Predicting the effect of impervious area change on water circulation rate in the Geum River watershed using HSPF modeling

Rapid urbanization in recent years has increased the area of impervious surfaces in South Korea. Increasing impervious areas increases the direct runoff when rain falls, which adversely affects the health of watersheds through increased pollution from nonpoint source pollutants. However, few studies have investigated the relationship between hydrological phenomena in broad watersheds and nonpoint sources of pollution. In this study, a watershed model to simulate the flow and water quality of rivers, with different impervious area coverages, was developed using the Hydrological Simulation Program – Fortran. The effect of different impervious area reduction scenarios on direct runoff and loading of nonpoint source pollution in the Geum River watershed, South Korea, was analyzed and the water circulation rate was used as a proxy for the health of the watershed. The impervious area coverage in the medium influence area of Gapcheon, where Daejeon Metropolitan and Sejong Cities are located, was 23.17%, approximately two times higher than that of other areas. Based on the impervious area reduction scenarios, the direct runoff, and a load of nonpoint source pollution showed maximum reductions of 51% and 41%, respectively. The water circulation rate showed a maximum improvement of 21%. Realistically, it is impossible to reduce the impervious area rapidly in regions with high proportions of urban development or built-up land. Therefore, low-impact development methods should be applied when conducting the development of public facilities or projects larger than a certain size.