The variation in continuity and geometry of macropores over the growing season can affect subsurface drain ‘tile’ water and nitrate-nitrogen (NO3-N) concentrations in tile water. This study analyzes the patterns of tile flows and nitrate-nitrogen (NO3-N) concentrations in tile water in relation to rainfall events using field measured data as well as invoking the macropore option of the Root Zone Water Quality Model (RZWQM98). The increase in NO3-N concentration in tile flow during the growing season in comparison with the decrease in NO3-N concentrations after the crop harvest following heavy rainfalls support the role of dual flow theory, soil matrix flow and preferential flow. These results and model simulations suggest that variation in macropore during the growing season can have significant effect on tile flow and NO3-N concentrations in tile water for soils similar to the study area.