Analysis of Red Soil Sediment Yield in a Small Agricultural Watershed in Ishigaki Island, Japan, Using Long-Term and High Resolution Monitoring Data

This paper analyzes the temporal variability in sediment yield in a small agricultural watershed located in the southwest portion of Japan. The relationships between the rainfall, runoff, and sediment variables were analyzed, and the suspended sediment concentrationdischarge hysteresis was used to interpret the mechanisms of sediment transport. A total of 227 flood events were derived from long-term high resolution discharge and sediment data that had been collected over the period of 2000-2015. Long-term analyses showed that there are highly variable hydrological and sediment responses on inter-annual, annual, seasonal scales. Sediment was strongly transported in the period of August-October, when the watershed was hydrologically and sedimentologically most active, followed by the February-April, MayJuly, and November-January periods. The sediment concentration for most flood events was relatively high, frequently exceeding 200 mg.L-1 and reaching as much as 1967 mg.L-1. The rainfall, runoff, and sediment variables showed significant correlations with each other. However, antecedent rainfall strongly influenced the efficiency of runoff generation, which further affected the sediment transport. Four types of hysteresis were observed: clockwise (48%), counter-clockwise (16%), figure-eight (25%), and complex (11%). The results confirmed the complexity and heterogeneity of both the hydrological and sedimentological responses in the studied watershed. Even though the analyses revealed the efficiency of the soil erosion-reduction schemes that had been implemented in the studied watershed, the red soil outflow is still relatively high. Therefore, additional measures (e.g., slope gradient modification, grass strips, and cover crop planting prior to summer sugarcane planting) are necessary to reduce soil erosion and sediment discharge.