Spatial- Temporal Dynamics of Iran’s Urban System (1956-2011)

1. Introduction
Since industrial revolution in 18th century, world population has increased exponentially, and it increased from one milliard in 1830 to more than seven milliards in 2013. Furthermore, urban population increased more rapidly than rural population, and it reached from 14 percent in 1900 to 47 in 2005 and to 61 percent in 2013. In fact, rural population growth has stopped. It is expected that all future population growth will happen in urban areas and especially in urban areas of developing countries. Although urbanization is a global event, this event is very dynamic in Iran. Record breaking urban growth has happened during the last five decades. During the past 55 years, urbanization increased from 31 percent in 1956, to 71 percent in 2011.Number of cities increased from 199 in1956 to 1139 in 2011. In fact, urbanization during the past decades, similar to other developing countries has been increasing, and population growth has happened in urban areas. Despite rapid increase in urban population and number of cities during the past 55 years, urban growth rate has not been the same and differences in urban growth rate have fundamentally changed the hierarchy of the cities. Some cities have experienced a raise in their status in the hierarchy as a result of economic, political, and environmental factors, while some other cities have experienced lower status in the hierarchy. To prevent unbalanced urban hierarchy and its consequent problems, study of urban network, urban hierarchy, type of functions, and their roles are necessary.
2. Methodology
This article examined changes in the cities’ hierarchies in Iran, spatial-temporal changes of urban system, using spatial data for the time periods from 1956 to 2015. Cities with population more than 100,000 in 2011 were studied. Changes in the rank of these cities in Iran’s urban hierarchy were examined. Advanced spatial analysis techniques such as Getis-Ord Gi* and Moran dispersion diagram were used to examine the regional differences and changes in the ranks of cities in Iran.
3. Results and Discussion
Until 1956, there was a relatively balanced distribution of population among cities. After 1956, fundamental changes in economic base, from agricultural economics to oil revenues, interferences of government in economic planning to the advantage of cities, land reforms, and changes in traditional production led to rapid trend in urbanization, but urban growth rates were not balanced. Urban hierarchy was influenced by different factors such as political, environmental, and technical changes.
Exploratory analysis of the changes in the cities’ hierarchy, using Getis-Ord G* shows that during 1956-1966, significant cluster of cities, located in Northwest, had descending rank in urban hierarchy. Government policies had impacts on the majority of these cities in western and northwestern areas and the economy was blooming in the cities. Presence of army in some cities such as Marageh had impacts on their physical development and money was injected indirectly by the governmental institutions to these cities economy. After 1956, a loss of ability to compete with other cities happened and these cities had lower rank in urban growth hierarchy. In 1956- 1966, no significant cluster of cities is observed with an ascending rank. Nevertheless, in Northern region of the country and specially Tehran metropolitan region, majority of cities have a positive z score, but they are not significant. In 1966- 1976, again, there were no significant differences among cities with ascending rank. However, first signs of centralization are observed around Tehran Megapole. In this decade, the only significant cluster with descending rank in Iran urban hierarchy is shifted from northwest to west. These cities are old cities mostly army-oriented and oil-oriented such as Masjed Solayman which were not able to keep their status in the urban hierarchy. In 1976- 1986, rapid urbanization happened in Iran and the first cluster of cities with positive z-score was formed around Tehran megapole. This includes cities that were considered as rural areas before 1956. In 1986-1996, the population who migrated to Tehran province was distributed in its rural areas because of the high values of land and housing. Therefore, after Tehran and Karaj city, Islam Shahr, Ghods, and Pakdasht were rural areas that gained population and were converted into cities. In this period, cities located in Tehran megapolitan region had rapidly ascending status in urban hierarchy. In this decade, a cluster of cities with negative z-score is also observed in southwest of Iran. In 1996- 2006, cities with positive z-scores around Tehran megapole were expanded. But the cluster of cities in border areas in west of the country had descending rank. With the passage of 5 decades of rapid urbanization in Iran, in 2006- 2011, the positive cluster around Tehran megapole has no longer that rapid urbanization, and no negative cluster is observed either. Since 1976, when the Islamic revolution and Iran-Iraq war happened, people migrated from fighting regions to other cities, urban population hierarchy had major changes. Cities in Tehran megapolitan region and its surrounding cities such as Khom and Khazvin had very rapid ascending rank and another cluster of cities had ascending rank in urban hierarchy. This cluster had not only ascending rank, but also expanded its borders to the adjacent provinces. Although, since 2006, with relative decrease in the urban population growth, and reduction in migration, this cluster had no longer that previous speed in growth and was limited to Tehran megapole region. Other cities have shown descending rank in the hierarchy. In western borders, clusters with descending ranks are observed. Totally, during 55 years of study (1956-2011), one can see the formation of center-periphery in urban hierarchy, with Tehran megapolitan region and its surrounding cities forming a cluster with positive z- score, while in other cities, negative move in ranking is observed.
Results of the research show that using the ranking indicator in urban hierarchy can solve the problems with other models such as zipf model and can also act as a suitable indicator to examine the changes in ranking based on the urban hierarchy through time. Also, the explorative analysis of spatial data with the assumptions of their relations, and spatial relationships can provide familiarity with the spatial and regional patterns.