MODIFICATION OF RZWQM FOR SIMULATING SUBSURFACE DRAINAGE BY ADDING A TILE FLOW COMPONENT

Fluctuating water table and subsurface drain flow components were inco rporated in the Root Zone Water Quality Model (RZWQM) to enable the mo del to simulate subsurface drain flows. Parameters in a modified model were calibrated using observed subsurface drain flows for 1990. Model performance was evaluated by predicting subsurface drain flows for 19 91 and 1992 by using the calibrated parameters and comparing the predi cted drain flows with observed subsurface drain flows for the same yea rs. The modified RZWQM model, in general, showed a good response to ra infall in terms of time of peak flows. However, the modified RZWQM mod el overpredicted total tile flows by an average of 13%, and the magnit udes;of peak tile flows were generally underpredicted. Selected soil p roperties (bulk density, macroporosity, and residue content) in the su rface horizon were changed to investigate tillage effects on tile flow s using the modified RZWQM. Four different tillage systems, chisel plo w (CP), moldboard plow (MB), no-tillage (NT), and ridge-tillage (RT), were considered. Predicted tillage effects on subsurface drain flows w ere consistent with the observed effects (i.e., maximum tile flow for NT and minimum tile flow for MB).