The dissipation and loss of atrazine and metolachlor from field plots and an agricultural watershed were compared for scale factors related to dissipation and loss. The watershed and the field plots, located in southwestern Ontario, were planted with corn and cash crops associated in the watershed. The studied areas were plowed by conventional methods. The watershed has well-drained Guelph loam and Embro silt loam and the field plots has a poorly drained Brookston clay loam. Dissipation rates of atrazine and metolachlor in the surface runoff and tile drainage from the field plots were faster than in runoff from the agricultural watershed. The applied atrazine and metolachlor losses in combined surface and subsurface runoff, were respectively 1.81, 0.33, 1.18 and 0.15%, for field plots and the watershed. The ratios between the herbicide input rate and output rate of the watershed and of the field plot are 2.33 and 2.76 for atrazine and metolachlor, respectively. These ratios illustrate that herbicide loss is independent of spatial variations, Leading factors identified from the study to be considered for scale effect are, soil organic carbon, water table depth, soil moisture content, soil textures, antecedent days, and time after application.
