The drought-risk effects of climate change are estimated using 5-km regional climate ensemble experiments, which are dynamical downscaling datasets of “Database for Policy Decision-Making for Future Climate Change, d4PDF” having 20-km resolution large ensemble climate simulations. The Yodo River Basin study area has water resources managed using highly developed water utilization facilities.

　For climate ensemble experiments, we applied a moving average method using a “moving-window” and a quantile-mapping bias-correction method applicable to large ensemble datasets. The storage volume of Lake Biwa, the largest lake in Japan, was evaluated to estimate drought risk in the Yodo River Basin. And the storage volume of other many water resource development facilities are evaluated too. For this research, water cycle processes such as snowfall, snowmelt, evaporation and water use in the Yodo River Basin are calculated using a distributed rainfall-runoff model. Furthermore, all amounts of demand set in the water resource development plan are supplied in calculations.

　Frequency distributions of the annual minimum level of Lake Biwa in the present climate and future climate are estimated as a safe degree of water use.

　Results revealed the following: 1) The lowest operating level of the Lake Biwa for water use (B.S.L.-1.5 m) is in the range of non-exceedance probability less than 0.1 in the present climate, and more than 0.5 in a future climate that is 4K warmer than the pre-industrial climate. 2) The probable 100-year drought annual minimum level is B.S.L.-3.04 m to -3.74 m in non-bias-corrected future experiments and B.S.L.-4.46 m to -5.92 m in bias-corrected future experiments.

　This study demonstrated the co-benefit of river management projects for both flood control and improvement of suburban agriculture farmland in the basin with long-term data of the river water level, ground water level, and soil moisture conditions (NDWI) in lowlands around the Chitose River Basin. Results show three main factors affecting the development of suburban agriculture: (1) The river flood control projects drastically reduced the frequency of inundation and associated soil runoff from farmlands during floods. (2) The projects also reduced river water and ground water levels during ordinary water discharge, leading to improvement of wet conditions in peat swamps around the midstream area of the Chitose River (KP22 km − 28 km). (3) Adding drainage improvement through agricultural land projects has greatly improved farmland soil wetness. Agricultural production diversity was achieved in the basin. Although record-breaking rainfall has not been confirmed for about 40 years, attention must be devoted to changes in future rainfall characteristics.

　A continuous 60-year hydrological simulation has revealed that snow water equivalent (SWE) in 2020 is the least in many river basins in Japan over past 60 years. To investigate significance and reasons of the record-breaking snow scarcity in 2020, we used a standardized index, which is computed by applying the Box-Cox transformation to annual SWE, precipitation, and temperature estimates averaged over individual river basins. The standardized index indicated that SWE was lowest due to the highest temperature in Kanto region. In contrast, low SWE scarcity in Hokkaido was caused not by warmer temperature, but by lower precipitation. These analyses revealed that a major factor of snow scarcity is different between Hokkaido and the other regions.