Calibration of Rural Watershed Models in the North Carolina Piedmont Ecoregion

This paper focuses on the calibration of four rural catchments in the North Carolina Piedmont ecoregion to USGS monitored flow records through the variation of infiltration and groundwater parameters. The. four catchments were constructed from geographical information system (GIS) data and field measurements as part of the Water Environment Research Foundation's Protocols for Studying Wet Weather Impacts and Urbanization Patterns. Calibration results for the Horton simulation are reported for individual events and through flow duration curves. Results from the calibration with groundwater are shown as flow duration and peak flow frequency exceedance curves. Results indicate that calibration by flow duration curves cannot be achieved for all events of record through the adjustment of watershed parameters like percent imperviousness, infiltration, overland roughness, and conduit roughness alone. This is due to the large amount of base flow and interflow volume not modeled in the Storm Water Management Model (SWMM) but monitored at the USGS station. The elimination of base flow on a monthly basis from the USGS monitored record produced a better fit between the monitored and modeled flows for runoff events larger than the one percent cumulative exceedance but events smaller and more frequent than this were still not represented realistically due to the presence of interflow. It was determined that modeling this interflow through groundwater with default aquifer materials and changes to the coefficients of the SWMM groundwater equation did not result in realistic calibrations. However, it was determined that changes in aquifer materials did result in realistic calibrations along the entire length of the flow-duration curve. This paper reports results obtained from the groundwater modeling of interflow, illustrating that groundwater modeling can effectively model interflow and generate a more realistic watershed calibration in stormwater modeling.