SIMULATING FLUCTUATING WATER TABLES AND TILE DRAINAGE WITH A MODIFIEDROOT-ZONE WATER-QUALITY MODEL AND A NEW MODEL WAFLOWM

The Root Zone Water Quality Model (RZWQM) is presently undergoing exte nsive evaluation at several locations, especially in the Midwestern st ates participating in the Management Systems Evaluation Areas (MSEA) p rojects, where it will be used for quantifying and analyzing effects o f agricultural management systems on ground and surface water quality. Presently, the model does not simulate fluctuating water tables in th e root zone and tile drainage conditions that occur in several parts o f the Midwest. To study the effects on water quality under such condit ions, we modified the water movement module (newly named RZWFLO) of RZ WQM. To test the modified numerical scheme, we also simulated the abov e conditions with a new more rigorous numerical model (WAFLOWM) which contains an optimizing dynamic gridding scheme. Both models employed t he recently proposed mass-conservative simulation technique for solvin g the one-dimensional soil water flow equation, but RZWFLO uses pressu re head as the dependent variable, whereas WAFLOWM uses hydraulic head . The models also differed in the number and placement of numerical no des, and in their approaches to the simulation of infiltration. The re sults from both models were evaluated against three years of field dat a from a site near Aurora, North Carolina, where water table depths we re measured for three drainage spacings. Both models predicted water t able depths which were comparable to those observed and those simulate d by previous models within errors most likely caused by neglecting sp atial variability of hydraulic properties and uncertainties in input d ata and parameters.