Hydraulic Gradients and Flow Rates of a Shallow Coastal Plain Aquifer in a Forested Riparian Buffer

Abstract

Water table gradients were measured and saturated flow rates estimated for a hillslope consisting of a tilled upland field and a downslope riparian forest buffer system located in the Gulf-Atlantic Coastal Plain Tifton-Vidalia Uplands. Three years of water table measurements and estimates of saturated hydraulic conductivity were used to evaluate and quantify saturated water flow gradients, directions, and rates. Forest treatments consisting of clear cutting, thinning, and no cutting were examined. The gradient of the water table from the top of the landscape to the bottom varied from 0.9 to 0.2%, less than the 1.5% land slope. The direction of groundwater flow generally followed the land slope. However, during summer months the hydraulic gradient within the forested buffer reversed direction. Water table data indicate the riparian area was saturated from January through March. During this time, flow direction in the shallow aquifer is from the top of the field to the stream bottom. During summer months, high rates of forest evapotranspiration created large water sinks in the shallow subsurface and large local hydraulic gradients. Examination of water table elevations indicates the seasonal water demand of the forest shifts the direction of shallow subsurface aquifer flow. During these periods flow direction within the riparian buffer was from the lowest landscape position to the riparian forest, reversed from winter months. Total subsurface flow within the hillslope was calculated as 35 mm yr1, 3% of average annual precipitation. Average groundwater linear velocity was calculated as 1.4 mm h1. Evapotranspiration loss was estimated as 67% of average annual precipitation.