Mapping Turbidity Spatial Distribution of Monobe River in Kochi, Japan, Using Satellite Images and Existing Turbidity Data

ABSTRACT Maintaining the environment and functionality of rivers is crucial for the natural environment and socioeconomic activities. In Kochi Prefecture, field measurements of turbidity have been conducted to contribute to countermeasures against prolonged turbid water in the Monobe River; however, a detailed understanding of turbidity across the river has not yet been achieved. Therefore, we attempted to propose a simple turbidity estimation equation based on satellite remote sensing using existing data and visualized the spatial distribution of turbidity in the Monobe River. For turbidity of over 50 FTU, the mean absolute error and mean absolute percentage error (MAPE) of the proposed turbidity estimation equation were 9.9 FTU and 8%, respectively, showing higher accuracy than previous studies, while the accuracy in turbidity of under 50 FTU was not improved for practical use (MAPE was over 49%). By applying the turbidity estimation equation to satellite images, the spatial distribution of turbidity was visualized from the estuary of the Monobe River upstream to the Nagase Dam. The results were consistent with known findings on turbidity dynamics, such as decreasing turbidity in a straight section and increasing it in a curved section, and the spatial distribution of turbidity was considered to be reasonably estimated. The visualization also revealed cases in which turbidity fluctuated greatly in sections where field measurements were not conducted (e.g., increases or decreases in turbidity due to river morphology). In the future, it is expected that the turbidity estimation based on satellite remote sensing will be used in combination with the conventional field measurement of turbidity to understand and analyze the detailed situation of turbidity occurrence in the Monobe River.