Hydrologic processes of forested headwater watersheds across a physiographaic gradient in the southeastern United States
Understanding the hydrologic processes is the first step in making sound watershed management decisions including designing Best Management Practices for nonpoint source pollution control. Over the past fifty years, various forest experimental watersheds have been instrumented across the Carolinas through collaborative studies among federal, state, and private organizations. One of the most notable theoretical hydrological advances that directly resulted from studies in this region perhaps was Variable Source Area Concept (VSAC) proposed by John Hewlett and others. VSAC offers a framework that explains the mechanisms of streamflow generation at the watershed scale and provides a basis for developing watershed management practices for minimizing negative impacts on stream water quality. Unfortunately, due to the dynamic nature of the variable source area, a zone that varies across space and time, it is rarely measured and quantified at the watershed scale. This paper presents findings from a stormflow monitoring study that spans a physiographic gradient from the mountain to the sea. This study suggests that the variable source area and stormflow flow characteristics were most influenced by antecedent soil moisture conditions, which reflect the controls of climate and topography. We found that the saturated area was rather small in the Appalachians and piedmont upland watersheds, but it could be rather large and variable in the lower coastal plain watersheds. Implications of these contrasting differences in VSA to watershed management are discussed.