Projected climate change impacts on water availability and hydrological droughts in Japan

The global community is increasingly alarmed by the impact of climate change, particularly in relation to the increased occurrence of droughts and the subsequent depletion of water resources in coming years. However, specific future projections using high-resolution climate simulations that focus on the  water resources and frequency and intensity of hydrological droughts in Japan is currently lacking. This study investigated the impact of climate change on water availability and dry spells in Japan using large ensemble regional climate projections derived from the database for Policy Decision making for Future climate change (d4PDF). Self-organizing maps were applied to atmospheric circulation fields to study the linkages between water availability and weather patterns (WPs) during summer in the present and future climate simulations. The climate projections exhibited a reduced water availability defined by precipitation and evapotranspiration in the majority of Japan, consistent with increases in both frequency and duration of dry spells. The impacts of climate change on water availability vary by WP, with a significant increase in dry conditions under WPs with intensified climatological Pacific high over eastern Japan. Our results suggest that future changes in water availability can be attributed to changes in WP-related precipitation/evapotranspiration under warming conditions and in WP frequency in relation to the Pacific high, recognized as thermodynamic and dynamic effects of climate change, respectively. The decompositions of climate change impacts by WP analogs revealed that thermodynamic and dynamic effects account for approximately 80 and 20% of water availability changes, respectively. In addition, the effects of climate change on hydrological drought in central Japan were examined using hydrological model simulations that were based on climate projections derived from an ensemble of high-resolution downscaling at a 5-km scale. The results indicated a decrease in the streamflow during summer as the climate change progressed, corresponding to an increase in drought years. In addition, the 1%ile values of streamflow decreased by 40 (15) % in the +4-K (+2-K) warming climate simulations. Moreover, there was a considerable increase in the number of consecutive hydrological drought days, reaching an unprecedented level. The yielded results are useful in the consideration of adaptive solutions that will ensure the sustainable use of water resources that balances economic and environmental demands.