Impacts of Spatial Distribution of Impervious Areas on Runoff Response of Hillslope Catchments: Simulation Study

This study analyzes variations in the model-projected changes in catchment runoff response after urbanization that stem from variations in the spatial distribution of impervious areas, interevent differences in temporal rainfall structure, and antecedent soil moisture (ASM). In this work, an ensemble of hypothetical imperviousness scenarios created for two small (<1ha) watersheds were incorporated into the gridded surface subsurface hydrologic analysis (GSSHA) model, which was calibrated against 41 runoff events under natural conditions. Each event was resimulated for each imperviousness scenario. Variations in the model-projected changes in runoff were characterized and related to temporal rainfall dispersion, ASM, and two metrics: (1)proximity of imperviousness from the outlet, and (2)normalized number of downstream pervious elements. Key findings include the following: First, interscenario variations in the simulated runoff were relatively subdued on an event-mean basis but were much wider for individual events. For example, the coefficient of variation (CV) was less than 7.8% for runoff peak but was beyond 20% for certain events. Second, the rate of increase in simulated runoff peaks with elevated imperviousness tends to be lower for events with higher temporal rainfall dispersion and ASM, with one of the largest events exhibiting the slowest rate of increase. Third, both metrics were found to be negatively correlated with simulated runoff depth. These findings point to the possibility of refining the model projection by incorporating indicators of overall locations of impervious areas, rainfall dispersion, and soil moisture conditions.