This study was designed to calibrate and evaluate the subsurface drain flow component of the Root Zone Water Quality Model (RZWQM; Version 2.5) for four tillage-systems: chisel plow (CP), moldboard plow (MB), no-tillage (NT), and ridge-tillage (RT). Measured subsurface drain flow data for 1990 was used for model calibration. Main parameters calibrated were lateral saturated hydraulic conductivity, and effective porosity. Subsurface drain flow predictions were made using calibrated parameters and compared with measured subsurface drain flows for 1991 and 1992. Measured subsurface drain flow data for all 3 yrs was obtained from the Nashua Water Quality Site in Iowa. The model, in general, showed a good agreement between measured and predicted subsurface drain flow values, although discrepancies existed for several days of a given year. Coefficients of determination calculated for predicted vs. measured daily subsurface drain flows ranged from 0.51 to 0.68 for 1990, 0.70 to 0.78 for 1991, and 0.54 to 0.69 for 1992. Simulated tillage effect on subsurface drain flows for 1991 and 1992 were consistent with those for calibrated year 1990 (maximum subsurface drain flow was observed under NT and minimum under MB). However, observed tillage effects varied from year to year, indicating a change in soil hydraulic properties, e.g., macroporosity. Other factors that could have caused the discrepancies between measured and simulated subsurface drain flows were: groundwater flux due to natural gradient, deep seepage, inaccuracies involved in the estimation of breakpoint rainfall data, and spatial variability in soil properties.