Submitted to: Proceedings of the TMDL Science Issues Conference
Publication Type: Proceedings
Publication Acceptance Date: November 16, 2002
Publication Date: November 16, 2002
Citation: TOMER, M.D., MEEK, D.W., JAYNES, D.B., HATFIELD, J.L. EVALUATION OF NITRATE-N FLUXES FROM A TILE-DRAINED WATERSHED. PROCEEDINGS OF THE TMDL SCIENCE ISSUES CONFERENCE. 2002. CD-ROM. Alexandria, VA.
Nitrate-N fluxes from tile-drained watersheds have been implicated in water quality studies of the Mississippi basin, but actual NO3-N loads from small-order watersheds over a long period are poorly documented. We evaluated discharge and NO3-N fluxes from an Iowa watershed (5134 ha) from mid-1992 through 2000, including the outlet and two tile-drained sub-basins (493 and 863 ha). The total NO3-N load from the catchment was 168 kg ha**-1, and 176 and 229 kg ha**-1 from the sub-basins. The outlet had greater total discharge (1831 mm), and smaller flow-weighted mean NO3-N concentration (9.2 mg L**-1) than the sub-basins, while the larger sub-basin had greater discharge (1712 vs 1559 mm) and mean NO3-N concentration (13.4 vs 11.4 mg L**-1) than the smaller sub-basin. Concentrations exceeding 10 mg L**-1 were common, but least frequent at the outlet. Nitrate-N was generally not diluted by large flows, except during 1993 flooding. The outlet, in fact, showed smaller NO3-N concentrations at low flows. Relationships between discharge and NO3-N flux showed log-log slopes near 1.0 for the sub-basins, and 1.2 for the outlet, considering autocorrelation and measurement-error effects. We estimated denitrification of sub-basin NO3-N fluxes in a hypothetical wetland using published data. Assuming temperature and NO3-N supply could limit denitrification, then about 20% of the NO3-N load would have been denitrified by a wetland constructed to meet USDA-approved criteria. The low fraction largely results from non-dilution of NO3-N by large flows. Agricultural and wetland BMPs are needed to achieve water quality goals in tile-drained watersheds.