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Contamination of surface and subsurface waters with nitrate-nitrogen (NO3-N) has been related to agriculture across the Midwestern USA. The SWAT model was recently modified to better describe NO3-N fate and transfer within tile and pothole systems. The modified SWAT was evaluated using measured data from Walnut Creek watershed (WCW) located in central Iowa. These data include 7 years of measured NO3-N loading data in stream discharge and 4 years of soil NO3-N data at 105-cm depth. The model was calibrated to the period of 1992 to 1995 and validated to the period of 1996 to 1998. Nash-Sutcliffe E values was used to evaluate the accuracy of the model. The results show that the pattern of predicted and measured NO3-N loads in stream discharge at the center and outlet of WCW during the validation period was reasonably close (E value of 0.85 and 0.70, respectively). However, the daily prediction of NO3-N loads in stream discharge were not as good as monthly (E = 0.51 for the center and 0.37 for the outlet of WCW during the validation period). The model reasonably simulated monthly NO3-N loads in subsurface flows (E values=0.79 and 0.68, respectively for the calibration and validation periods), though improvement is needed in the simulation of daily subsurface NO3-N loads (E values = 0.47 and 0.38). A reasonably good pattern between measured and predicted soil NO3-N (for 105-cm depth) was found for all simulated soil types. The E values for predicted NO3-N in Okoboji, Clarion and Canisteo soil profiles during 1992 to 1995 were 0.61, 0.73 and 0.68, respectively.