Because of increased heavy rainfall that is occurring along with climate change, some concern exists that damage caused by river flooding will spread. Countermeasures against damage are urgently needed. However, increased heavy rainfall along with climate change and the degree of current countermeasures differ depending on the river. The necessity of countermeasures and their degree require examination for each river. Therefore, it is necessary to detect effects of climate change and identify changes in risks for rivers. For that purpose, we developed a method of detecting climate change effects on river levels and changes in flood risk by selecting rivers to exclude, as much as possible, anthropogenic effects other than climate change. Specifically, from a water information system, we selected rivers that are unaffected by dams and stations upstream of the rivers to exclude urbanization effects. Then, we used Mann-Kendall tests to analyze trends for time-series analysis to ascertain whether higher water levels were recorded with the increased heavy rainfall and whether the annual maximum river level and the frequency of river levels at risk of flood were increasing. Results demonstrated that river levels have risen because of climate change effects in five of the seven target rivers. Flood risks also increased. Applying this method for each river, climate change effects on the river level were detected.
Changes in maximum and minimum daily runoff (no-snow season) caused by forest decline in the Minami-dani catchment, Tatsunokuchi-yama experimental watershed and No. 2 and No. 3 catchments, Kamabuchi experimental watershed were analyzed using paired catchment experiments. For the No. 3 catchment, the maximum daily runoff during the period affected by logging was about 8 mm day-1 greater, on average, than in the period without the effect. This value was considered not to be affected whether the rainfall was high or low at the rainfall event that caused the maximum daily runoff. Results obtained in the case of Minami-dani caused by wild fire and pine wilt disease were similar to those obtained for the case of No. 3 catchment. The increase in maximum daily runoff was thought to be mainly attributable to the forest tree disappearance. For the No. 2 catchment, the maximum daily runoff during the period affected by logging also tended to be greater than during the period when logging did not affect it. However, results suggest that greater rainfall during rainfall events producing maximum daily runoff is associated with a greater increased value. For the No. 2 catchment, about 20 % of the basin area became bare land because of slope failure that occurred after logging. Along with forest tree disappearance effects, results suggest that forest soil disappearance effects must also be considered.