A simple tile drainage model was incorporated into a recent version of the CROPGRO-soybean model. The model was calibrated and validated using four years of data from the Water Quality Research site, located at the Iowa State University Northeast-Research Center near Nashua, Iowa having two soil types (Readlyn and Kenyon). The data consisted of measurements of soil water contents at different depths and times, cumulative monthly tile flow, and final crop yield from 36 different 0.41 ha plots that were individually tile drained. The model was calibrated to give the best fit between the predicted and measured tile flow, soil water content, and crop yield from 1993 and 1994 for each soil type. It was validated by using data from 1995 and 1996 for each soil type. Predicted soil water contents, tile drainage and soybean yield matched measured values very well over all years for these two soil types. The results of this study show that the modified CROPGRO model does a very good job in simulating soybean yields and water dynamics for Readlyn and Kenyon soil in the experiment.