Research on Characteristics of Turbid Water Effluent from Paddy Fields during the Paddling and the Transplanting Period

Turbid water inflow from paddy fields to river and lake especially during “spring agricultural practice in paddies” period which consists of plowing, paddling, and transplanting, is causing concern as one of the non-point source pollution. In a paddy watershed (4 ha) located on Lake Biwa basin, the investigation of suspended solid (SS) concentration and SS load at the end of the drainage and visual observation of progress in spring agricultural practice were carried out from the middle of April to the end of May 2004. During the investigation period, SS concentration and load showed two peaks. Relation between SS peaks and progress in spring agricultural practice suggests that movement of soil particles with water leakage through percolation contributes to the first peak, emerging between the beginning of irrigation and the practice of paddling. The causes of the second peak, emerging between the completing of paddling and the transplanting, are suggested to be overflow during paddling and artificial drainage just before transplanting. The loads during spring agricultural practice are 71 kg/ha/day for usual SS fraction (>1.0µm) and 104 kg/ha/day for fine SS fraction (>0.3µm). Many approaches for reducing the turbid water so far are mainly planed to measures against the second peak which is controllable by agricultural practices, however, the results indicated that it is also important to take effective measures against the first period.