ارزیابی قوانین زیپف و گیبرات در سیستم شهری ایران (مقاله علمی وزارت علوم)
درجه علمی: نشریه علمی (وزارت علوم)
آرشیو
چکیده
توزیع اندازه شهرها در یک نظام شهری بیانگر چگونگی و کیفیت سیستم شهری از لحاظ توزیع جمعیتی شهرهاست و روند تحول توزیع را در طول زمان نشان می دهد. هدف اصلی مطالعه حاضر بررسی توزیع اندازه شهرها در سیستم شهری ایران طی سال های 1395-1335 است. در این راستا دو قاعده مهم در ادبیات توزیع اندازه شهرها -قانون زیپف و قانون گیبرات- مورد آزمون قرار گرفته است. بررسی ها در دو بخش شهرهای بزرگ و کل شهرهای موجود در سیستم شهری ایران صورت گرفته است. نتایج حاکی از آن است که در کل سیستم شهری ایران، قاعده زیپف تایید نمی گردد. همچنین قدرمطلق ضریب رابطه رتبه اندازه (ضریب زیپف) در تمامی سال ها کوچکتر از یک بوده و دارای روند نزولی است که بیانگر نابرابرتر شدن توزیع اندازه شهرها در طول زمان است. نتایج آزمون قاعده گیبرات نیز نشان می دهد که از سال 1335 تا 1365 قاعده گیبرات در سیستم شهری ایران برقرار است. اما از سال 1365 به بعد این قاعده برقرار نبوده که نشان دهنده واگرایی روند رشد شهری در سیستم شهری ایران است. همچنین مطابق نتایج، با در نظر گرفتن شهرهای بزرگ، رشد شهری ایران در تمامی سال ها از قانون گیبرات تبعیت می کند و اندازه شهرها دارای توزیع زیپف با ضریب یک است.Evaluating the Validity of Zipf’s and Gibrat’s Laws in Iranian Urban System
The size distribution of cities in an urban system indicates the quality of the urban system in terms of population distribution of cities, and shows the evolution of distribution over time. The main purpose of this paper is to study the city size distribution for the Iranian urban system for the period 1956-2016. In this regard, the two most important laws in the city size literature - Zipf's and Gibrat's laws- were examined. Analysis have been performed in two parts: large and all cities in the urban system of Iran. The results show that Zipf’s law does not hold for urban system of Iran. Furthermore, the absolute value of coefficient of rank-size is less than one and has a downward trend, which shows that city size distribution is becoming more unequal over time. The results of testing Gibrat's law show that from the year 1956 to 1986, Iran’s urban growth obeys Gibrat's law, while from 1986 onward, this rule does not hold, which indicates a divergence in the trend of growth rate of urbanization in Iran. Moreover, our results show that for the large cities, the urban growth follows the Gibrat's law and the distribution of city sizes is Zipifian with coefficient of one. IntroductionIn studying any urban system, what is of paramount importance is the study of urban hierarchy. Studying the distribution of city sizes and the laws governing it helps in understanding the urban hierarchy and finding optimal conditions. The examination of the distribution of city sizes from the past to the present has received a great deal of attention from researchers. In fact, explaining how city size distribution works is recognized as one of the most important theoretical issues in analyzing urban and regional problems (O'sullivan, 2018). The distribution of city sizes, which actually represents the distribution of population among cities, can be a tool for understanding economic efficiency. If we define the size of each city by its population, the distribution of city sizes in an urban system reflects the manner and quality of the urban system in terms of population distribution and shows the trend of distribution changes over time. Zipf's and Gibrat's laws are among the most important and commonly used methods for studying the distribution of city sizes. This study reviews the complete studies conducted in this area and examines Zipf's and Gibrat's laws in the urban system of Iran. Methods and DataThis applied research analyzed secondary data from the national censuses of Iran. It focused on the population of cities in Iran from 1956 to 2016. The data and information were obtained from the General Population and Housing Census, conducted by Statistical Center of Iran. FindingsThe results will be presented in two sections: testing the Zipf’s rule and testing the Gibrat's law:1). Zipf’s Rule test: The Zipf coefficient has a significant difference from one in the urban system of Iran, and Zipf's Rule was not supported. The absolute value of the Zipf coefficient was below one in all years, indicating the divergence of urban growth trends in the urban system of Iran. Additionally, the Zipf coefficient decreased over time, revealing an increasing inequality in the city sizes distribution over time. This suggests that some cities grew too large, and the gap in city sizes in the urban hierarchy widened. 2). Gibrat's Law test: The results of this study showed thaat Gibrat's Law applied to cities with a population above 200,000 in all years. This implied that the growth of the size of these cities was independent of their initial size. However, for all cities, it is found that Gibrat's Law applied to the urban system of Iran from 1956 to 1986, but not after 1986, and we observed a divergence trend in the urban growth of the country in these years. In other words, since 1986, large cities have grown at a faster rate, and small cities have grown at a slower rate, resulting in an increase in the population (size) difference between small and large cities in the urban system of Iran. These results were consistent with the findings of the rank-size rule test. Conclusion and DiscussionBased on the most recent census in 2016, only 25 cities account for more than half of the urban population in the country. To be more specific, Iran had 1242 cities in 2016, of which 8 cities had over 1 million population, 90 cities had between 100,000 and 1 million population, and 1144 cities had less than 100,000 population. These facts indicate an uneven population distribution in Iran's urban system. The current study’s results show that the city size distribution in Iran's urban system is unequal and this inequality is growing over time. Additionally, the long-term urban growth trend is divergent. That is, larger cities have higher population growth rates while smaller cities have lower population growth rates, resulting in a divergent size of cities in the long term and agrowing population (size) gap between small and large cities over time. Based on the results and analyses of the current study, it can be argued that the government should implement effective policies to balance the distribution of city sizes. Small cities are not big enough to enjoy economies of scale, and large cities in the urban system increase living costs such as congestion and pollution. Therefore, the overall urban system efficiency declines, and it is essential to implement policies to decrease the population of very large cities and increase the appeal of living for individuals and businesses in medium and smaller cities. By expanding infrastructure investments in medium and small cities and adopting suitable policies to attract population to them, the existing size difference between large cities and lower-ranked cities in the urban system can be reduced.
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