Red-soil erosion and runoff have led to agricultural and environmental problems in Okinawa, and effective measures which can reduce sediment yield in the farmland are necessary. For the purpose, advanced model for predicting sediment yield and transport is required. The authors carried out long-term continuous observation for sediment yield and transport at Nagura watershed, Ishigaki Island. Observation items were precipitation, water depth, flow velocity, turbidity, and SS concentration. Observed data contain 31 rainfall events for 1.5 years and some characteristics of sediment runoff were grasped such as sediment runoff variations due to timing of flood and tide. In these studies, the Water Erosion Prediction Project (WEPP) model was tested to verify their applicability in predicting sediment yield on farmland and sediment runoff at watershed. The calculated sediment runoff was reasonable compared with observed one. Consequently, the applied result of the model using the climate data for the past 13 years has specified the severe eroded hillslopes and the severe sediment runoff months.
