Evaluation of Hydrology and Water Quality in a Large Watershed in North Carolina's Lower Coastal Plain Following the Hurricanes and Related Storms of 1999.

Abstract

Quantifying and recording the impacts of large storm events is important to watershed management. Large concentrations of nutrients and pollutants may be lost from a watershed in runoff during these events.

This project documents the hydrologic and water quality response of a lower coastal plain watershed to the weather events of 1999 compared to 1996 through 1998. The watershed, which includes agriculture, natural forest, and managed forest, was instrumented to measure water quantity, water quality, and weather. Data was collected from 3 subwatersheds.

In 1999 the watershed received approximately 555 mm of rainfall during September and October associated with hurricanes, the wettest such period in 49 years. Prior to the hurricanes, the watershed experienced a dry spring and summer (565 cm for Feb.-Aug.). The period from February through August of 1999 was the third driest such period in 49 years.

The forested subwatershed (S4) has mostly organic soils with a large amount of drainable pore space, compared to the mineral soils of the agricultural subwatershed (T4). These pores were filled by the first hurricane (Dennis), which effectively made two passes over the watershed. During the first occurrence of hurricane Dennis, the forested subwatershed produced no flow, the agricultural watershed produced 2.9 mm of flow, and the mixed-use watershed (C2) produced 2.4 mm of flow. With the second pass of Dennis, the pores of forested soils were filled and 10.8 mm of flow occurred. In the agricultural subwatershed and the mixed-use subwatershed five to six times more flow occurred (59.1 mm and 64.5 mm, respectively). For hurricane Floyd, the flow response was similar for all subwatersheds since the soils were saturated (108.9 mm from S4, 164.2 mm from T4, and 163.3 from C2).

Daily flow rates measured across the research watershed were greater during hurricane Floyd than for any other time in the four-year study. Daily flows observed for the agricultural subwatershed were generally greater than for the forested subwatershed or for the mixed-use subwatershed throughout the study, and during the hurricanes of 1999. Total annual flow from the agricultural and forested subwatersheds in 1999 was similar to average annual flow for the three previous years, but that flow was concentrated in the months of September and October. Sixty-four and 66 percent of the 1999 annual flow occurred in the months of September and October for the forested and agricultural subwatersheds respectively.

Daily sediment and nutrient loads measured across the research watershed were greater during hurricane Floyd than for any other time in the four-year. In general, the two month period of hurricanes (1999) produced TN and TP loads comparable to loads for an entire year.

Total annual nitrogen load from an agricultural subwatershed was 18 kg⁄ha in 1999, compared to the 4-year annual average load of 15 kg⁄ha. Total annual nitrogen load from a forested subwatershed was 15 kg/ha in 1999, compared to the 4-year annual average load of 16 kg⁄ha. The nitrogen export observed in the forested subwatershed was high compared to other studies, likely due to the organic soils. Total annual phosphorus load from an agricultural subwatershed was 0.9 kg⁄ha in 1999, the same as the 4-year annual average load. Total annual phosphorus load from a forested subwatershed was 0.1 kg/ha in 1999, the same as the 4-year annual average load.

An agricultural area receiving swine lagoon effluent produced on average 44 kg/ha of TN, 30 kg⁄ha NO3, and 2 kg⁄ha TP. In 1999 the annual export was 106 kg⁄ha of TN, 73 kg⁄ha NO3, and 3 kg⁄ha TP.