Is snowmelt runoff timing in the Japanese Alps region shifting toward earlier in the year

Inter-annual variations of snowmelt runoff timing in 15 basins across central Japan were analyzed across 30 years, from 1980–2009, to determine if mountain hydrology has been affected by global warming. Observed daily river discharge was utilized to calculate center time (CT) of mass of flow. CT was found to be occurring significantly earlier in the year at two northern basins, with a rate of change of around five days per decade. While decreasing trends in CT in the other basins were not significant, negative correlations between CT and winter temperature was significant except for the central to northeastern basins. The effect of winter warming on snowmelt runoff was more significant in northern basins on the Sea of Japan side, where CT also correlated with the flowering date of cherry trees. Positive correlations between precipitation and discharge were stronger in southern basins, disturbing winter warming effect on spring discharge. These findings support the notion that winter warming accelerates snowmelt runoff, although yearto-year fluctuations were more pronounced than progressive warming over the three decades. Our results highlight interbasin differences in hydrological response to climatic change, serving to validate down-scaling of climate simulations over the Japanese Alps region.

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