Underlying rules of evolutionary urban systems in Africa

Professor Peng Gong, Vice-President (Academic Development) of HKU, and Professor Limin Jiao of Wuhan University are the corresponding authors of the paper. Gang Xu and Mengyan Zhu of Wuhan University are the co-first authors. Other contributors include Assistant Professor Bin Chen and Professor Xiaoling Zhang from the Faculty of Architecture at HKU, Assistant Researcher Zhibang Xu of the Institute of Urban Environment, Chinese Academy of Sciences, Professor Xuecao Li of China Agricultural University, Dr Muhammad Salem of Cairo University, Associate Professor Patrick Brandful Cobbinah of the University of Melbourne, and Dr Neema Simon Sumari of Sokoine University of Agriculture in Tanzania.

Urban systems are typically complex systems, and their evolution follows a series of universal laws that can be expressed through concise mathematical models, such as the rank-size rule (Zipf's law), the law of proportionate growth (Gibrat's law), and scaling laws. However, previous studies have primarily focused on the universality of these laws, paying insufficient attention to how these rules might vary at different stages of urbanisation or how these models can convey the changes in city systems over time.

Africa, recognised as one of the most rapidly urbanising regions worldwide, boasts a rich array of natural geographical features. The influences of colonial history, socio-economic structures, and multiculturalism have shaped the unique urbanisation processes within African countries. This diversity has led to considerable variations in urban evolution across the continent, offering a multifaceted sample for the study of urban system development.

Starting from a unified urban definition, the research team utilised remote sensing data on urban population and land use to comprehensively analyse the evolution of over 9,200 towns and cities across Africa over the past 70 years (1950-2020). Applying the theories of Zipf's law, Gibrat's law, and scaling laws, the study first verified the universality of urban system laws in African cities and then focused on their specificity and the explanatory power of model parameters. The parameters of Zipf's law revealed a shift from dispersed to concentrated urban populations in Africa, with an increasingly concentrated trend toward larger cities. This shift was corroborated by the parameters of Gibrat's law, which highlighted differences in population growth rates among different-sized cities. The scaling law results showed that, unlike in Western developed countries, land development in large African cities, particularly in East and West Africa, has been more extensive. However, as urban systems matured, economies of scale in land use gradually emerged, aligning with theoretical expectations.

The study's contribution lies in revealing the underlying rules, specificity, and geographical explanations of African urban system evolution from a complex systems perspective. It provides an analytical framework to understand the laws of urban system evolution and regional urbanisation processes, offering new insights for global urban science research.

The research was supported by the National Natural Science Foundation of China, the Hong Kong Research Grants Council, and the Croucher Foundation of Hong Kong.