CityReads│Big Cities and Small Town, Which Grow Faster?
This study analyzes the hierarchical structure and the urban growth process for the seven countries including the five BRICS countries,USA and Europe, with reference to Zipf’s and Gibrat’s model.
Pumain, D., Swerts, E., Cottineau, C., Vacchiani-Marcuzzo, C., Ignazzi, A., Bretagnolle, A., Delisle, F., et al. (2015). Multilevel comparison of large urban systems, Cybergeo : European Journal of Geography.
Source：http://cybergeo.revues.org/26730
IForms of Urban Hierarchy and Growth Processes: Zipf's Law and Gibrat's Model
There is a vast corpus of literature about the forms taken on by urban hierarchies, and in particular the descriptions that refer to Zipf’s rank-size rule or by a lognormal statistical distribution. Recent syntheses have not reached a consensus on the universal nature of this rule, nor on the factors that might explain its wide applicability, or the main reasons for the variations that are observed.
Gibrat’s model is a first approximation by a stochastic repartition of the growth rates of individual cities in an urban system. It predicts the statistical form of urban hierarchies and their persistence over time. This model assumes that cities grow in a manner that is proportional to their size. Despite considerable fluctuations in growth rates from one city to another over short time spans, long-term growth averages out at the same level for all cities in a given system.
Urban theories and models are too rarely tested on sets of data that are properly defined and standardized, thus it produces many contradictory results and controversial papers in urban studies. A first obstacle to the construction of scientific knowledge on this issue is the extreme heterogeneity of the samples of cities that have been used to perform the tests, and sometimes the doubtful quality of the definitions and delineations used to measure city size. We think of crucial importance to establish solid and replicable results from sound data that are made comparable by using a common theoretical background for defining and delineating cities. We compare urban systems in seven among the largest countries in the world: China, India, Brazil, Europe, the Former Soviet Union (FSU), the United States and South Africa to illustrate urban systems in almost all continents at different stages of the urbanization process, with reference to Zipf’s and Gibrat’s model.
Harmonised databases derived from an evolving concept of the city are a prerequisite for comparative urban studies.We define a city as a place in which the daily activities of most residents are concentrated. Its delineation constitutes a spatial "envelope" that evolves through time, generally in expansion. For each time period, we select an urban delineation that is suited to the local regime of socio-spatial interaction, including administrative, morphological and functional definition. The present development of urban databases for the seven study zones is based on common principles, while at the same time allowing for adaptation to local constraints and political and administrative contexts. Figure 1 showed the population databases of seven selected countries over long time spans.
Table I sums up the final content of these databases and the method used to adapt our generic definition to the information available in each country. The urban entities included are those with more than 10,000 inhabitants at the date indicated.
At the start of the 21st century, the numbers can vary from a few hundred to several thousands, but they remain fairly closely linked to the total urban population of the country, where a simple linear regression adequately fits, which means that the degree of concentration of the urban population varies little in the present-day world. The United States appear as an exception, with far less urban units than expected from their total urban population. That huge concentration of the urban system can be explained by the historical process of settlement in the “New world” as well as by the very large size of the elementary spatial units aggregated in SMA’s.
The value of the rank-size slope is an index of size inequalities of cities. The lowest values are observed in countries that have been populated for a long time (India, China, Europe) and the highest values for countries that were settled more recently, higher transportation speed enabling a wider spacing between settlements as well as larger urban concentrations emerging on sparser spatial distributions of rural population (South Africa,the United States). The fairly high values for the Soviet Union could be explained by its relatively late industrialisation compared to Europe, and a more recent urban development of its Eastern areas in the Asiatic part, while Brazil escapes the general pattern with a moderate degree of urban concentration (see Table II).
The qualitative variations in shape of the size distribution are explained above all by the diversity of he politico-administrative organisation of the territories concerned (see Figure 3). In countries that have been run under socialist regimes aiming at restraining urban growth, there is a levelling-off of the curves. Conversely the countries with the most marked macrocephaly (South Africa, India, Brazil) are those that have allowed their metropolises the greatest latitude.
