Management of Agricultural Drainage Canals for Reducing Outflow Load

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Nitrogen, phosphorus and carbon contents in the bottom sediment in an agricultural drainage canal in a paddy-field district around neighboring Lake Biwa were investigated in the present study. Nitrogen content in the sediment was high during the irrigation period and low during non-irrigation period. The temporal variation of carbon was similar to that of nitrogen. It was shown that phosphorus adsorption by the sediment depended largely on water temperature. It was concluded that water management practices (dredging of the sediment and closing of the floodgate) at the beginning of non-irrigation period was important for reducing effluent load from the paddy-field district.