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Nitrogen and Phosphorus Attenuation within the Stream Network of a Coastal, Agricultural Watershed

Institute of Marine Sciences, University of North Carolina at Chapel Hill, 3431 Arendell Street, Morehead City, NC 28557

View larger version (31K): [in a new window]   Fig. 1. Location of the study site in eastern North Carolina.

View larger version (18K): [in a new window]   Fig. 2. Planform view of the drainage area of the Southwest Creek watershed, including canals and ditch configuration of the modeled watershed.

View larger version (12K): [in a new window]   Fig. 3. Frequency distribution of discharge data from the watershed at Southwest Creek, August to December 2003.

View larger version (12K): [in a new window]   Fig. 4. Distribution of runoff volume and nutrient loads between base flow and storm flow periods, August to December, 2003.

View larger version (15K): [in a new window]   Fig. 5. Mass transfer velocity (Vf) of (A) NH4+ in ditches, (B) PO43– in ditches, (C) NH4+ in canals, and (D) PO43– in canals; error bars represent one standard error. No net uptake of PO43– was measured on 3 Sept. 2003.

View larger version (21K): [in a new window]   Fig. 6. Empirical relationship between the fraction of the (A) NH4+ load, and (B) PO43– load retained within the stream network and upstream discharge; lines show a polynomial equation fit to the model output. No measured discharge exceeded 2500 L s–1.

View larger version (9K): [in a new window]   Fig. 7. Cumulative predicted mass of NH4+ and PO43– entering the modeled stream network (In) and observed values exiting the watershed (Out) during storm events. The bar representing "In" is based on the model simulations using the median Vf (mass transfer velocity) from experimental injections, and error bars represent simulations using Vf values at the 25th and 75th percentile.

View larger version (20K): [in a new window]   Fig. 8. Dissolved inorganic N/P ratios observed at the watershed outlet upstream of the Southwest Creek estuary. Predicted values are derived from the upstream NH4+ and PO43– concentrations estimated using the retention model (Eq. [5]) for the lower 900 m of canal; predicted values incorporate the downstream observed NO3– values. Arrows indicate storm events.