HYDRAULIC GRADIENTS AND FLOW-RATES OF A SHALLOW COASTAL-PLAIN AQUIFERIN A FORESTED RIPARIAN BUFFER

Water table gradients were measured and saturated flow rates estimated for a hillslope consisting of a tilled upland field and a downslope r iparian forest buffer system located in the Gulf-Atlantic Coastal Plai n Tifton-Vidalia Uplands. Three years of water table measurements and estimates of saturated hydraulic conductivity were used to evaluate an d quantify saturated water flow gradients, directions, and rates. Fore st treatments consisting of clear cutting, thinning, and no cutting we re examined. The gradient of the water table from the top of the lands cape to the bottom varied from 0.9 to 0.2%, less than the 1.5% land sl ope. The direction of groundwater flow generally followed the land slo pe. However, during summer months the hydraulic gradient within the fo rested buffer reversed direction. Water table data indicate the ripari an area was saturated from January through March. During this time, fl ow direction in the shallow aquifer is from the top of the field to th e stream bottom. During summer months, high rates of forest evapotrans piration created large water sinks in the shallow subsurface and large local hydraulic gradients. Examination of water table elevations indi cates the seasonal water demand of the forest shifts the direction of shallow subsurface aquifer flow. During these periods flow direction w ithin the riparian buffer was from the lowest landscape position to th e riparian forest, reversed from winter months. Total subsurface flow within the hillslope was calculated as 35 mm yr(-1), 3% of average ann ual precipitation. Average groundwater linear velocity was calculated as 1.4 mm h(-1). Evapotranspiration loss was estimated as 67% of avera ge annual precipitation.