Sustainable urban system structure evaluation in sparsely populated areas: case study of the Qinghai-Tibet Plateau in China.

Urbanization in sparsely populated areas is critical for sustainability. The Qinghai-Tibet Plateau is a typical example of an ecologically fragile region that plays a crucial role in China's ecological safety and water resource protection. We use a social network analysis to illustrate the structure of the urban system on the plateau and find that the agglomeration and diffusion capabilities of the core nodes are weak, which presents an obstacle to the sustainable development of the urban system. Nevertheless, we find that the intermediate nodes-which serve multiple integration functions for the various cultures, ethnic groups, and religions in the region-are core nodes that divide the dispersed cities and towns into four subgroups that function as small worlds (The cities and towns within one subgroup connect closely and each subgroup is organized independently somehow from others). Based on this finding, we suggest implementing a "double-layer" urban system to promote the sustainable urban development of the Qinghai-Tibet Plateau. This "double-layer" system breaks the usual urbanization trend, in which cities grow in size and their distribution becomes more concentrated; instead it provides a feasible way to maintain urban sustainability in a sparsely populated area.