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Abstract
Hydrological connectivity describes how different portions of the landscape interact in some form of water-mediated transfer of matter or energy. Hydrological connectivity between portions of the landscape (hillslopes, wetlands, streams) has major implications on the environmental functioning of a watershed as a whole. Understanding what governs these interactions can provide insight into individual watershed processes and can be used for more robust watershed management. In this dissertation, there are three separate but related studies primarily looking at the influence of topography and watershed structure on the flow of water and solutes at three low-relief, permeable, and groundwater-driven headwater catchments of Upper Fourmile Creek, which drains the Savannah River Site in the Upper Atlantic Coastal Plain of South Carolina. The first study investigates the longitudinal depiction of watershed structure and characteristics like topography, stream networks, groundwater levels, and land use through three case studies. These case studies provide examples of applying longitudinal depictions to stream networks, watershed hydrologic behavior and land use distributions. This longitudinal depiction of flow networks revealed large variability in slope-area relationships for low-order streams, clearly shows the influence groundwater connections have on the permanence of surface water features and quantifies the effect of land use relative to topographic position. The second study introduces a topographically based, GIS-driven, two-dimensional groundwater model for predicting water table position and groundwater travel times. This model was introduced and then used in Upper Fourmile Creek to estimate water position and compared to groundwater records, and then used to calculate groundwater travel times to provide context for nitrate transport on site. The third study investigates the fate and transport of excess nitrogen in the form of nitrate from an intensively management short-rotation woody crop plantation through the riparian zone of an intermittent low-gradient blackwater stream in Upper Fourmile Creek. Two years of measuring nitrogen in topographically positioned well network indicated rapid transformation and denitrification within the forested wetland valley. Shallow groundwater denitrification in this riparian area reduced more than 90% of excess nitrogen originating from nitrate fertilizer.