This paper evaluates the effectiveness of alternative agronomic practices at improving water quality in agricultural fields. A spatial process model was calibrated using monthly flow and NO 3-N losses from a 14 ha commercial farm in southeastern Minnesota. The study area was monitored for NO 3-N losses from 1999-2003. Spatial patterns in crops, topography, fertilizer applications and climate were used as input to drive the model. Monitoring data from 1999-2001 were used for calibration and data from 2002-2003 were used in validation of the model. For the calibration period, the observed and predicted flow and NO 3-N discharges were in good agreement with r 2 values of 0.86 and 0.86, respectively. During validation, the observed and predicted flow and NO 3-N discharges were in excellent agreement with r 2 values of 0.92 and 0.91, respectively. The calibrated model was used to evaluate the effects of rate and timing of N fertilizer application on NO 3-N losses. Reductions in NO 3-N losses are possible by changing the application timing from fall to spring. For an application rate of 180 kg/ha (160 lb/ac), there was an 8% reduction in NO 3-N losses when switching from a fall to spring application. Further reductions in NO 3-N losses require reductions in N application rates. A twelve percent reduction in nitrate losses was found when the application rate was reduced from 180 kg/ha (160 lb/ac) to 135 kg/ha (120 lb/ac).