Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 28, 2002
Publication Date: April 25, 2003
Citation: TOMER, M.D., MEEK, D.W., JAYNES, D.B., HATFIELD, J.L. EVALUATION OF NITRATE-N FLUXES FROM A TILE-DRAINED WATERSHED IN CENTRAL IOWA. JOURNAL OF ENVIRONMENTAL QUALITY. 2003. V. 32. P. 642-653. Interpretive Summary: Nitrate losses from agricultural watersheds in the Midwest are the largest contributors to excess nitrogen the Mississippi River basin and ultimately low oxygen conditions in the Gulf of Mexico. However, the loads of nitrate lost from small, artificially drained watersheds in the region have not been well documented. We evaluated the flow of water and nitrate from Walnut Creek, a 12,700 acre Iowa watershed, from 1992 through 2000. The mouth of the stream and two tile-drained sub-basins were monitored. Nitrate concentrations exceeded the 10 mg L**-1 drinking water standard more than 70% of the time at the two sub-basins, but only about 30% of the time at the outlet. Nitrate was generally not diluted during high flows, except during 1993 flooding. Smaller nitrate concentrations were found at the mouth during low flows. Water and nitrate flow rates changed similarly in the sub-basins, but nitrate concentrations increased with flow at the stream mouth. Processes that remove nitrate in streams are apparently more effective at low flows. We also estimated that only about 20% of the nitrate could be removed by denitrification in a hypothetical wetland constructed to meet USDA-approved criteria. Consequently, wetlands may reduce nitrate loads during low flows, but may not substantially reduce large loads carried with large flows. This research demonstrates that wetlands alone may not achieve the desired reductions in nitrate loads. Such reductions will require agricultural systems to become much more efficient in nitrogen use. The results of this research will be important to farmers and action agencies when designing systems to reduce nitrate contamination of streams.
Technical Abstract: 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 cumulative 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 11.3 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 actually 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 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 BMPs are needed to achieve water quality goals in tile-drained watersheds.