Compared to the United States and Europe, the BRICS countries stand out for the particular shape of the upper part of their urban hierarchies: Russia is the only BRICS country to present a case of urban primacy. While all the others except China are characterized by macrocephaly comprising two to four cities that are clearly discontinuous with the rest of the distribution.
Top part of the urban hierarchy is not sufficient to characterise the degree of concentration of an urban population, and it is relevant to look at the other parts of the distribution. China rank first with a total urban population (500 millian) and number of cities over 1m inhabitants(66). Although population concentration in China is still moderate, the massive size of the country and the closeness of certain cities one to the other suggests that several large conurbations or megalopolises are likely to develop with 30 to 40 million inhabitants each, around the Pearl River Delta, in the regions round Shanghai, or between Beijing and Tianjin. India, ranking second for the weight of its total urban population (around 400 millian) ranks only 4th for the number of cities with more than a million inhabitants( 44), it is outranked by the USA ( 51) for an urban population of only 287 million, which is therefore much more concentrated spatially. In contrast again, Europe where the urban population is more or less the same than in the USA (291 million) has only 39 cities of over a million inhabitants, and the degree of concentration is similar to that of the former Soviet Union (173.5 million city-dwellers and 28 cities of over a million), or Brazil (161 and 23 respectively).
We tested the hypotheses of the Gibrat model for the intervals between these two dates for which city population data is known in our bases. The hypothesis for generating a lognormal distribution stipulates that the variations in growth rate at each time interval do not depend on city size, and are distributed randomly from one period to another.
Overall, the process observed in the BRICS complies with the model. Everywhere, the correlation between city size and growth is low or absent. Whether at national level or for the main regions, the hypotheses of the Gibrat model are verified, and from the start of the 20th century. There is a discrepancy with the model, particularly in periods of vigorous growth, for the USA throughout the 19th century and in Europe after 1950, where there is a positive temporal autocorrelation of the growth rates. In Russia the process also appears in the course of the second phase of industrialisation in the 1930s and in the two decades during which there was a trend towards metropolisation of the largest cities and a cumulative decline of certain specialised cities. A consequence is that inequalities in city sizes are growing faster in real urban systems than according to a pure Gibrat’s rule.
To conclude, the urban hierarchies and growth processes in a variety of large urban systems all over the world including the BRICS countries do share generic common features despite major differences in their history as well as territorial and political organization. That is why Zipf’s law and Gibrat’s model albeit purely statistical models remain rather good standard references enabling international comparisons for a synthetic description of empirical urban hierarchies and urban growth processes, even though they do not directly provide an explanation for the underlying generative geographical processes.
IIUrban Transition and Cities Trajectories
Understanding city size distribution and moreover urban growth processes cannot be done without referring to the history of each urban system. W. Zelinsky (1971) used an analogy with the demographic transition, urban transition, to describe the universal change from a dispersed and homogeneous spatial repartition of population in rural habitat towards much more concentrated diversified and hierarchized forms in urban settlements. The process which generally accompanies economic development started roughly at the beginning of 19th century in first industrialized countries and around 1950 in the less developed ones.
Table III shows countries that experimented earlier urban transitions (Europe, Former Soviet Union, USA, Brazil) have much lower average urban growth rates during the last forty years than countries that are still in the exponential stage of the growing curve of their urbanization rate. China is with an average urban population growth rate of over 5% per year over 40 years, followed by South Africa ( 3.2%)and Indian( 2%). In the USA and in Brazil growth is still over 1%, while in Europe and Russia it is just 1% or much lower.
