Ground Water Storage Effect on Streamflow for a Southeastern Coastal Plain Watershed

Ground water is known to substantially affect streamflow in regions where the two are interconnected. Likewise, streamflow can rapidly recharge surficial aquifers. Because of this interaction, accurate predictions of streamflow require accurate data on the relationships between streamflow and shallow aquifer conditions. Ground water levels in the surficial aquifer and streamflow were examined for a six-year period for two small watersheds in south-central Georgia within the Coastal Plain Physiographic region of the United States. Precipitation for the six-year period was below average. The shallow water table was elevated during average precipitation conditions, but receded as precipitation decreased. During the three-year period from 2000 through 2002, the riparian buffer was fully saturated approximately 40% of the time. Lower available aquifer storage was observed for late winter and early spring due to higher rainfall and lower evapotranspiration (ET). During recharge periods, which generally coincided with the period from December to March, the ground water gradient from the upland to the stream was approximately equal to the 3% land surface slope. During other times of the year, the gradient fluctuated dramatically as a response to individual precipitation events, streamflow, and focused recharge. Based on ground water discharging to the stream for 40% of the year, ground water discharge accounted for approximately 4% of the annual precipitation for the period from 2000 through 2002. Aquifer saturation, the fraction of the available aquifer storage filled by ground water, was found to have a substantial effect upon streamflow. During periods when the aquifer was fully saturated, observed storm-generated streamflow peak rates were greater and of shorter duration. Comparison of similar magnitude events occurring in 1998 indicated that peak discharge was up to three times greater under saturated aquifer conditions than under nonsaturated conditions. Watersheds with substantial pond acreage appear to extend storm-flow over longer durations. These data indicate the impact of aquifer and pond storage on peak and total discharge in coastal plain watersheds.