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THE EFFECTS OF LAND-USE/LAND-COVER, GEOMORPHOLOGY AND CLIMATE ON MAGNITUDE AND TIMING OF NUTRIENT EXPORT AND LOADING RATES IN THREE COASTAL PLAIN WATERSHEDS
Citation:
Lehrter, J. AND J. R. Pennock. THE EFFECTS OF LAND-USE/LAND-COVER, GEOMORPHOLOGY AND CLIMATE ON MAGNITUDE AND TIMING OF NUTRIENT EXPORT AND LOADING RATES IN THREE COASTAL PLAIN WATERSHEDS. Presented at Water Environment Federation 77th Annual Conference, New Orleans, LA, October 02 - 06, 2004.
Impact/Purpose:
Conference presentation
Description:
Watershed nitrogen (N), phosphorus (P), organic carbon (OC), and total suspended sediment (TSS) export rates were determined in 18 sub-basins of three watershed-estuarine systems over two annual cycles (2000 and 2001). The three watersheds all drain to the Mobile Bay estuary and differ primarily by the amount of surface area that is forested, urban, and agricultural. The 18 stations, representing the pour-point of each sub-basin, were visited 40 times during the study and water samples were collected and analyzed for TSS and the suite of particulate and dissolved, inorganic and organic components of N, P, and C. Waterborne loads for these constituents were calculated as the product of empirical and modeled flow rates and constituent concentrations. A watershed hydrology model was used to estimate daily flows in ungauged sub-basins while an empirical multiple regression model was implemented to calculate daily in stream constituent concentrations. Highest DIN and TN export rates were observed in the watersheds with the greatest density of agricultural activity. TN export rates also occurred in predominately forested wetland sub-basins. DIN and TN export rates ranged from .046 to 7.42 and 1.51 to 11.27 kg ha -1, respectively. Highest TP export rates were from basins having intermittent to low flow rates and from forested wetland basins. TP export rates ranged from 0.05 to 0.62 kg ha-1 yr-1. An urbanized sub-basin, undergoing active human-mediated disturbance, had the highest TSS export rates, >100 kg ha-1 yr-1. Inter-annual variation in precipitation also influenced export coefficients with predominately larger export rates for constituents having a positive correlation with flow in the wetter year of 2001 than in 2000. The loading ratios of DIN:TOC, shown previously to determine the trophic status of estuaries, were found to increase with the percent of sub-watershed area in urban and agricultural land-use. While high variability was observed in these data, clear patterns of sediment and nutrient export related to biophysical aspects of the watershed landscape and aquatic biogeochemical N, P, and C processing were observed.