Modeling ground-water hydrology is critical in low-gradient, high water table watersheds where ground-water is the dominant contribution to streamflow. The Hydrological Simulation Program-FORTRAN (HSPF) model has two different subroutines available to simulate ground water, the traditional ground-water (TGW) subroutine and the high water table (HWT) subroutine. The HWT subroutine has more parameters and requires more data but was created to enhance model performance in low-gradient, high water table watershed applications. The objective of this study was to compare the performance and uncertainty of the TGW and HWT subroutines when applying HSPF to a low-gradient watershed in the Coastal Plain of northeast North Carolina. One hundred thousand Monte Carlo simulations were performed to generate data needed for model performance comparison. The HWT model generated considerably higher Nash-Sutcliffe efficiency (NSE) values while performing slightly worse when simulating the 50% lowest and 10% highest flows. Model uncertainty was assessed using the Average Relative Interval Length (ARIL) metric. The HWT model operated with more average uncertainty throughout all flow regimes. Based on the results, the HWT subroutine is preferable when applying HSPF to a low-gradient watershed and the accuracy of simulated stream discharge is important. In situations where a balance between performance and uncertainty is called for, the choice of which subroutine to employ is less clear cut. / Master of Science