RZWQM SIMULATION OF NITRATE CONCENTRATIONS IN SUBSURFACE DRAINAGE FROM MANURED PLOTS

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) c oncentrations in subsurface drain water-from continuous coin for Iowa soils, Measured values of subsurface drain flow NO3-N concentrations i n drainage water; and residual NO3-N in the soil profile from three ch isel plow plots were available for the growing seasons of 1993 and 199 5, The measured values were used to evaluate the RZWQM. Several parame ters of RZWQM were calibrated to provide satisfactory subsurface drain flow, nitrate in drainage water and nitrate in soil profile for the g rowing season of 1993, The calibrated parameters were then used to sim ulate subsurface drain flows, its NO3-N concentrations, and residual N O3-N content in the soil profile as affected by manure application for the gr-owing season of 1995. Simulated subsurface drain flows, NO3-N concentrations, and total residual NO3-N contents were compared with t he measured values. Predicted daily subsurface drain flows by the RZWQ M were close to the observed flows, Annual fetal subsurface drain flow s predicted by the model were also close to the observed values (diffe rence 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 concentra tions predicted by the RZWQM were also in close agreement with the mea sured values for 1993 and 1995 (within a difference of -3.0%). Linear regression (zero interception) between the predicted values for the po oled darn (average of three plots for two years) and the measured data gave an R-2 value of 0.88 with a slope of 0.96. The predicted soil NO 3-N contents in 0-1.2 m soil profile were also ill close agreement wit h the measured values in the field. The overall results of this study indicate that RZWQM is capable of simulating various rate of manure ap plications in different weather and soil conditions.