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Streamflow Nutrient Dynamics on Coastal Plain Watersheds

USDA-Agricultural Research Service, Southeast Watershed Research Lab., P.O. Box 946, Tifton, GA 31793.

^* Corresponding author.

ABSTRACT

Streamflow and water quality parameters were measured on five watersheds (262 ha-11 487 ha) of the Little River in the coastal plain near Tifton, GA. Concentrations of nitrate, ammonium, chloride, ortho-P, total P, total N, and electrical conductivity were determined. Loads and flow-weighted concentrations were calculated for the five watersheds and for six subwatersheds formed by subtraction of smaller watersheds from larger ones. Mean concentrations and total loads calculated from samples taken every 3 h were not significantly different from samples taken every 12 h on two watersheds. In general, nitrate was enriched and chloride diluted relative to baseflow during storm events. This was attributed to reduction of nitrate in soils and aquifers of riparian areas. Both nitrate loads and concentrations were decreased during low flow years, whereas chloride concentrations were significantly higher in a year with abnormally low flows. Ortho-P concentrations increased during storm events, but total P showed the opposite response. Enrichment of NO₃-N and ortho-P and dilution of Cl and conductivity were due, in part, to concentrations in rainfall. Nutrient loads on the 11 watersheds and subwatersheds were similar except for one subwatershed that had the lowest percent of cropland but the highest loads. These high loads were due to exceptionally high flow rates in 1983 and 1984 following clear cutting of forest on nearly 25% of the subwatershed in late 1982. Nutrient loads in Streamflow were not correlated with the percent of land in spring-planted row crops but were correlated with land in winter wheat, which is fertilized in late fall or early winter. Conditions that change hydrologic responses of watersheds may change the nonpoint sources of nutrients by changing water volumes and by altering processes that create nutrient concentrations in Streamflow.