Prediction of NO3-N losses with subsurface drainage water from manured andUAN-fertilized plots using GLEAMS

Excessive application of swine manure to a field over long durations can in crease nitrate-nitrogen (NO3-N) leaching as a result of accumulation of mor e nutrients in the root zone than the subsequent crops may need. the object ive of this study was to use the GLEAMS (V2.1) model to compare measured ve rsus simulated effects of swine manure application with urea-ammonium-nitra te (UAN) on subsurface drain water quality from beneath long-term corn (Zea mays L.) and soybean (Glycine max L.) plots. Four years (1993-1996) of fie ld data from an Iowa site were used for model calibration and validation. T he SCS curve number and effective rooting depth were adjusted to minimize t he difference between simulated percolation below the root zone and measure d subsurface drain flows. Model predictions of percolation water below the root zone followed the pattern of measured drain flow data, giving an avera ge difference of 10% and -5% between predicted and measured values for manu red and UAN-fertilized plots, respectively, for four years from 1993 to 199 6. Model simulations for overall NO3-N losses with percolation water were c omparable to measured NO3-N losses with subsurface drain water gibing an av erage difference of 20% for manured plots. The model overpredicted NO3-N lo sses, particularly for soybean on plots which received manure in the previo us year. Predicted NO3-N losses with subsurface drainage from fertilized pl ots were much lower than measured values with an average difference of -32% . The best fit line with zero intercept showed correlation coefficients of 0.73 and 0.66 between monthly predicted and measured NO3-N losses with subs urface drain flows for manured and UAN-fertilized plots for four years from 1993 to 1996, respectively. The results of the study show that the N-trans formation processes and the associated rate factors based on soil temperatu re and soil water levels may need to be refined for consistent simulation o f NO3-N losses with subsurface drainage water when fertilized with either s wine manure or UAN for corn production.