Abstract
In the watershed of the Ogouchi Reservoir, there are water source forests managed by the Tokyo Metropolitan Government Bureau of Waterworks and privately owned forests. The water source forest is expected to be effective in preventing sediment runoff, while the privately owned forest is concerned about sediment runoff due to lack of maintenance, which is a major concern in dam management. In this study, we attempted to extract and analyze the effects of major events by using long-term time series data based on periodic water quality surveys conducted by the water resource management office. Specifically, we attempted to evaluate the data by introducing a new evaluation method using a polar coordinate system to evaluate the scatter plot of the long-term time series data. As a result of the analysis using the devised evaluation index, the effects of the slope, tree density, and understory vegetation on the long-term effects of the water source forest and the forest condition affecting the prevention of sediment runoff were clarified based on the behavior of the ratio θ of flow rate specific turbidity.
