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The influence of fine sediment discharge from paddy fields on river water turbidity was investigated on the alluvial plain of the Eai River, Northeast Japan. The study comprises three phases: (1) Field measurements in a drainage channel from a rice paddy plot (3.7 ha); (2) Analysis of river turbidity data; (3) Discussion of the relationship between the two data sets. During data processing, turbidity was transformed to suspended solid (SS) flux for quantitative discussion, and particular attention was paid to the “farming calendar”, which is a rice production manual prepared nationwide by farmers cooperatives. The data obtained in the drainage channel were analyzed using the Hayashi’s quantification method II and the Galerkin method to derive an empirical equation for daily SS flux from the paddy plot. The results clearly showed the dependency of SS flux on the stage of the farming calendar as well as the daily precipitation. Analysis of the available river turbidity data showed that SS flux increased in the river reach where the paddy field discharge was concentrated; that the relationship between turbidity and SS in the downstream river reach was consistent with the relationship from the solution of paddy field sediment measured in the laboratory; and that the river turbidity occasionally exceeded the critical level for fish adaptation during the mid-summer drying period of the farming calendar. Application of the empirical equation for SS flux derived from the paddy plot measurements to the tributary watershed scale (3,100 ha) successfully reproduced the order of turbidity variation during the irrigation period in the downstream river reach.

Impact of fine sediment discharge from paddy fields on river water quality: a case study focusing on the rice farming calendar