Study on influence of natural and social environment on sediment yield in the mountainous area

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Recently, in Japan forestry is decline because its profit falls down drastically for reasons that price of timbers get cheap in spite of increase of forest management cost, forestry workers decrease and get aged. Therefore, forests become to be left frequently after deforestation, resulting in serious devastation of hill slopes and torrents. In this study, sedimentation of Miyakawa-dam where located in Mie prefecture, and changes of deforestation and forestation with time was grasped to clean influence of forest practice on sediment yield. Using these results, sediment yield predicting model considering annual precipitation and forest practice conditions was constructed. And this sedimentation model was applied to Miyakawa-dam basin. As the result of this simulation, comparison of the observed and calculated sedimentation showed that this simulation model was adequate for the predicting the sediment yield in Miyakawa-dam basin.Furthermore, in order to understand effects of forest roots (living - root and decayed - root), which are a portion in primary factor to make soil layer structure heterogeneous, on hydrologic processes, that is, water movement in soil layer, field sprinkling experiments were carried out on a hillslope in the Kamiyadono basin where located in upper stream of Shigenobu-Liver. The following matters were confirmed by these field experiments. Rainwater concentrates on the surroundings of forest roots. And, its tendency is enlarged in proportion to an increase of rainfall intensity. Equivalent hydraulic conductivity at surrounding of forest roots is 1.1〜3.2 timeses as large as one of the part of soil matrix.