The long-term goal of this research is to improve tools for predicting water quality in agricultural watersheds with significant sub-surface drainage. Specifically, USDA-ARS' Root Zone Water Quality Model (RZWQM) was investigated as a tool for predicting the effects of agricultural management practices on nitrate loads in tile drainage in east central Illinois. Sensitivity of the model was explored to identify those input parameters with the greatest influence on tile flow, nitrate in tile drainage, and crop yield. Studies were made within the contexts of two hydraulic descriptions of a silty clay loam soil and two crops, corn and soybeans. Simulated tile flow proved to be most sensitive to drain spacing and soil hydraulic properties. In addition to the parameters affecting tile flow, tile nitrate-N was also greatly influenced by soil macroporosity. Biomass required to achieve a leaf area index of 1.0 had the greatest impact on crop yield. More information is needed regarding the statistical distributions of the input parameters. If the model is to be used to predict nitrate loads in tile drainage, particular care should be taken in selecting saturated, lateral hydraulic conductivity and Brooks-Corey soil properties.
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