Abstract
The runoff characteristics of pesticides were observed every three days for four months in the Yodo River and its two tributaries, and monthly for their traverse distributions in the Yodo River while flowing down from the junction of the upper three big rivers to the Torikai Big Bridge. Simultaneous rice transplanting and subsequent application of pesticides in catchments caused high concentrations and loadings of pesticides. When the first-half stage of storm runoff events after pesticide application was observed, the concentrations and loadings of pesticides in the river were very high. The concentration peaks of pesticides were lower and broader in the Yodo River than in its tributaries due to the time lag in local pesticide application in its big catchment area. The peak times and heights of many pesticide concentrations on the right and left sides, and in the center in the traverse section of the Yodo River differed from one another owing to their differences among the three big upstream rivers and small midstream tributaries. The frequency distributions of pesticide concentrations in the fixed observation period became the logarithmic normal distributions due to many unditected low values. The runoff risks of pesticides in the rivers could be evaluated by the maximum concentration, ΣC·ΣT, cumulative loading per catchment area, and cumulative ditected concentration.
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