Alteration of the prairie pothole ecosystem through installation of subsurface tile drains has enabled the U.S. Corn Belt to become one of the most agriculturally productive areas in the world but has also led to increased nitrogen losses to surface water. The literature contains numerous field plot studies but few in-depth studies of nitrate exports from small, tile-drained catchments representative of agricultural drainage districts. The objectives of this study were to quantify hydrology and nitrate-nitrogen (NO-N) export patterns from three tile-drained catchments and the downstream river over a 5-yr period, compare results to prior plot-, field-, and watershed-scale studies, and discuss implications for water quality improvement in these landscapes. The tile-drained catchments had an annual average water yield of 247 mm yr, a flow-weighted NO-N concentration of 17.1 mg L, and an average NO-N loss of nearly 40 kg ha yr. Overall, water yields were consistent with prior tile drainage studies in Iowa and the upper Midwest, but associated NO-N concentrations and losses were among the highest reported for plot studies and higher than those found in small watersheds. More than 97% of the nitrate export occurs during the highest 50% of flows, at both the small catchment and river basin scale. Findings solidified the importance of working at the drainage district scale to achieve nitrate reductions necessary to meet water quality goals. They also point to the need for implementing strategies that address both hydrology and nitrogen supply in tile-drained landscapes.

The site‐specific nature of P fate and transport in drained areas exemplifies the need for additional data to guide implementation of conservation practices at the catchment scale. Total P (TP), dissolved reactive P (DRP), and total suspended solids (TSS) were monitored at five sites—two streams, two tile outlets, and a grassed waterway—in three agricultural subwatersheds (221.2–822.5 ha) draining to Black Hawk Lake in western Iowa. Median TP concentrations ranged from 0.034 to 1.490 and 0.008 to 0.055 mg P L−1 for event and baseflow samples, respectively. The majority of P and TSS export occurred during precipitation events and high‐flow conditions with greater than 75% of DRP, 66% of TP, and 59% of TSS export occurring during the top 25% of flows from all sites. In one subwatershed, a single event (annual recurrence interval < 1 yr) was responsible for 46.6, 84.0, and 81.0% of the annual export of TP, DRP, and TSS, respectively, indicating that frequent, small storms have the potential to result in extreme losses. Isolated monitoring of surface and drainage transport pathways indicated significant P and TSS losses occurring through drainage; over the 2‐yr study period, the drainage pathway was responsible for 69.8, 59.2, and 82.6% of the cumulative TP, DRP, and TSS export, respectively. Finally, the results provided evidence that particulate P losses in drainage were greater than dissolved P losses. Understanding relationships between flow, precipitation, transport pathway, and P fraction at the catchment scale is needed for effective conservation practice implementation. Core Ideas Single events accounted for the vast majority of annual P and total suspended solids export. Frequent, low‐depth events resulted in extreme P and total suspended solids losses. Particulate P losses in drainage waters can exceed dissolved P losses.

A sub‐model for the Soil and Water Assessment Tool (SWAT) is developed to predict Escherichia coli levels in the streambed sediment as well as in the water column. New formulations to estimate the levels of E. coli in streambed sediment and the water column are derived. These equations include calculations of E. coli resuspension from the streambed sediment to the water column, E. coli deposition from the water column to the streambed sediment, E. coli growth in the streambed sediment and the water column, and instream E. coli routing. These formulations were programmed in FORTRAN and integrated into SWAT. The modified SWAT model was applied to Squaw Creek Watershed, Iowa, to predict E. coli levels in the stream. Escherichia coli concentrations in the streambed sediment and the water column were monitored extensively in this watershed, and observations were used to verify the model predictions. The model proposed here can predict E. coli concentrations in streambed sediment as well as in the water column. Approximately 58% of the predictions of E. coli levels in the bed sediment were within 1 order of magnitude from the observed value, and in the water column 83% of the predictions of E. coli levels were within 1 order of magnitude. Results suggest that the proposed model will help predictions of instream bacterial contamination.

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The ability to accurately simulate flow and nutrient removal in treatment wetlands within an agricultural, watershed-scale model is needed to develop effective plans for meeting nutrient reduction goals associated with protection of drinking water supplies and reduction of the Gulf of Mexico hypoxic zone. The objectives of this study were to incorporate new equations for wetland hydrology and nutrient removal in Soil and Water Assessment Tool (SWAT), compare model performance using original and improved equations, and evaluate the ramifications of errors in watershed and tile drain simulation on prediction of NO 3 -N dynamics in wetlands. The modified equations produced Nash-Sutcliffe Efficiency values of 0.88 to 0.99 for daily NO 3 -N load predictions, and percent bias values generally less than 6%. However, statistical improvement over the original equations was marginal and both old and new equations provided accurate simulations. The new equations reduce the model's dependence on detailed monitoring data and hydrologic calibration. Additionally, the modified equations increase SWAT's versatility by incorporating a weir equation and an irreducible nutrient concentration and temperature coefficient. Model improvements enhance the utility of SWAT for simulating flow and nutrients in wetlands and other impoundments, although performance is limited by the accuracy of inflow and NO 3 -N predictions from the contributing watershed. Editor's note: This paper is part of the featured series on SWAT Applications for Emerging Hydrologic and Water Quality Challenges. See the February 2017 issue for the introduction and background to the series.(KEY TERMS: wetlands; nitrate-nitrogen; tile drainage; SWAT; nutrient reduction.) Ikenberry, Charles