Abstract：The rapid urbanization and urban sprawl in China require a scientific understanding of complex urban growth patterns and processes.As urban evolution involves various actors, different patterns and process, complex dynamics, and multi-scale influences, the authors argue that scientific understanding must be based on elaborated complexity theory and a multidisciplinary framework. This paper first proposes a conceptual model for defining urban evolution. It is supposed that urban evolution occurred in a specific period from time t1 to t2; apparently the evolution of urban is closely related to three systems: Planned Urban System, Developed Urban System, and Developable Non-Urban System. Planned Urban System is a spatial and conceptual system that results from a spatial planning scheme, preparing organized space and activities for urban evolution in the future. Developed Urban System is a highly complex social and economic system, offering current activities rather than space for urban evolution to come. Developable Non-Urban System is a typical physical and ecological system, primarily providing possible opportunities and potential for urban evolution in space. Urban evolution can be defined as a system resulting from the complex dynamic interactions among the three systems.
Secondly, this paper analyses the complex interaction of urban evolution by using the complex hierarchy theory. The complex interaction of urban system can be summarized as five interweaving levels: policy, actor, behavior, process and pattern.In the terms of hierarchy theory, understanding a single level must consider its lower and upper levels as they are comparatively closely linked. Then, the complexity characters of urban evolution such as heterogeneous composition, temporal complexity, spatial complexity and decision-making complexity are distinguished as separate domains.
In the end, this paper discusses the complex laws of urban evolution which involve the dependence of initial values: the different initial states of cities will evolve to different results although they follow the same rule; the dominance of evolution rules: if the power of evolution rules are strong enough, different urban states will evolve to the similar ones; the irreversibility of evolution process: once a city has evolved to a relatively stable state, it will be very hard to change it to the initial state even then the driving force which dominants such evolution process has disappeared; the fluctuation of evolution pattern: the evolution pattern of cities is analogical to the pattern of water when a stone is thrown in it; and temporal-spatial periodicity of evolution: the economic periodicity of cities lead to the seasonal evolution of urban spatial structures and forms; and the uncertainty of evolution path: the economic, social, technological and natural power, etc., will lead to the uncertain evolution paths of cities.
