WATER BALANCE OF DRAINED PLANTATION WATERSHEDS IN NORTH CAROLINA

A 3-year study to evaluate the effect of thinning on the hydrology of a drained loblolly pine (Pinus taeda L.) plantation was conducted in eastern North Carolina. The study utilized a paired watershed design with a 40-ha thinned watershed (WS5) and a 16-ha control watershed (WS2). Data from the field experiment conducted from 1999-2002 was used to perform a water balance over pre- and post-thinning monitoring periods to gain a better understanding of the interaction of hydrologic components on drained forested organic soil watersheds. Components included in the water balance were watershed outflow, change in soil water storage in the soil profile (PS), and evapotranspiration (ET) (dry canopy transpiration + soil water evaporation + interception losses). Outflow accounted for 10 percent of the total observed precipitation for WS2 and 15 percent for WS5. The PS based on field measurements was 20 and -3 mm over the entire study period for WS2 and WS5, respectively. Gross ET during the 3-year study period (2000-2002) represented 89 percent of total precipitation for WS2 and 85 percent for WS5 based on the field water balance. The field-based water balance was compared to DRAINMOD predicted water balance components over the entire study period. Predicted water balance components; outflow, PS, and ET, were in close agreement with field-based water balance components over the study period. The differences in water balance components for the watersheds are primarily attributed to the reduced ET on WS5 as a result of the thinning operation.