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 agricultural watersheds. To date, no multipurpose agricultural water quality model has seepage zones incorporated into its process-level representations. We chose to test two widely used models of agricultural water quality, the Groundwater Loading Effects of Agricultural Management Systems (GLEAMS) and the Root Zone Water Quality Model (RZWQM), by seeing how well each predicted solution pesticide concentration and loss in surface runoff from two agricultural fields: one with and one without seepage zones. Daily simulated atrazine and metolachlor concentration and loss in surface runoff from both calibrated and default (or non-calibrated) GLEAMS and RZWQM were compared with three years of measured data from the two fields. The results of the study show that GLEAMS and RZWQM using default input parameters were not capable of predicting atrazine and 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 runoff from both fields (coefficient of determination >0.52, index of agreement >0.83, and modeling efficiency >0.53) and can be used for assessing the effects of seepage zones on pesticide loss in surface runoff from agricultural fields.