Forest management of Japan has been depressed in recent years. It is due to low-level stumpage price, low profit in forestry production, and decreasing number of forestry workers, who has been aged. And depressed forest management frequently causes serious devastation of hill slopes and torrents. This study cleared an influence of annual varietion of deforestation area and reforestation area on sedimentation of Miyakawa-dam, Mie prefecture. 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.
Hydrologic properties of soil layer are determined by various kinds of heterogeneities (for example soil pipes, cracks, cobbles and forest roots etc). And these heterogeneities give large influences to hydrologic processes in soil layer. Therefore, it is important to estimate accurately effects that heterogeneities give for water movement in soil layer. In spite of importance of these heterogeneities in soil layer, few studies concerned with these phenomena. 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. 1) Rainwater concentrates on the surroundings of forest roots. And, its tendency is enlarged in proportion to an increase of rainfall intensity. Runoff from surrounding of forest roots in steady state increases approximately with rainfall intensity. Runoff from surrounding of forest roots can be approximated lineally with rainfall intensity. Comparison of the observed and calculated runoff from surrounding of forest roots showed that the formula constructed in this study is adequate for the predicting the runoff from surrounding of forest roots. 2) Equivalent hydraulic conductivity at surrounding of forest roots is 1.1-3.2 timeses as large as one of the part of soil matrix. From these results, it is expected that rainwater concentrate on surrounding of forest roots.
Decision-making of evacuation from natural disasters, especially sediment-related disasters is surrounded by many uncertainties. A simple response system and a simple decision model are introduced in order to specifically analyze the uncertainty of forecast precipitation, which has a significant impact on the decision. The response system has precipitation as its input variables, geomorphologic and geological aspects as its parameters, and a signal of occur/not occur as its output. The decision model constitutes of a decision node (evacuate/not evacuate), uncertainty nodes (rain exceeding a critical amount), and consequence nodes (presumed loss of lives and cost of evacuation) . Operational forecast and actual precipitation used by Japan Meteorological Agency (JMA), which are called “Very Short-Range Forecast of Precipitation” and “Radar-Automated Meteorological Data Acquisition System (AmeDAS) Precipitation” respectively, are examined to verify their quantitative accuracy for the period from January 1998 to January 2001. Conditional probabilities of a precipitation amount exceeding a given critical amount when a certain amount of rain was forecast are obtained through the verification. The conditional probabilities have to go beyond the cost-loss ratio of evacuation decision-making in order for the evacuation decision to stand rational. The accuracy level of 1-hour operational forecast precipitation barely meets the required level indicated by cost-loss ratio for 5 by 5km square mesh, when a perfect response system is assumed. Importance of further research on (subjective) cost-loss ratios of actual decision-makers and sizes of areas in concern came to our attention.