城乡界面动态模型研究

城乡界面在地域上处于城市和乡村交界处,在功能上起到城乡要素交流的空间载体作用,其发展对城乡协调意义重大。基于系统动力学,从要素流的角度构建城乡界面的理论框架,通过构建数量模型来刻画城乡界面动态演化机制。模型求解结果显示：① 在不同发展阶段,城乡界面受到内生动力和外生动力不同程度的影响。② 随着城乡界面的发展,其效应由负效应向正效应过渡,效应大小取决于其循环积累能力大小。③ 平衡态受城乡要素交换率、自身内生动力和循环积累能力的共同影响;城乡界面的平衡态是恒定的,但具有空间尺度效应。④ 开关模型刻画了界门对城市系统和乡村系统的作用机制。据此,分别从时间尺度和空间尺度对中国城乡系统的发展提出政策建议。

Research on dynamic model of urban-rural interface

The spatial location of the urban-rural interface (URI) is at the junction of urban and rural areas. In function, URI is the spatial carrier of communication for urban and rural elements. The development of URI is of great significance to the coordination of urban and rural areas. First, from the perspective of elements flow, this research established a theoretical framework of URI, which is based on system dynamics, to extend the urban and rural system to a three-body framework: urban area-URI-rural area. In this framework, the structure of urban and rural system is composed of three subsystems including urban system, URI system and rural system. And among them, URI system has its unique structure and function. From this meaning, URI consists of three parts including boundary wall, interface and boundary gate. For its function, it mainly plays the role of defense and exchange in the urban and rural system. The formation mechanism of URI reflects the forces of urban and rural systems to URI, and the interaction mechanism reflects the reaction of URI to urban and rural systems. Second, on the basis of the theoretical model, we built a quantitative model to characterize the dynamic evolution mechanism of URI by using methods of quantitative geography. The model solution shows that: (1) In different stages of development, URI is affected by these different degrees of endogenous and exogenous dynamic power. In early stage of URI's formation, the development power is mainly derived from endogenous dynamics. With the development of URI, exogenous dynamics increased gradually, and functioned with endogenous dynamics. Moreover, the gap between two dynamics is narrowed and the development of URI is accelerated. After URI achieves the equilibrium, exogenous dynamics reduce gradually and endogenous dynamics become the main driving force. (2) With the development of URI, the effects transited from negative to positive. The value of effects depends on the cyclic accumulation capacity of URI itself. In early stage of URI's formation, the ability of cyclic accumulation is weak, so the effect of URI is minor, and may even be negative. With the increasing ability of the cyclic accumulation, the negative effect transfer to positive one and increase gradually. (3) The balancing condition of URI is mutually affected by the exchange rate, endogenous dynamics and cyclic accumulation ability of urban-rural elements. The equilibrium of a certain URI is stable and has spatial scale effects. According to the research on the stochastic dynamic model, we found that the random noise term would influence the speed of equilibrium state of certain URI, instead of the equilibrium state itself, which proved the argument that the equilibrium state of a certain URI is stable. Equilibrium solutions of the dynamic model further characterize that its equilibrium state is not exactly equal at different spatial scales. (4) The switch model depicts the function of boundary gate, and the interface equilibrium is the critical point to decide whether we switch the gate open or closed. When the boundary gate is open or closed depends on whether URI reaches the critical value of its equilibrium state. According to the above conclusions, the paper puts forward policy recommendations respectively from temporal and spatial scales. From the temporal perspective, because of the impetus, the effect of URI and the role of boundary gate are different in different development stages of URI, we should take discretionary measures. From the spatial perspective, the equilibrium of URI is different at different spatial scales, so different development policies should be implemented at different spatial scales of URI.