Smaller cities have large impacts on West Africa's expanding urban system

Abstract

Urban expansion studies are often biased toward large cities with populations greater than 1 million, while small and medium cities with smaller populations are overlooked. We mapped and assessed the expansion of 1603 cities across Ghana, Togo, Benin, and Nigeria using impervious surface data generated from Landsat from 2001 to 2020. We classified pixels with >20 % impervious as developed with 93 % overall accuracy. Total developed area increased 2.3-fold from 4001 km² in 2001 to 9402 km² in 2020. Of the expanded area, more than half (54 %) occurred in small and medium cities, and 73 % was sprawl. The mean annual expansion rate was 4.6 %. Expansion rates and sprawl-to-infill ratios were higher in small and medium cities than in large cities, and the annual expansion rates of large cities decreased over time while those in small and medium cities were stable or increased. Proximity to large cities also increased smaller cities' expansion rates in Nigeria, but more remote cities had higher expansion rates in Benin and Togo. Although much attention is focused on large megacities, these results show that smaller and more numerous cities contribute substantially to urban expansion and need to be incorporated into regional assessments of urban growth and its impacts.