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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

View larger version (87K): [in a new window]   Fig. 1. Map of Walnut Creek watershed showing monitoring locations for the watershed (330-stream site), two tile-drained subbasins (210- and 230-tile sites), the stream network, and known locations of tile mains. The 310-stream site, which provided data for flow calibrations (see text), is also shown.

View larger version (38K): [in a new window]   Fig. 2. Annual summaries of (A) precipitation, (B) streamflow, (C) flow-weighted NO3–N concentration, and (D) NO3–N flux for the three monitoring stations. Summaries are cumulative across dates given in Table 1, except precipitation, which is totaled for the entire year from 1993 through 2000. In particular, note that the record in 1992 begins after the 201st day.

View larger version (39K): [in a new window]   Fig. 3. Distribution of cumulative water flows (top), flow-weighted NO3–N concentrations (center), and cumulative mass export of nitrate N (bottom), according to date, given the period of record described in Table 1. The plots in the right column are the same but omit the 1993 flood year.

View larger version (36K): [in a new window]   Fig. 4. Distribution of NO3–N concentrations by day of year for each of the monitoring stations, with data from the 1993 flood year distinguished. Nondetects (<1 mg NO3–N L-1) are plotted at 0 mg L-1.

View larger version (30K): [in a new window]   Fig. 5. Frequency–duration plots for water flows, NO3–N concentrations, and NO3–N fluxes for the three monitoring stations.

View larger version (23K): [in a new window]   Fig. 6. Relationships between water flows and NO3–N fluxes for the 210- and 230-tile drains, and the 330-stream site. Iso-lines (10 and 1.0 mg L-1) provide orientation to interpret NO3–N concentrations. Individual samples with NO3–N concentrations less than 1.0 mg L-1 (the detection limit) are plotted at 0.5 mg L-1. Best-fit lines shown are those determined using the reduced major axis method (coefficients given in Table 3).

View larger version (23K): [in a new window]   Fig. 7. Cumulative frequencies of water flows, NO3–N fluxes, and estimated denitrification in a hypothetical wetland at the 210-tile site, sorted according to water flow rate. Denitrification estimates are based on a scenario of zero detention for the wetland, under which temperature and NO3–N supply rate could limit denitrification.