In Japan, mountains occupy about 70% of the land area and most of them are covered by snow in the winter season. Future projections of runoff in snowy mountain areas are very important and often focus on the middle or late 21 century, whereas this study focuses on the immediate future and assesses the impact of climate change on the monthly runoff of four snowy mountainous basins. In this study, the immediate future was defined as the period 2006–2020 and uses future climate projection under the emissions scenario RCP8.5 of MIROC5, which was provided in ISI–MIP. The differences in the rate of GCM precipitation and that in GCM temperature between the historical period (1991–2005) and the immediate future (2006–2020) were applied to generate the future precipitation and temperature of each study basin. The conceptual hydrological model developed for mountainous basins (Fujimura et al., 2011) adopted in this study consists of the Diskin–Nazimov infiltration model and the storage–discharge relationships. To evaluate the reliability of the immediate future projection, the following three criteria were considered: (a) the simulated mean annual water balance for each study basin should be adequate, (b) the simulated flow duration curve should reproduce the observed flow duration curve, and (c) the relative errors between the simulated and observed hydrographs should be reliable. The results show that the difference the between observed runoff and the projected runoff could not be definitively detected by Student's t-test; however, the trends of the simulated increase/decrease monthly of runoff approximately reproduced the observed monthly runoff in 39 out of 48 months in the four basins.
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