A dual effect of urban expansion on flood risk in the Pearl River Delta (China) revealed by land-use scenarios and direct runoff simulation

Since the late 1970s, the Pearl River Delta (PRD) in China has undergone a rapid transition from an agricultural landscape to a metropolitan area. The rapid urbanization has not only increased the area of impervious surfaces in the central plain; it has displaced farmland to the hilly peripheral areas. The objectives of this study are to: (1) analyze how these changes in land use in the PRD have influenced flood incidence over the past 20 years and (2) explore possible changes in flood incidence in the coming two decades. An integrated simulation of the land-use changes and the hydrological processes is employed to investigate the impact of urbanization on the volume of direct runoff. Historical flood records are used for validation. The simulation results indicate that land-use change in the PRD has markedly increased direct runoff over the past two decades. Changes in direct runoff generation and in land use are significantly correlated with historical floods at the county level. These results suggest that the increase in floods stems from the dual effect of urbanization on land use through the expansion of impervious surfaces and the displacement of farmlands to the hilly outskirts. The simulations suggest that the dual effect would continue in a future of business as usual. Stopping farmland replacement would likely reduce the increase in direct runoff generation, and this reduction would be augmented if changes in farmland replacement were combined with compact city development.