CALIBRATION AND EVALUATION OF SUBSURFACE DRAINAGE COMPONENT OF RZWQM V.2.5

This study was designed to calibrate and evaluate the subsurface drain Bow 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 calibrat ed were lateral saturated hydraulic conductivity, and effective porosi ty, Subsurface drain flow predictions were made using calibrated param eters and compared with measured subsurface drain fleas for 1991 and 1 992. Measured subsurface drain Bow data for all 3 yrs was obtained fro m 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 ye ar, Coefficients of determination calculated for predicted vs, measure d 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 thos e for calibrated year 1990 (maximum subsurface drain Bow was observed under NT and minimum under MB), However, observed tillage effects vari ed from year to year, indicating a change in soil hydraulic properties , e.g., macroporosity. Other factors that could have caused the discre pancies between measured and simulated subsurface drain flows were: gr oundwater Bur due to natural gradient, deep seepage, inaccuracies invo lved in the estimation of breakpoint rainfall data, and spatial variab ility in soil properties.