Hydrologic and Water Quality Effects

Forest operations such as harvesting, thinning, and site preparation can affect the hydrologic behavior of watersheds on poorly drained soils. The influence of these operations conducted on organic soil sites can be more pronounced than on mineral soil sites due to the differences in bulk density and soil moisture relationships that exist between mineral and organic soils. This article reports the results of a study to evaluate the effect of thinning on the hydrology and water quality of an artificially drained pine plantation watershed on organic soils in eastern North Carolina. Outflow, water table depth, and water quality were monitored over a 3-year study period from paired 40 ha and 16 ha 15-year-old loblolly pine (Pinus taeda L.) plantations located in Washington County near Plymouth, North Carolina. Thinning increased daily outflow and peak flow rates based on a paired-watershed study design. Mean daily outflow doubled and peak flow rates increased 40% on the thinned watershed in relation to the control. Treatment effects were also observed on nutrient loads following the thinning operation. Phosphorous, TKN, and TSS loads increased following thinning, while nitrate-nitrogen loads decreased following thinning. These differences in hydrologic behavior are primarily attributed to the reduction in evapotranspiration that resulted from thinning.