Delineating Urban Growth Boundaries Using ABM Model: The Case Of Chongqing

It is an increasingly prominent problem that the ecological environment is destroyed and arable land is eroded due to the rapid urbanization, this offer an opportunity to establish the system of Urban Growth Boundary(UGB) with strong binding. As national new urbanization was put forward, introducing UGB is urgent to control the urban sprawl and guide the rational growth. Taking Chongqing as a case, this paper attempted to integrate the simulation of ABM model and the delineation of UGB to explore the delineation of UGBs in com plex mountainous setting. Particularly, this study combined the rigorous boundaries for the purpose of ecological protection and the elastic boundaries for providing new space for urban growth. The method can effectively protect the environment and farmland, and better to predict spatial layout of urban growth within a reasonable period,and investigate the effect of different agents on the urban growth boundary, and explore the determination of the urban growth boundary under different government. Meanwhile, the paper is a reference case to provide a method and idea for determining the urban growth boundary.This paper firstly described the applications of the theory of UGB and ABM model in the urban growth, and a combination of both would be followed. Secondly, taking the urban city of Chongqing as an example, the process of urban expansion would be analyzed both from the urban land-use conversion, the urban expansion mode and the urban expansion form, and analyzed the impact to urban expansion from ecological boundaries and the borders of planning. The ecological boundary involve in the basic farmland protection areas, the border of Green Line, the border of ecological control. Thirdly, the ecological control would be delineated according to rigid ecological constraint factors, and then the rigid urban growth boundary. Finally, the elastic urban growth boundary should be determined based on the ABM test,and based on the feasible model, different government roles should be setted including general government, cautious government, developmental government and minimalist government, to simulate the urban expansion in the future 2025 and to compare every role, the elastic urban growth boundary would be determined through comprehensive amendments.The results indicated that:(1) a.From 1978 to 2011, the construction land of Chongqing urban city increased rapidly and the arable land was eroded seriously especially after becoming municipality. b. Chongqing’s spatial expansion was primarily driven by extension growth whose rates are over 50%. The leapfrog urban growth increased a little in recent, although it declined in the past while the infilling urban growth was generally on the rise. c. From 1978 to 2011, the change of cohesion index for urban clusters had different trends, such as decreasing, increasing, U-shape fluctuation respectively. d. Most of the urban development occurred at the middle slopes, and then a proportion of urban development was found at ridges, lower slopes, upper slopes, flat slopes and valley.(2)The effects of the basic farmland protection areas, the border of Green Line, the border of ecological control to urban expansion was rigid; while the effect of the urban planning was flexibility.(3) After the test of model accuracy, the delineation of UGB based on ABM model was feasible. The overall accuracy of the model reached 82.92 percent, the simulation results and the actual situation were broadly consistent which can reflect the trend of urban growth on the whole.(4) The construction land of Chongqing urban city would reach1066.17 km2 in 2025. Four scenarios of UGBs had their respective advantages and shortcomings. Specifically, the scenarios of cautious government paid attention on the moderate speed of development, the compact urban form and the minimal loss of farmland, thus being consistent with the goals of planning permits. The scenarios of general government had good performances similar to that of cautious government. The scenario of developmental government focused on rapid urban development, thus not conforming to master plans and leading to outward growths. The scenarios of minimalist government was evident in sprawling, without rigorous zoning and regulations. However, it should be noted that the results of UGBs simulated by ABM should be adjusted and adapted to satisfy the goals of urban planning, such as polycentric urban development.