In-stream nonpoint source nutrient prediction with land-use proximity and seasonality

This research developed multiple regression models relating land use to in-stream concentrations of total nitrogen (TOTN) and total phosphorus (TOTP) in eight, low-order watersheds on the coastal plain of South Carolina, The study area (4860 km(2)) included dominant land-use categories of agriculture, forest, urban, and wetland comprising the lower portion of the Lake Marion drainage. Land-use data were obtained from a pre-existing GIS database derived by classification of satellite images. The models partitioned land-use categories according to distance from stream channels using a series of buffer zones around each stream, Effects of point source contributions were removed from observed in-stream concentrations so that nonpoint source effects could be more clearly delineated. All models except two were significant at P < 0.05, The models for TOTN (r(2) from 0.25-0.63) explained more variability of stream nutrient concentrations than those for TOTE (r(2) from 0.16-0.39), Greater predictive strength for TOTN than TOTP likely reflects differing pathways from terrestrial to aquatic systems, Land close to the stream channel (<150 m) was a better predictor of nutrient concentrations than land away from the channel (>150 m), Land-use change scenarios (converting forest and wetland to agriculture) support the conclusion that management of stream water quality will be most effective with emphasis on riparian and adjacent lands. Seasonal models were generally significant (P < 0.05) and demonstrate that the seasonal profile of stream nutrient concentrations is dependent on the mosaic of land uses in a specific subbasin.