Assessing the Performance of HSPF When Using the High Water Table Subroutine to Simulate Hydrology in a Low-Gradient Watershed

Modeling groundwater 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 groundwater (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. Monte Carlo simulations were performed to generate data needed for model performance comparison. Both models performed well when simulating the 10% highest daily average flow rates. However, neither model performed well when simulating the 50% lowest daily average flow rates. The HWT model significantly outperformed the TGW model when simulating daily average flow over the full three-year simulation period, an indication that the HWT model out-performed the TGW over the full range of simulated flows. Model uncertainty was assessed using the Average Relative Interval Length (ARIL) metric. The HWT model exhibited slightly more combined model structure and parameter uncertainty than the TGW model. 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 the paramount concern.