Quantifying Watershed Loads to a Low Relief, Coastal Plain Estuary, the New River Estuary, N.C

Watershed modeling is an important tool for quantifying the inputs of fresh water, sediments, and nutrients into receiving estuaries and potential changes in those loads under scenarios including changes in land use and climate. There are a variety of existing watershed loading models available, from simple to complex, but a spectrum of these models have yet to be applied and compared in a low relief, coastal plain setting. This project has been conducted as part of the Defense Coastal/Estuarine Research Program (DCERP), which has focused on the impact of Marine Corp Base Camp Lejeune (MCBCL) and activities in the surrounding watersheds on the New River Estuary (NRE), located in southeastern North Carolina. As part of DCERP, nine sub-watersheds on MCBCL with contrasting land use were monitored to allow computation of freshwater, sediment, and nutrient loads to the NRE. In the current project, these loads were used to assess the performance of two existing watershed models using the Environmental Protection Agency’s (EPA) Better Assessment Science Integrating point and Nonpoint Sources (BASINS) 4.0 modeling suite: the relatively complex, temporally-resolved Hydrologic Simulation Program-Fortran (HSPF), and the relatively simple, annually-resolved Pollutant Loading (PLOAD) model. For both models, the 2001 National Land Cover Data were used for analysis; this dataset was compared to the recently released 2006 NLCD dataset and changes were found to be small. Monthly HSPF model output generally followed precipitation trends, and tended to over-predict freshwater stream flow and under-predict sediment and nutrient loads. PLOAD reproduced annual loads of total nitrogen within measured ranges, under-predicted annual loads of total suspended solids, and was less successful at predicting PO43- loads. Results from HSPF and PLOAD were combined with those from six other modeling approaches applied during DCERP to complete a spectrum of models from simple to complex. Model output from HSPF and PLOAD was scaled up to estimate loads entering the NRE from that portion of its watershed lying on MCBCL. Model estimates suggest that approximately 5-6% of the total nitrogen entering the NRE from external sources originates from the MCBCL watershed, a value on the lower end but within the range of estimates from other models applied to the system. Scenarios were run within HSPF to investigate how the conversion of forested land to impervious surfaces on MCBCL may alter existing loads; the model was relatively insensitive to changes in impervious surfaces. Neither PLOAD nor HSPF predicted nitrogen, phosphorus, and sediment loads better than the other watershed models applied during DCERP. The results of this study combined with development of other models suggest that simpler models, such as PLOAD, are able to estimate loads to the NRE as well as more highly technical models, such as HSPF, and that regardless of model choice a focus on loads at the annual scale is most justifiable