DISSIPATION AND LOSS OF ATRAZINE AND METOLACHLOR IN SURFACE AND SUBSURFACE DRAIN WATER - A CASE-STUDY

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 associate d in the watershed. The studied areas were plowed by conventional meth ods. The watershed has well-drained Guelph loam and Embro silt loam an d the field plots has a poorly drained Brookston clay loam. Dissipatio n rates of atrazine and metolachlor in the surface runoff and tile dra inage from the field plots were faster than in runoff from the agricul tural watershed. The applied atrazine and metolachlor losses in combin ed surface and subsurface runoff, were respectively 1.81, 0.33, 1.18 a nd 0.15%, for field plots and the watershed. The ratios between the he rbicide input rate and output rate of the watershed and of the field p lot are 2.33 and 2.76 for atrazine and metolachlor, respectively. Thes e ratios illustrate that herbicide loss is independent of spatial vari ations, Leading factors identified from the study to be considered for scale effect are, soil organic carbon, water table depth, soil moistu re content, soil textures, antecedent days, and time after application .