Abstract: Urban metabolism is an organic process in cities which output products, services and waste with the input of resources, energy, and human efforts. As a complex open dissipative system, urban system, including environmental resources, economy, and social development, can be modeled with a range of metabolic processes. Such problems as resources and energy shortage, ecological damage to the environment, human life quality drops, and economic overgrowth, may be due to metabolic disorder in regional and urban development. This article reviewed the progress in the studies of urban metabolism, such as concept perspectives, methods and index system of urban metabolism. Current research field in the urban metabolism can be divided into three parts: macro- regional simulation of urban metabolic systems, metabolic responses of urban land expansions, and micro- scale metabolic measures of communities. As the field of urban metabolism expands, the traditional view of a stationary urban system is giving way to a more dynamic eco-system which includes social and economic facets of the urban systems. Material flow analysis and energy flow analysis are the main research methods. Material flow analysis tracks all of the material, energy and water flows within the urban metabolic systems. Energy flow analysis, especially the Emergy theory, normalizes all products and services to equivalents of one form of energy that enables all of these resources to be compared on a common basis. These two methods have been used to provide additional information to complement economic theory from the viewpoints of resource, energy, and material inputs and outputs, in order to further inform public policy for urban development and sustainability. In addition, other methods include human appropriation of net primary production (HANPP) and multi-scale integrated analysis of societal metabolism (MSIASM). As far as urban metabolic efficiency indicators are concerned, the core is to measure input-output ratio, involving two major categories: indicators of resource input and indicators of output. These two major indicators can be further divided into the indicators of cities' resource consumption, environmental pollution, economic development and social welfare, et al. The integration of them can reflect the overall systematic efficiency and environmental efficiency. Based on in-depth review of urban metabolism development, this article forecasts the trend of application in the urban sustainable metabolism: (1) It is necessary to develop and optimize regional metabolic management policies. Study of urban metabolism is an integral part of monitoring regional environment and provides measures indicative of a region's sustainability. (2) The urban metabolism not only is the basis of an urban daily accounting framework, but also goes a long way towards monitoring urban operations based on the development of the processes in the mathematical models of the urban metabolism. (3) Urban metabolic study helps improve the urban metabolic-oriented green community planning and design. It is possible to reduce environmental impacts by tracking the energy and material flows in urban design. Finally, suggestions on research of urban metabolism are also proposed: focus on the coupling of urban metabolism and industrial development; innovate the mechanism of urban metabolic changes; explore metabolic efficiency differences among different cities; build up a reasonable index system of sustainable metabolism.

Key words: review, sustainable urban development, urban geography, urban metabolism