Evaluation of the root zone water quality model using field-measured data from the Missouri MSEA

The USDA-ARS Root Zone Water Quality Model (RZWQM) is a comprehensive compu ter model developed to simulate water, chemical, and biological processes i n the root zone of agricultural management systems. The model is capable of evaluating the effects of various cropping and management practices on sur face and ground water quality. In this study, the performance of RZWQM Vers ion 3.2 was evaluated for a claypan soil, particularly surface runoff and c hemical loss to surface runoff predictions. The model was calibrated and ev aluated using data collected from the Missouri Management Systems Evaluatio n Area (MSEA) and the Kingdom City runoff plots. Soil water predictions of the model compared well with those measured, particularly at the 15-, 60-, 75-, and 90-cm soil depths. In most cases, corn (Zea mays L.) and soybean [ Glycine max (L.) Merr.] yield predictions were within 15 and 20%, respectiv ely, of those measured. Using the macropore option (constant cracking) grea tly improved the prediction of chemical losses to seepage. Annual runoff si mulated for corn and soybean under conventional and no-till systems was ade quately predicted. The model underpredicted large runoff events and overpre dicted runoff events that occurred after long dry periods when soil crackin g was a dominant factor. The model overpredicted NO3-N concentrations in ru noff but underpredicted concentrations in near-surface soils. Predicted and measured atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine- 2,4 -diamine] and alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)ace tamide) concentrations in surface runoff compared well, particularly when t he computed runoff was close to that measured. In this study, the model was run using the option of constant cracking in the soil. To improve the pred ictions of agrichemical losses to runoff and seepage, RZWQM should include the capability to predict variable soil cracking based on soil moisture.