عنوان مقاله [English]
نویسندگان [English]چکیده [English]
With the occurrence of industrial revolution in 18th century, most rapid changes and evolutions happened. Urbanization process and urban growth had impacts on all countries in the world. Urban population reached to 49 percent in 2005 compared to 13 percent in 1900. Recent forecasts show that in 2030, about 60 percent of world population will live in the cities (United Nations, 2010). Although, the urbanization and urban growth are global events, its impacts are more considerable in developing countries. High percentages of urbanization have occurred in developing countries compared with other parts of the world and changes have been considerably different among countries. But totally, urbanization processes are happening more rapidly in developing countries. Evidences about reductions in population growth rate in the largest cities shows that spatial decentralization is happening in the form of multi-nuclei megapoles. But this event does not reject this fact that urbanization is happening in the whole world.
Urbanization process in different parts of the world is the result of complex social, economic, political, technological, geographical, cultural factors at global and local levels. This process in different stages through the time, according to the local context, is producing different patterns in different patterns of the world. Opposite to decentralized pattern of urban changes in advanced societies, centralized processes of urbanization and urban growth are dominating in developing world. High level of concentration of population or high share of urban megapoles in urban population is the main characteristic of rapid urbanization in many developing countries. This increasing centralization creates urban primacy pattern. Urban primacy pattern, with its primate city and few other large cities have caused numerous problems in quality of life, national industries, management and institution building, social integration and sustainability.
Although, urbanization is a global event, this event is considerably dynamic in Iran, so that a unique urban growth has happened in Iran in the past five decades. During the past 55 years, the urbanization level has increased from 31 percent in 1956, to more than 71 percent in 2011. Parallel to rapid urbanization level, the number of cities has also increased from 199 cities in 1956 to 1139 cities in 2011(Iranâs statistics Center). In fact, urbanization in Iran, during the recent decades, similar to other developing countries has been increasing and the population has concentrated in urban areas. Despite rapid urbanization and increase in the number of cities in the past 55 years, urban growth rate has not been the same and differences in urban growth rate caused major changes in Iranâs urban system. Some cities have gained higher ranks in the urban hierarchy as a result of economic, political and environmental factors, while some other cities have lost their status.
Urbanization processes in different parts of the world are the results of socio-economic, political, technological, geographical and cultural factors at local and global level. These processes are producing different urban patterns in different parts of the world, considering time and context. Current paper is the result of research about factors affecting urbanization, processes and patterns. Results show that urbanization process and urban growth in Iran has passed through two stages of centralization and decentralization. Also, urban primacy pattern is observed and Tehran is the primate city. In 1980s, the problems of centralization in first rank cities ended in decentralization. But this decentralization has clustered dispersions. One of the resulting spatial patterns is Tehran urban megapole. Explorative analysis of spatial data shows the formation of center-periphery pattern in Iranâs urban network. At the national level, Tehran is considered as the center and other surrounding cities are considered as peripheries. At regional level, similar center-periphery pattern is observed. Top rank cities are centers and other surrounding cities act as peripheries.
To prevent unbalanced urban growth and its consequent problems, study of the hierarchical network, their functions and their roles is necessary.
Despite fundamental differences between urbanization and urban growth in developing and developed countries, processes and patterns of urbanization and urban growth pass through similar path. In all countries, urbanization process and urban growth, in their spatial-temporal dynamics, are affected by social, economic, technological, geographical and cultural factors at global and local level. Two major steps of centralization and dispersion occur. Each of these stages follows particular conditions, considering conditions and characteristics of each country.
The stage of concurrent centralization happens with urban growth and it is at a time that urban growth happens in a country as the result of internal and external changes and has an ascending trend. In this stage, urban growth happens in large urban centers, drains population, resources, and capitals from national level. The process of centralization causes centralization and convergence of population and activities urban areas, and as a result, formation of megapoles and urban primacy pattern in different countriesâ urban system. In other words, the formation of megapoles and urban primacy pattern are a reflection of the spatial centralization in different countries. In this stage, one or two citiesâ population grow rapidly and separates itself from other cities.
