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TECHNICAL REPORTS
Surface Water Quality

Evaluation of Nitrate Nitrogen Fluxes from a Tile-Drained Watershed in Central Iowa

USDA Agricultural Research Service-National Soil Tilth Laboratory, 2150 Pammel Drive, Ames, IA 50010

^* Corresponding author (tomer{at}nstl.gov)

Received for publication April 15, 2002. Nitrate N fluxes from tile-drained watersheds have been implicated in water quality studies of the Mississippi River basin, but actual NO₃–N loads from small watersheds during long periods are poorly documented. We evaluated discharge and NO₃–N fluxes passing the outlet of an Iowa watershed (5134 ha) and two of its tile-drained subbasins (493 and 863 ha) from mid-1992 through 2000. The cumulative NO₃–N load from the catchment was 168 kg ha^-1, and 176 and 229 kg ha^-1 from the subbasins. The outlet had greater total discharge (1831 mm) and smaller flow-weighted mean NO₃–N concentration (9.2 mg L^-1) than the subbasins, while the larger subbasin had greater discharge (1712 vs. 1559 mm) and mean NO₃–N concentration (13.4 vs. 11.3 mg L^-1) than the smaller subbasin. 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 showed smaller NO₃–N concentrations at low flows. Relationships between discharge and NO₃–N flux showed log–log slopes near 1.0 for the subbasins, and 1.2 for the outlet, considering autocorrelation and measurement-error effects. We estimated denitrification of subbasin NO₃–N fluxes in a hypothetical wetland using published data. Assuming that temperature and NO₃–N supply could limit denitrification, then about 20% of the NO₃–N would have been denitrified by a wetland constructed to meet USDA-approved criteria. The low efficiency results from the seasonal timing and NO₃–N content of large flows. Therefore, agricultural and wetland best management practices (BMPs) are needed to achieve water quality goals in tile-drained watersheds.

Abbreviations: DOY, sequential day of year

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