The Root Zone Water Quality Model (RZWQM, V 3.25) was used to simulate the effect of swine manure applications on nitrate-nitrogen (NO3-N) concentrations in subsurface drain water from continuous corn for Iowa soils. Measured values of subsurface drain flow, NO3-N concentrations in drainage water, and residual NO3-N in the soil profile from three chisel plow plots were available for the growing seasons of 1993 and 1995. The measured values were used to evaluate the RZWQM. Several parameters of

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... ZWQM were calibrated to provide satisfactory subsurface drain flow, nitrate in drainage water and nitrate in soil profile for the growing season of 1993. The calibrated parameters were then used to simulate subsurface drain flows, its NO3-N concentrations, and residual NO3-N content in the soil profile as affected by manure application for the growing season of 1995. Simulated subsurface drain flows, NO3-N concentrations, and total residual NO3-N contents were compared with the measured values. Predicted daily subsurface drain flows by the RZWQM were close to the observed flows. Annual total subsurface drain flows predicted by the model were also close to the observed values (difference over two years was -3.9%). The predicted NO3-N concentrations in subsurface drainage water followed the observed trends well for years 1993 and 1995 for all three plots. The annual average NO3-N concentrations predicted by the RZWQM were also in close agreement with the measured values for 1993 and 1995 (within a difference of -3.0%). Linear regression (zero interception) between the predicted values for the pooled data (average of three plots for two years) and the measured data gave an R2 value of 0.88 with a slope of 0.96. The predicted soil NO3-N contents in 0-1.2 m soil profile were also in close agreement with the measured values in the field. The overall results of this study indicate that RZWQM is capable of simulating various rate of manure applications in different weather and soil conditions.