Hydrology and Water Quality of a Drained Loblolly Pine Plantation in Coastal North Carolina
This paper evaluates 17 years (1988-2004) of hydrologic and 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 30 years at the end of (2004). The 17-year average rainfall of 1538 mm was 11% higher that the 50-year (1951 – 2000) long-term data of 1391 mm observed at the nearest US Weather Bureau station in Morehead City, NC. Annual rainfall (and drainage outflow expressed as a % of rainfall) varied from as low as 852 mm (5% outflow) in the driest year (2001) to as high as 1308 mm (56% outflow) in the wettest year (2003), with an average of 33%. The daily outflows occurred much more frequently in winter resulting in 62% of rain as outflow than other seasons. The drainage outflow (runoff) on this watershed was primarily subsurface flow to drainage ditches and was related to water table depth (R2 = 0.75), which in turn, also correlated well with rainfall (R2 = 0.69). The water table depths tended to be close to the surface mostly during the winter and early spring, and during summer tropical storms, but were as deep as 2.5m during the dry summer-fall periods. 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, indicating that the ET may not have been limited by soil moisture conditions during the study period at the site. This was also indicated by the estimated average annual ET of 997 mm, which was 2.8% higher than Penman-Monteith based annual potential ET (PET) for a grass reference that varied between 782 mm to 1097 mm with an average of 970 mm. Although nitrogen (N) levels in drainage water were elevated after commercial thinning of the stand in late 1988 followed by fertilization, these elevated levels reduced substantially by 1995. Average annual concentrations of total N ranged from 0.57 mg L-1 to 2.0 mg L-1 with a long term average of 1.11 mg L-1. Annual average values for total P ranged from 0.01 to 0.06 mg L-1, with an average of 0.03 mg L-1. The average annual total N and P loadings were (6.9+-5.8) kg ha-1 and (0.17+-0.12) kg ha-1. Both concentrations and annual loadings were similar to other forested sites in the region.