). The SIUS density of ZZ is 8.39 times that of SMX, which is the least dense city. For steel stock per capita, the per capita SIUS in Henan Province is 4.56 t/cap. Among the cities, the city with the largest per capita SIUS is JY, reaching 6.92 t/cap, mainly because JY has the smallest population in Henan Province. The city with the lowest per capita SIUS is ZK (3.24 t/cap), mainly because ZK has the lowest per capita GDP in the province. Comparing the coefficient of variation (CV) between these two data sets, the data for per capita SIUS present a CV of 22%, and the density of SIUS presents a CV of 54%. This finding indicates the spatial heterogeneity within the prefecture-level cities of Henan in terms of steel stocks' density.  
In-use stocks not only satisfy societal needs but also serve as a driving force for material recycling [40]. _China's Recycling of Waste Materials_ policy [6] emphasizes the need to optimize the industrial layout of resource recycling based on resource endowment,  
Figure 5: Spatial structures of Henan’s prefecture-level cities: (**a**) steel-in-use stocks per sq. km, (**b**) per capita SIUS.  
Figure 6: Correlation of SIUS with per capita GDP and population density.  
industrial distribution and waste characteristics. Differences in the quantity and types of scrap steel in prefecture-level cities in Henan Province are closely related to the variability in the distribution pattern of SIUS. Consequently, it is essential to consider the spatial distribution of SIUS in order to optimize the recycling layout of scrap steel resources. Both SIUS density and per capita SIUS in prefecture-level cities provide critical indicators for optimizing the future layout of scrap resources.