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Objectives of this study were to calibrate and validate a water quality model for monthly flow and NO3-N losses, and evaluate a set of alternative nutrient management practices to reduce NO3-N losses in an agricultural watershed. A dynamic watershed scale spatial modeling approach that uses ADAPT, a field scale model and GIS was calibrated to predict monthly flow and NO3-N losses from a sub-watershed of Seven Mile Creek in south-central Minnesota. It is a 4029-ha watershed with over 85% of the total area under agriculture. Calibration and validation of the model were done using monitoring data from 2000-2002 and 2003-2004, respectively. For the calibration period, the model predicted mean monthly flow and NO3-N losses of 0.38 m3/s and 4.04 kg/ha, respectively, against measured flow and NO3-N losses of 0.48 m3/s and 3.77 kg/ha, respectively. For the validation period, the predicted mean monthly flow and NO3-N losses were 0.29 m3/s and 2.93 kg/ha, respectively, against measured flow and NO3-N losses of 0.18 m3/s and 1.37 kg/ha, respectively. Long-term simulations were made for a wide range of climatic conditions between 1955 and 2004 to evaluate the effects of fertilizer management practices on the NO3-N losses. A 35% reduction in NO3-N losses was observed when application rate and timing were changed from a fall application of 179.3 kg/ha to a spring application of 112 kg/ha.