Climate Change Impacts on River Discharge Regimes and Hydrological Drought in Japan: Insights from Nationwide Hydropower Climate Projections

Despite growing concern over climate‑driven hydrological extremes, nationwide and regime‑specific assessments of future drought and river‑discharge changes in Japan remain limited. In this study, we examined the impacts of climate change on river discharge at major hydropower dam sites in Japan using hydrological simulations based on a 5-km resolution ensemble climate dataset. We applied clustering to the seasonal evolution of river discharge and evaluated the impact of each cluster, thereby revealing strong regime-dependent differences that challenge the feasibility of uniform adaptation. By integrating analyses of interannual changes in hydrological variables, we found a pronounced decline in water availability along with increases in both flood and drought frequencies. Notably, the number of consecutive hydrological drought increased by approximately 1.3 times under 2K warming and 1.7 times under 4K warming relative to present-day climate conditions. Amplified drought conditions emerged in regimes typical of the Sea of Japan side of western Japan and heavy-snow mountainous regions, where concurrent increases in evapotranspiration and decreases in precipitation were likely to intensify drought risk. Seasonally, hydrological drought increased most in December–January on the Pacific side and in August–September on the Sea of Japan side. These regional and seasonal contrasts were closely linked to global sea surface temperature warming patterns that modulate atmospheric large-scale circulation, thereby governing drought severity in Japan. Collectively, our findings highlight the need to tailor water-use adaptation strategies to specific regional and seasonal river discharge regimes under future climate warmings, underscoring the importance of regime-aware planning for hydropower and water resource management.