Long-term hydrology and water quality of a drained pine plantation in North Carolina

Long-term data provide a basis for understanding natural variability, reducing uncertainty in model inputs and parameter estimation, and developing new hypotheses. This article evaluates 21 years (1988-2008) of hydrologic data and 17 years (1988-2005) of water quality data from a drained pine plantation in eastern North Carolina. The plantation age was 14 years at the beginning of the investigation (1988) and 34 years at the end (2008). The 21-year average rainfall of 1517 mm was 9% higher than the 50-year (1951-2000) long-term average of 1391 mm observed at the nearest U.S. Weather Bureau station in Morehead City, North Carolina. Annual rainfall varied from 852 mm in the driest year (2001) to 2331 mm in the wettest year (2003) during the study period and was affected by several hurricanes and tropical storms. The runoff coefficient (ROC; drainage outflow expressed as a fraction of rainfall) varied from 0.05 in the driest year to as high as 0.56 in the wettest year (2003), with an average ROC of 0.32. Annual outflow (runoff) on this watershed was primarily subsurface flow to drainage ditches and was strongly correlated with rainfall (R2 = 0.81). Outflows were greater, more continuous, and longer in winter than in other seasons. Outflow in winter was 59% of rainfall on average. March was the only month that never produced zero outflow. The lowest mean outflow occurred in the spring and was significantly different from the other three seasons. Consistent with theory for subsurface drainage, outflow from this poorly drained land is dependent on water table elevation and occurs when the water table is within about 1.1 m of the surface. The water table tended to be close to the surface during the winter and early spring with low ET demands, and during summer with hurricanes and tropical storms producing large outflows, but was drawn down to depths much deeper than the drains during long dry periods in summer and fall. As a result, annual outflow and annual average water table depth were only weakly correlated (R2 = 0.52). There was no relationship (R2 = 0.01) between the annual average water table depth and the annual average evapotranspiration (ET), calculated as the difference between annual rainfall and outflow. The estimated average annual ET of 1005 mm was close to the Penman-Monteith based average annual potential ET (PET) of 1010 mm for a grass reference. Although nitrogen (N) levels in the drainage water were elevated after fertilization of the stand in late 1988, these elevated levels declined substantially by 1995. Average annual concentrations of total N ranged from 0.51 to 2.23 mg L-1 with a long-term average of 1.10 mg L-1. Annual average values for total P ranged from 0.01 to 0.12 mg L-1 with an average of 0.04 mg L-1. The highest average annual concentrations for N and P occurred in 1989 (N) and 1990 (P) following fertilization in spring of 1989. The average annual total N and P loadings were 6.5 ±5.3 kg ha-1 and 0.17 ±0.11 kg ha-1, respectively. Both concentrations and annual loadings were similar to other forested sites in the region. These long-term data should be useful for assessing the effects of land use change and management treatments on the hydrology and water quality of similar lands in the coastal region.