Hydrology of Poorly Drained Coastal Watersheds in Eastern North Carolina

A 10,000 ha lower coastal plain land near Plymouth in eastern North Carolina has been intensively monitored since 1996 to measure hydro-meteorological parameters including outflows and quality of water drained from fields and subwatersheds with varying land management practices. This study summarized the data for a six-year period (1996-2001) for a 2950 ha forested, a 710 ha agricultural subwatershed and a 8140 ha watershed comprised of agricultural, forested, and riparian lands. The period covered a wide range of weather conditions from a dry year with annual rainfall of 775 mm in 2001 to a wet year with 1512 mm of rain in 1996 with two hurricanes. While 1998 with 1242 mm of annual rain experienced a wet winter and a prolonged dry summer-fall, the conditions were opposite in 1999 (1302 mm of rain) with a dry winter-spring and three hurricanes in the summer and fall. A near normal rainfall (1219 mm of rain) was observed in year 2000. The average annual PET for the site was estimated to be 1000 mm. Variability in annual rainfall was found to have greater effect than the land use type on annual outflows drained from these three watersheds. The average annual runoff/rainfall ratio for the managed pine forest watershed was the lowest compared to two other watersheds. Both the magnitude and frequency of peak flow rates were highest for the agricultural watershed, as expected. Average annual ET, calculated as difference of rainfall and outflow, was 922mm, 714 mm, and 727 mm for forested, agricultural and mixed land use watersheds, respectively. Annual ET estimated by the method suggested by Zhang et al. (2001) were in close agreement with the water balance for all six years when a plant-available water coefficient value of 3.0 was used for the managed pine forest. However, further tests of this ET model are suggested in other watersheds. These results will be valuable for estimating nutrient exports from the watershed as well as verifying watershed scale hydrologic and water quality models.