306-2 Comparison of Land Use/Land Cover Effects on Streamflow Variation in the Piedmont and Coastal Plain Physiographic Provinces of the Southeastern United States

Abstract:

Urbanization continues to reshape watersheds across the southeastern United States, and the hydrologic response of streams to land development varies strongly with physiographic setting. This study compares streamflow behavior in five Piedmont watersheds (Sope Creek, Mountain Creek, Marsh Creek, Little Sugar Creek, and Difficult Run) and four Coastal Plain watersheds (Santee River, Pee Dee River, Savannah River, and Roanoke River) over a 26-year period (1995-2021), using USGS gauging records and National Land Cover Database (NLCD) classifications from 2011 and 2016. Mean, low (25th percentile), high (95th percentile), storm, and base flow were computed annually for each site and evaluated against land cover composition (developed, forest, agriculture, and wetland percentages) and against time via linear regression.

Land cover change was generally modest across both provinces over the study period, though agricultural land increased more in the Piedmont, while forested area and wetland extent showed measurable shifts in the Coastal Plain, including an increase in wetland-classified area between 2011 and 2016. Piedmont streams displayed no statistically significant trend in flow metrics over time at most sites, though Little Sugar Creek showed a marginally significant increasing trend in average flow (p = 0.05). Piedmont streamflow patterns showed a common decline around 1999 and a peak near 2004, coinciding with major regional hurricane activity. Coastal Plain streams, by contrast, showed a shared decline in streamflow around 2004, along with substantial variability in high flow and skewed baseflow distributions, most pronounced in the Santee River. Correlations between baseflow and land cover percentage (developed, wetland, forest) were not statistically significant across sites. These results suggest that physiographic setting, more than land cover change alone, strongly governs streamflow extremes and variability across the region.

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