Evaluating the effects of topography and land use change on hydrological signatures: a comparative study of two adjacent watersheds

Abstract. Watershed hydrological processes are significantly influenced by land use/land cover change (LULCC) and watershed characteristics such as topography. This study comparatively investigates the impacts of terrain slope and urbanization-driven LULCC on hydrological processes in two adjacent subtropical watersheds but with distinct terrain and land-cover conditions within the Greater Bay Area (GBA) of China. We developed an Integrated Surface-Subsurface Hydrological Model (ISSHM) using the Simulator for Hydrologic Unstructured Domains (SHUD), which was calibrated using data from river and groundwater flow monitoring stations in the watersheds. The calibrated model facilitated simulations to assess how terrain slope and LULCC affect surface runoff, subsurface flow, evapotranspiration (ET), and infiltration. Our results indicate that slope impacts hydrological processes differently in watersheds with varying characteristics. In mountainous areas, there are consistent high correlations between slope and annual surface runoff, infiltration, and subsurface flow across all watersheds. However, at lower elevations, the hydrological responses of steeper watersheds correlate weakly with local slope. Furthermore, urbanization (increase in impervious areas) has led to significant increases in annual surface runoff and significant decreases in annual infiltration and ET across all watersheds, especially in those with steeper slopes. On the other hand, in watersheds with gentler slopes, the annual increase in surface runoff is less than the percentage increase in impervious area, suggesting a buffering capacity of these flatter watersheds against urbanization. However, this buffering capacity is diminishing with increasing annual rainfall intensity.