The process of centralization, as a subsection of urbanization and urban growth process, affected by a collection of internal and external factors are replaced by another process, called decentralization. In large cities, lack of economies of scale beyond a point, plus congestion, high land value, pollution, new opportunities for investment in other areas, government policies to direct economic growth in other areas, affects polarization impacts, and the process of decentralization starts. In this stage, decentralization process causes dispersion of population and related functions and changes in megapoles and large centralized cities. Richardson calls this process as the return of centralization or decentralization. Richardson believes that in this stage, secondary cities in a region, compared with major city have faster population growth. In other words, in this new trend, megapole expansion happens in a number of cities and towns within a wider megapolitan region with different distances from the major city. This trend causes the decentralization process.
The result of analysis shows that until 1976, no major changes are observed in Iranâs urban hierarchy. Since 1976, with the major changes as the result of Islamic revolution, Iran-Iraq war, and migration of population from the fighting regions to other cities and rapid urbanization, Iranâs urban hierarchy experienced major changes. Cities located in Tehran megapole and its surrounding cities such as Khom and Khazvin gained higher rank in urban hierarchy and a cluster of cities with high urban growth was formed. This cluster not only had more rapid ascending move in urban hierarchy. But its borders were extended to the surrounding provinces. But, after 2006, with the relative reduction in urban population growth in each year, and reduction in speed of migrations, a shrink happened in this clusterâs borders, and it was limited to Tehranâs megapole. Majority of other cities in the country have experienced stable and descending trend in urban hierarchy. Totally, during 55 years, 1956-2011, evidences of a center-periphery pattern are observed. Tehran megapole, and its surrounding cities have formed a cluster of cities with positive z-score while other cities in the country experienced, descending rank, with negative z-score. Although, the number of cities was increased in Iran, from 199 in 1956, to 1139 in 2011. But the major urban growth has happened in large and mid-size cities. In fact, large and mid-size cities had population growth as a result of natural population growth and rural migration during the past 55 years. Spatial-temporal changes of urban growth in Iran shows that large cities have absorbed the material, capital and population flows and play the major role in their regions. As a result of such flows, urban clusters or megapolitan regions, with the dominance of a megapole are formed or are forming gradually. In this study, DMPS satellite images were used. These images are taken during the night. In fact, the lights during the nights are recorded. Results are shown in figure 6. Dark areas on the map show large urban cores. Further away from these cores, the colors are brighter because the degree of lights during the night decreases with the distance from urban cores. The comparisons of maps show the emergence of major urban clusters between, 1992 - 2012.
Urbanization and urban growth as a global event, has affected all countries in the world. The degree of the impact of these events, as a country has reached a peak during the past few decades. Knowing and understanding this event and its impacts, needs the presentation of a framework that urbanization and its consequent urban growth are considered as cause and also as effect. The interaction between various internal and external factors and consequent changes, have created the urbanization in Iran and each time period, different stages are observed. The first stage is a process of urbanization and urban growth in a developing country. With the changes in factors affecting the urbanization and urban growth, the process of centralization converts to second stage of urban growth, which is decentralization. The formation of megapolitan regions reflects this decentralization.
Results of study show that population urban growth has been transferred to smaller cities. In fact, after a period of rapid urban growth in large cities, higher urban growth rates have happened in mid-size and small cities. But spatial-temporal changes in Iranâs urban growth show important points. Until 1986, no significant stable cluster is observed in Iran. It means that urban hierarchical system in Iran has urban primacy pattern which consists of one city absorbing all flows. After 1986, as a result of implementing decentralization policies, for the first time, significant clusters of urban growth are observed in Iran. This cluster exists around Tehran megapole. In fact, since 1981, because of decentralization policies, decentralization process in the form of cluster dispersion has occurred. Rapid growth has happened in cities around Tehran, and formed Tehran megapole. This significant cluster has been stable through the time and its radius has increased through the time. Therefore, the center-periphery pattern is formed. The same pattern can be observed in many regions within Iran.