Evaluation of the Root Zone Water quality model using data from the Iowa MSEA

Before simulation models can be used to evaluate the benefits of alternative management systems, their accuracy needs to be examined. This study evaluated the performance of the Root Zone Water Quality Model (RZWQM Version 3.2) against crop yield and water, nitrate, and herbicide fate and transport during four years of a cora-soybean [Zea mays L.-Glycine max (L.) Merr.] rotation. The model was calibrated and initialized using default values supplied with the model and adjusting model parameters to obtain stable organic matter and microorganism, pools. The model was further calibrated to match crop yields for the first two years of measurements. For the second two years, predicted soybean yield was within 60 kg ha -1 (3%) of measured yield but predicted corn yield was underpredicted by 790 kg ha -1 (10%). Disagreements between modeled and measured yields may be due to factors not included in the model such as disease and insect damage. The model estimated total soil nitrate content well, although nitrification rates during winter were overestimated. Predicted nitrate leaching out of the root zone agreed well with measured losses to tile drainage. Predictions of herbicide residues with the one-parameter lumped dissipation model option were best after reducing the expected half-lives of metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one], or atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine], by a factor of four. The two-parameter lumped half-life model option improved the prediction for atrazine but not for metribuzin. Predictions of herbicide losses in drainage water were overestimated in the second year following herbicide application. Neither nitrate nor herbicide concentrations in the soil profile were modeled satisfactorily. Invoking the macropore option of the model did not consistently improve model prediction of herbicide leaching. Overall, the model shows promise in predicting the interplay of climate, soil, and management on crop yield and nitrate fate, but evaluation over a wider range of conditions is needed.