The transport of three herbicides, pyriminobac-methyl, imazosulfuron and pyraclonil from a watershed that includes 40 ha of paddy fields to a drainage canal was monitored in the Lake Biwa basin, Japan. Based on the intensive monitoring of all paddy plots and in the drainage canal conducted on 3 days during and after the application period, the passage of herbicide discharge from the paddy fields to the drainage canal was separated into surface runoff obtained from field observations and percolation flow calculated from the herbicide mass balance. Surface runoff from paddy plots immediately after herbicide application, discharging a large volume of paddy water, or both processes in conjunction had a significant effect on herbicide discharge. Without surface runoff, paddy fields discharged a consistently high amount of herbicide gradually by percolation flow. These results suggest that considerable amounts of herbicides were discharged into the drainage canal through percolation even if appropriate water management to prevent herbicide surface runoff was practiced.
Keywords: herbicide discharge; paddy fields; percolation flow; surface runoff.