Evaluation of the RZWQM for simulating tile drainage and leached nitrate in the Georgia piedmont

Models have become an important tool for evaluating the impact of agricultural management practices on water quality. We evaluated the Root Zone Water Quality Model (RZWQM version 1.3.2004.213), for simulating the drainage and NO3 leaching under conventional and notillage management practices in cotton (Gossipium hirsutum L.) production and rye (Secale cereale L.) cover cropping practices in a Cecil (kaolinitic, thermic, Typic Kanhapludult) soil in Georgia, USA. We calibrated the model for the drainage and NO3 leaching in maize (Zea mays L.) production and for cotton development and water use in a previous study based on experimental data collected from 1992 through 1993 at Watkinsville, GA. For the current study, we used an independent data set collected from 1997 through 2000. Differences in measured and simulated the drainage were 926 mm in conventional tillage and 712 mm in no-tillage treatments. Measured and simulated values of leached NO3 were different by 62 and 73 kg harespectively, for the two tillage treatments. Some of the differences in simulated drainage compared with the calibration study could be attributed to differences in simulated evapotranspiration and runoff. Comparing the simulated and calculated water balances and winter rye production of the calibration study with the current study, however, the effects of winter cover cropping practices during the 4-yr period at the study site since the model was calibrated have affected the amount of soil water available for drainage and NO3 leaching at the depth of the drains.