Patterns of runoff behaviours in concentration and loading of pesticides from paddy fields have been intensively observed in the catchment area of Rikimaru Reservoir, mainly during the regular application period of fungicide during the summer. The runoff characteristics of three pesticides (pyroquilone, isoprothiolane and flutolanil) from paddy fields to the drainage river differed from one another due to their water solubilities. The linear correlation between the water solubilities and the amounts of pesticide runoff loadings from paddy fields to the drainage river and reservoir during dry weather days was confirmed as good on log-log paper. To reduce the risk to raw water for drinking in the reservoir the method of irrigation water management for paddy fields should be improved.

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