Implications of groundwater hydrology to buffer design in the southeastern U.S.
The objective of this study was to examine the hydrologic processes of shallow groundwater to better define and design forest riparian management zones in headwater streams of two contrasting terrains in the southeastern U.S. We employed two long-term experimental watersheds, WS80 (206 ha) and WS77 (151 ha) at the Santee Experimental Forests in South Carolina, and WS2 (12 h) at the Coweeta Hydrologic Lab in the southern Appalachians in North Carolina. These two separate research sites represent a low-gradient and a steep mountain terrain, respectively. Groundwater table monitoring (biweekly) over three years (1992-1994) at the two low-gradient watersheds suggest that the temporal water table variability is extremely high for most of the 84 wells while the spatial variability is smaller (<0.5 m). These two watersheds periodically had saturated areas a water level less than 30 cm extending much farther than the stream riparian zones. On average, 23.0% of wells in WS80 and WS77 were saturated during the 122 visits. In contrast, only one of the nine wells located in the zero order streams showed brief saturation during storms at the mountain watershed over a three-year monitoring period (2005-2007). We found that coastal plain watersheds that received variable rainfalls and had high evapotranspiration (ET) rates had highly variable flow patterns (i.e., either very dry or wet). In contrast, Coweeta watershed (WS2) that received high rainfall but lower ET, had year-round continuous flows. The source of stormflows at the low-gradient watersheds was saturation excess overland flows that often extended beyond the riparian zones. In contrast, subsurface quick flows dominated the stormflows in Coweeta WS2 that has a narrowly confined riparian zone with saturated areas along the 1st order stream channel. We argue that the existing forestry Best Management Practices (BMP) rules on buffer width developed from upland watershed studies may not be adequate in protecting water quality for the low-gradient coastal watersheds. Overland flow in steep upland watersheds rarely occurs, thus attention should be focused on managing subsurface flows and stream bank protections.