Annual patterns of mountain runoff and its long-term trends in Japan based on dam inflow data

Annual patterns in mountain runoff in Japan and its long-term trends were analyzed from dam inflow observational data. In this study, inflow into dams, which is often found in mountainous areas of Japan, was regarded as runoff in mountainous areas. Cluster analysis based on dam inflow revealed that mountain runoff in Japan has distinct regional characteristics. For dams on the Japan Sea side, inflow was predominately during the snowmelt season. For dams on the Pacific Ocean side, inflow was predominately during the summer rainy season. Greater inter-annual variability in annual inflow was identified for dams on the Pacific Ocean side due to the substantial inflow variability in summer. An analysis of dam inflow trends found annual dam inflows showed no significant trends for the majority of dams. In contrast, inflow in March and May at many dams in snowy regions showed notable changes. The findings suggest that the total available water resources in mountainous regions in Japan have not seen significant changing trends, but that climate warming has already led to changes in the timing of dam inflows (i.e. mountain runoff) due to earlier snowmelt in snowy regions in Japan.