Calibration and evaluation of the root zone water quality for simulating tile drainage and nitrate leaching in the southern Piedmont
Abrahamson Stark, Deborah Ann
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Simulation models are used to evaluate the impact of alternative agricultural management practices on soil and water resources that would normally require expensive and labor intensive experimental techniques. However, models must first be evaluated for the system of interest in order to provide a credible account of the impact of different land management practices on these resources. The Root Zone Water Quality Model (RZWQM) was developed to provide a comprehensive simulation of root zone processes that affect water quality, and to respond to a wide range of agricultural management practices. The latest version of the model, v. 1.3.2004, was evaluated in this study for simulating tile drainage and nitrate leaching in maize and cotton production systems under conventional and no tillage management practices. The model accurately simulated tile drainage and nitrate leaching in maize production for conventional tillage management practices in a Cecil soil after calibration. Average cotton production and daily water use were also accurately simulated during the critical peak bloom period for the cotton growth calibration. There were no differences between simulated tile drainage with and without macroporosity in the model, which supports the field research at the study site. When the model was tested with an independent data set for cotton production, tile drainage and nitrate leaching, it over predicted tile drainage and leached nitrate by large amounts under both conventional tillage and no tillage management practices. However, the patterns of tile drainage, leached nitrate, and cotton development were well correlated with observed values. The differences in simulated and observed tile drainage and leached nitrate appeared to be due to 1) the under estimation of simulated ET for the cotton and winter rye crops and, 2) the differences in the amount of soil water and soil nitrogen available for tile drainage and nitrate leaching at the study site during the winter months compared to the period when the model was calibrated. Suggested improvements to the model include a user option to simulate vegetative growth into the reproductive stage for indeterminate crops such as cotton. In addition, model simulations of cover crop development for annual winter cover crop management practices could be improved by processes that allow soil water and nitrogen uptake to respond to various perturbations in soil water and nitrogen under a wider range of rainfall and climate conditions exhibited by annual winter rye from one growing season to the next. Guidelines or standard protocols used for calibrating a model may also be addressed with more interest because of our efforts in this study.