Flows and connections of metabolic nodes in Beijing-Tianjin-Hebei Region and its cities: A study at two scales.

The metabolic processes of cities and their embedded regions have received great attention, but it is still unclear how the metabolic processes change at the scale from cities to urban agglomerations. In view of the lack of multi-scale research in the field of urban metabolism, this study took Beijing-Tianjin-Hebei region, one of the urban agglomerations with largest economic scales in China, as a case to construct metabolic network models at two scales of city and urban agglomeration. The material transfers between nodes were calculated, and the connection degree index was put forward in the ecological network analysis to quantify the influence of a single node on the network when multi-level transfers were considered. On this basis, the similarities and differences of metabolic nodes at the two scales were analyzed. The results showed that nearly 97% of the volume of material transfers in the urban agglomeration was concentrated within the cities, among which the transfer volumes of Tangshan, Handan, and Shijiazhuang were more than 600 Mt. Manufacturing and environment were the major contributors to material transfers. The connection degrees of nodes had both commonness and differences at the two scales. In general, the connection degrees at the urban scale were relatively homogeneous, while their difference was large at the urban agglomeration scale. The connection degrees of nodes in Langfang were prominent at the urban agglomeration scale. The connection degrees of environment and manufacturing ranked top 3 at both scales. Meanwhile, the connection degree of energy conversion at the urban scale was relatively high, while its influence was replaced by mining sector at the urban agglomeration scale. The analysis of material metabolic nodes in Beijing-Tianjin-Hebei region can provide theoretical supports to position the key points in the process of material utilization in the cities or the urban agglomeration, and help to identify the breakthrough points for subsequent regulatory.