Using Satellite Observations to Understand Urban Expansion Dynamics of the West Africa Urban System

The West Africa urban system is a global hotspot of rapid urbanization and associated increases in impervious surfaces, including sprawl that expands the urban footprint and infills within the urban boundary. Urban sprawl poses challenges for food security, biodiversity, climate change, services provision, and urban planning for policymakers. Previous studies have quantified urban expansion in West Africa using global, national, and local level impervious cover datasets. However, these data are inconsistent in their estimation of urban change and less suitable for regional assessments because of their coarse spatial and temporal scales and the need for locally calibrated data. The Landsat archive provides consistent data for mapping and projecting urban expansion at a 30m resolution but is challenging to use in West Africa due to data gaps. Previous studies of urbanization have focused on the largest major cities but ignored the more abundant but smaller secondary cities. However, secondary cities are a substantial component of the urban system, containing 46% of the urban population in West Africa. This study quantified sprawl versus infill expansions in 1591 secondary compared with 17 major cities in four rapidly urbanizing West African countries (Benin, Ghana, Nigeria, and Togo). We used the Random Forest regression model and LandTrendr temporal segmentation algorithm to develop a consistent and regionally calibrated annual urban land cover product from the Landsat archive. Preliminary results from Ghana show that annual percent impervious cover had a 0.82 predicted-observed correlation and 10% percent mean absolute error. Based on a threshold of 20% impervious cover, the total developed area in major cities changed from 470 km2 in 2000 to 1085 km2 in 2021 with an annual growth rate of 4%, while secondary cities changed from 220 km2 to 675 km2 with an annual growth rate of 5% over the same period. The majority of expansion in both city types is sprawling with gradual infilling. Although the total area of secondary cities remains smaller than major cities, they are growing more rapidly and accounting for a larger percent of developed area over time. Thus, the significant impact of sprawling secondary cities on human populations and the environment is expected to increase.