Prediction of Pesticide Losses in Surface Runoff From Agricultural Fields Using Gleams and RZWQM

Seepage zones have been shown to be of critical importance in controlling contaminant export from agriculturalwatersheds. To date, no multipurpose agricultural water quality model has seepage zones incorporated into its process-levelrepresentations. We chose to test two widely used models of agricultural water quality, the Groundwater Loading Effects ofAgricultural Management Systems (GLEAMS) and the Root Zone Water Quality Model (RZWQM), by seeing how well eachpredicted solution pesticide concentration and loss in surface runoff from two agricultural fields: one with and one withoutseepage zones. Daily simulated atrazine and metolachlor concentration and loss in surface runoff from both calibrated anddefault (or non-calibrated) GLEAMS and RZWQM were compared with three years of measured data from the two fields. Theresults of the study show that GLEAMS and RZWQM using default input parameters were not capable of predicting atrazineand metolachlor concentration and loss in surface runoff from the fields with and without seepage zones (modeling efficiency<0.16). Site-calibrated GLEAMS and RZWQM predicted atrazine and metolachlor concentration and loss in surface runofffrom both fields (coefficient of determination >0.52, index of agreement >0.83, and modeling efficiency >0.53) and can beused for assessing the effects of seepage zones on pesticide loss in surface runoff from agricultural fields.