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Adequate knowledge on the movement of nutrients under various agricultural practices is essential for developing remedial measures to reduce nonpoint source pollution. Mathematical models, after extensive calibration and validation, are useful for obtaining such knowledge and to identify site-specific best agricultural management practices. A spatial-process model that uses GIS and the ADAPT, a field scale daily time-step continuous water table management model, was calibrated and validated for flow and nitrate-N discharges from a 365 ha agricultural watershed in Walnut Creek, IA. This watershed was monitored for nitrate-N losses from 1992-1997. Spatial patterns in crops, topography, fertilizer applications and climate were used as input to drive the model. The first half of the monitored data was used for the calibration and the other half was used in validation of the model. For the calibration period, the observed and predicted flow and nitrate-N discharges were in excellent agreement with r2 of 0.88 and 0.74, respectively. During the validation period, the observed and predicted flow and nitrate-N discharges were in good agreement with r2 of 0.71 and 0.50, respectively. For all six years of data, the observed and predicted annual nitrate-N losses for the entire simulation were in excellent agreement with nitrate-N losses of 26 and 27 kg/ha, respectively. The calibrated model was used to investigate the long-term impacts of nitrate-N losses to changes in the rate of nitrogen fertilizer applications.