To elucidate storm runoff mechanisms in a mountain catchment, observations of soil water and spring runoff were conducted of a slope in a forested small catchment that had sufficient hydrological observational data. During the wetting stage of the soil layer in a rainfall event, the effect of spatial heterogeneity within the soil layer on the soil water distribution was large. However, in all zones where the soil layer was wetted to the bottom, the temporal change of rainfall was transmitted rapidly to the deep soil layer through pressure-head propagation. The zones were regarded as runoff contribution areas. In the runoff recession stage after the end of rainfall, the pressure head and the volumetric water content decreased in response to the decrease in the runoff rate. Based on correspondence between their monotonical decreases, we compared the total volume of spring runoff with the reduction in soil moisture in the runoff contribution area. We inferred that the reduction in soil moisture caused the discharge of spring water. When comparing the event described above with another event with a similar total rainfall but a larger runoff before the rainfall event, the latter showed slower runoff recession than the former, which was explained reasonably by differences in the expansion of the runoff contribution area because of the soil wetness conditions before the event.

　Because of increased extreme precipitation events caused by climate change, floods have occurred frequently in many parts of Japan. Earlier studies have suggested that floods accelerate population decline and that they adversely affect local economies, but most are case studies and include no discussion of the general relation between floods and population decline. For this study, encompassing 41 areas in Japan that have been affected by floods, we examined 1) the estimated number of population outflows after floods, 2) factors that exacerbate population outflows, and 3) demographic changes in flooded areas. The results demonstrated that the degree of damage to houses affected the willingness of people to move out of the area, that elderly people tended to stay in the area whereas farmers tended to move out, and that 0–20% of the affected people moved out every year. This study is expected to provide useful knowledge for local governments that are adversely affected by depopulation, and to help them cope with further population decline after floods.

　In addition to a transpiration model that incorporates the plant root distribution, a baseflow model and a piezometric groundwater flow model were newly developed to investigate the effects of horizontal groundwater flow on the hydrological cycle of the watershed. Both models are based on the the transpiration model and on the soil moisture deficit (SMD) concept. The baseflow model can represent the occurrence of baseflow. The piezometric groundwater flow model can represent the increase or decrease of SMD in different parts of the watershed, with consideration of the piezometric groundwater flow within the watershed. The model was applied to the Kitadani of the Tatsunokuchi-yama Experimental Watershed of the Forestry and Forest Products Research Institute, by separation of it into an upstream and a downstream area. Results show that, the model was able to represent the process of maintaining baseflow during periods of no rainfall. Results demonstrated that the formation of the variable source area is explainable as a process of SMD formation between the upstream and downstream areas because of piezometric groundwater flow.

　Kohama Pond and the Kakita River are adjacent spring systems situated at the southeastern foothills of Mt. Fuji. To characterize their hydrochemical variability, water quality was systematically monitored from May to November 2024 under both fair-weather and post-precipitation conditions. Parameters measured included air temperature, water temperature, pH, RpH, electrical conductivity, and the concentrations of sulfate, chloride, nitrate-nitrogen, and silica. Previous studies have demonstrated that both springs are predominantly recharged by groundwater originating from Mt. Fuji, with minor contributions from the Hakone and Ashitaka Mountains. Nevertheless, continuous hydrochemical monitoring of these systems has not previously been reported.

　Results indicate that Kohama Pond exhibited significantly higher water temperature and electrical conductivity relative to the Kakita River (p < 0.05). No statistically significant intra-site differences were observed between fair-weather and post-rainfall conditions. However, sulfate concentrations decreased in Kohama Pond and increased in the Kakita River following heavy rainfall, and this inter-site difference was statistically significant (p < 0.05).

　The Inter-Sectoral Impact Model Intercomparison Project (ISIMIP) is an international project aimed at comparing impact models for climate change globally across different sectors. Through ISIMIP, multiple impact models from many fields have used the latest climate and socioeconomic scenarios in common. A large-scale multi-model ensemble climate impact projection, including its associated range of uncertainty, has been constructed for use in research and policy planning for climate change. As described in this paper, ISIMIP features and overall progress are explained in addition to the specific activities and achievements in each field. Based on the latest international trends, implications for Japanese researchers are also discussed.