Effects of controlled drainage on the hydrology of drained pine plantations in the North Carolina coastal plain

This paper presents results of a 5 year study to characterize the hydrology (rainfall, interception, evapotranspiration (ET), soil water storage, drainage rate, lateral seepage, and water table fluctuations) of three identical drained, pine-forested watersheds in Carteret County, North Carolina. During the 2 year calibration period (1988-1989), all three watersheds were operated in conventional drainage mode with the weirs in the outlet ditch approximately 1.0 m below the soil surface. About 17% of the total rainfall was intercepted and subsequently evaporated and 53% was removed by transpiration and evaporation from the soil during this period. Drainage removed about 28% and the remaining 3% was lost by lateral seepage. During the 3 year controlled drainage treatment period (1990-1993), drainage in Watershed 2, managed for tree growth, was reduced to 21% of gross rainfall as compared with 30.5% for Watershed 1 under free drainage. Watershed 3, managed to minimize offsite impacts, yielded 26% of gross rainfall as drainage. Interception loss accounted for about 14.5% of the gross rainfall. ET amounts computed as the residual in a water balance, were 50%, 60%, and 55% of total rainfall for Watersheds 1, 2, and 3, respectively. The effects of controlled drainage on water table depths, drainage and ET were demonstrated for seasonal and year-to-year variation in rainfall. The controlled drainage treatments affected both drainage volumes and daily peak outflow rates. The treatment in Watershed 3 was more effective in reducing peak outflow rates.