Rainfall and flow discharge relationship in Japanese rivers: Effects of climate change on hydrological processes

Spatiotemporal change in precipitation induced by climate change can be a concern for riverine disasters. The relationship between precipitation and flow discharge in the 8 rivers from northeast to southwest of Japan (either mainstream or tributary) in which neither manmade dam nor reservoir is present was investigated based on observed data at the stream gauging stations managed by the Ministry of Land, Infrastructure, Transport and Tourism of the Japanese government. Observed data of air temperature, precipitation, etc. by the Japan Meteorological Agency vicinity of the gauging station in the 8 rivers basins were also used in the analyses. The precipitation concentration index (PCI) which is the Gini coefficient of precipitation during a year, and the Gini coefficient of flow discharge were computed for about half a century. The results reveal that annual maximum flow discharge can be related closely to the extreme rainfall events such as annual maximum daily or hourly precipitation. Obtained trends of the PCI were all positive and statistically significant at the 1% level, indicating that an in – equality in rainfall distribution during a year has been accelerated as the air temperature has risen in the basins. Whereas obtained trends of the Gini coefficient of the flow discharge were either positive or negative, and very weakly correlated with the trends in the PCI. Temporal precipitation distributions in a year have changed in the 8 rivers basin, i.e. light rain days (0 – 1 mm/day) have increased whereas rain days with 1 – 10 mm/day have decreased, while no such trend is seen in flow discharge in the 8 rivers. The interaction between surface and subsurface flows, and soil moisture may play important roles in moderating the effects of spatiotemporal change in precipitation. The flow discharge, however, can increase immediately in response to the precipitation when rainfall intensity is sufficiently strong.