تحلیلی بر نظام سکونتگاه‌های شهری کلان منطقه مرکزی در سال‌های 90-1335

1- Introduction
Lack of balance and centralization is one of the features of the third world countries which is consequence of results of polar growth policies. Population of cities (size of cities) has been vastly distributed among different habitats with different sizes and patterns in different regions. These fully different patterns specify the regions in which growth of cities occur without any plan and with unequal policies. During this trend, the largest cities will have the fastest growth rates leading to emergence of population, economic, social and political dominance of one or more cities over other cities inside urban system of a country or region. Lack of balance between rural and urban centers in different social-economic fields will disrupt spatial order of habitats on the one hand and rapid growth of some important cities. Since dispersion of settlement points in zone of the land and lack of cohesion in the spatial structure is one of the features of urban system in the country, therefore, it is very important to study it in all regions of the country to identify the links and differences.
This research studies condition of the urban habitats located in central metropolis including Isfahan, Yazd and ChaharMahal and Bakhtiari in 1956 to 2011 to mention balance of population distribution and dispersionof metropolitan urban points of the region in different urban classes.
2- Methodology
The research method is descriptive-analytical and the required information has been collected with documentary and library method and population distribution has been studied in metropolises of central region with population models such as rank-size rule, entropy coefficient, Gini coefficient and Lorenz Curve. The demographic data has been collected based on detailed results of population and house census of 1956 to 2011 and analyzed with the mentioned models
3– Discussion
By calculating Urban Primacy index in central region and its provinces, it was observed that Isfahan and Yazd provinces had Urban Primacy degree of higher than the normal limit in all years which indicated high population centralization in Isfahan and Yazd primate cities (capitals of the province) but Urban Primacy index is normal and desirable in ChaharMahal and Bakhtiari Province. Urban Primacy index has been between 3.6 and 4.8 in the region which is higher than the normal limit but it has decreasing trend since 1996 later on which indicates balance of population distribution in the first and second cities of the central region i.e. Isfahan and Yazd and decrease of urban primacy dominance of Isfahan compared withthe past years. By studying Lorenz Curve in the region, we see that the closer we get from 1956 to 2011, the farther the Lorenz Curve will be from the normal lime indicating that population balance in 1956 is better than the next years and has been lost in 2011. Considering calculation of Gini coefficient, it is analyzed that population distribution is semi-balanced in metropolitan points of the central region to 2006(0.5-0.75) and has lost its balance in 2011(0.75-1) and it has generally undesirable condition.
Entropy coefficient in the central region was higher than 1 in all years but 1986 which was higher than that of Isfahan, Yazd and ChaharMahal and Bakhtiari Provinces and indicated better population balance in the metropolitan points of the region than its provinces. This coefficient in 1986 is lower than that in other years and tends to zero which indicates more centralization of metropolitan system of the region and lack of balance in population distribution in these years compared with other years.
During all periods, real population curve of the cities is placed below the model population curve which indicates low population of all cities compared with rank-size model. In all periods, there is relatively high reverse correlation between rank logarithm and size logarithm (population) of the cities. Logarithmic distribution of the metropolitan system of central region has been directed to lack of balance during 1956-2011.
4– Conclusion
Results of the research indicate that there are very small cities (villages- towns) with very low population and few large cities with very high population. Isfahan is regarded as the top primate city in metropolitan system of this central region due to political, administrative and financial centrality during these years and has very long distance from another size of the metropolises of the region. Metropolitan system of the central region has been balanced to some extent only in terms of centralization and urban primacy indices and also entropy but it is highly distant from normal limit in other studied models such as rank-size, Lorenz Curve and Gini coefficient. Generally, there are lack of balance, centralization and phenomenon of urban primacy in the urban system of the region. Therefore, one can help distribute urban hierarchy optimally in metropolitan level of the central region by providing services and reinforcing small and medium cities and reducing growth of large cities in the region.