Assessing long-term urban surface water changes using multi-year satellite images: A tale of two cities, Dhaka and Hong Kong

Water is a fundamental component of an urban environment. Management of water resources is important to facilitate a liveable environment and urban sustainability. Several factors affect water resources, including urbanization, climate change and seasons. Moreover, the nature of urban expansion and unsustainable water management practices have been associated with water scarcity, loss of biodiversity and increase of flood risk. Knowledge of the changes in urban surface water in relation to changes in seasons, land covers, anthropogenic activities, and topographical characteristics are important for managing watersheds and urban planning, and developing adaptation strategies to address environmental challenges posed by urbanization. However, existing studies rarely consider all the above factors when monitoring surface water changes in the urban environment. To address this problem, this study uses satellite images from multiple seasons and years, and assess the changes in surface water in relation to changes of several important factors, e.g., seasons, urbanization, land cover and topography. Moreover, this study applies advanced geostatistical tools to assess the local relationship between changes in surface water and the driving factors, and compares the findings in two cities Dhaka (Bangladesh) and Hong Kong with a large contrast in many aspects. First, seasonal influence in the distribution of water area is evaluated. Second, land cover classifications are assigned, and then the contingency matrix and decadal maps are formulated to investigate and evaluate the influence of urbanization on the occurrence and transitions in surface water. Third, an advanced geostatistical regression model is used to investigate the spatially varying relationships for change in surface water in relation to change in other land covers and topography. The investigations confirm that the temporal and seasonal variation, and urbanization induced land cover changes largely affect surface water. In addition, topography influences the nature of the city's expansion, which in turn entails the changes in surface water. The flat terrain of Dhaka facilitates the easy invasion of water bodies and horizontal expansion, in contrast, the steep terrain of Hong Kong prohibits horizontal expansion thus it's surface water is more stable than Dhaka.