These trends are partly determined by the stage reached by the different countries in the urban transition,but they are also influenced by their particular history of urbanization: in China political action, in particular that affecting migrations, for a long time put brakes on the explosion of urbanisation , far more markedly than in India where it was rather social and family ties that slowed migration from rural areas. South Africa has remained at an average level among African countries since the end of Apartheid, on the one hand because of internal migrations from the former Bantustans, and on the other because of its attractiveness towards foreign migrants. Brazil and the USA being among the "new" countries in terms of waves of settlement had a former urbanisation rate systematically higher than in "old world" countries in Europe and Asia. The slowing in urban growth rates has been more marked in Russia since the 1990s, as a result of decreases in the total population, resulting from the dismantling of the Soviet Union.
The weights of urban population of these countries in the total urban population of the world have changed as shown in Figure 4 that compares their evolution over almost half a century. These contrasted evolutions explain the very rapid turnover in rankings among the mega-cities. In the 1960s, the three largest cities by size were located outside the BRICS, with populations of 10.6 million for New York, 8.9 for Greater London, and 7.2 for Paris. By the end of 20th century, this ranking had been completely overturned, with Sao Paulo joining or overtaking New York with some 20 million inhabitants, and 4 cities in the BRICS taking top places among the others: Delhi (17 million), Shanghai (16), Kolkata (16), Mumbai (15), Moscow (14), Beijing (14), and Rio (12).
According to the orientation of their trajectories in relation to the city system to which they belong, cities can be grouped generally in two types, "winners" and "losers", but a stable type also appears in India, China, the former Soviet Union and South Africa. These cities that maintain their relative weight in the system are often long-standing cities with administrative functions – certain State capitals in India, provincial capitals in China regional capitals in Russia, and medium-sized cities in South Africa.
In Brazil almost all the large metropolitan areas have strongly ascending trajectories. The recent dynamic thus has the effect of accentuating the hierarchical inequalities in the country. India too exhibits this process of reinforcement at the top of the urban hierarchy. In contrast, in the other countries, what can be seen is a form of ”catching up” by the smaller cities and peripheral areas. In China two processes are seen, where the large cities in the East are gaining weight in the system while medium sized cities in Xinjian province and Inner Mongolia are developing fast, illustrating the catching-up by peripheral regions.
There is a long-term trend in most city systems whereby it is mostly the small urban entities that show relative decline. It is because smaller cities are more likely to be distant from the main waves of innovation, or else to be highly specialised in declining sectors of activity, so that they lose their influence on local markets as a result of acceleration in the speed and capacity of transport systems. The only exception appears in China where classes of cities with an ascending profile are made up mainly of small cities, a third of which in Special Economic Zones in which innovating activities have been set up and to which populations migrate. However cities with a "winning" trajectory are mainly located in the immediate vicinity (roughly less than 200 km) of large metropolitan areas, for instance Guangzhou or Shanghai. The importance of policy in urban dynamics can thus be seen in the creation of new cities, at the same time preserving a degree of spatial and historical coherence with the earlier trends in the city system. In a territory where urbanisation is long-standing, like China, these new urban developments fit themselves into the previous urban spatial pattern, while in ”new” countries like Brazil, the USA and South Africa, urban creations ran alongside the settlement of new territories.
IIIConclusion
This paper for the first time provides a comparable overview of the systems of cities in the five BRICS countries. Using Zipf’s distribution of city sizes and Gibrat’s urban growth models as benchmarks for the comparison, we have demonstrated that the dynamic urban processes in BRICS during the last fifty years were rather similar to those observed for instance in Europe or the United States.
Of course differences do exist, but they relate to the specific developmental pathway of these countries, including the relative delay in the urban transition compared to more developed countries, which explains their very high mean urban growth rates – the case of Russia being excepted. History matters too for differentiating the evolution of urbanisation rates, which registered higher values earlier in Russia and Brazil compared to South Africa, China and India.
When shifting from the macro-scale of countries to the micro-scale of individual cities, the most striking fact is the diversity of urban trajectories that exhibit contrasted patterns of booming growth or relative decline everywhere.