Variability and change of precipitation and flood discharge in a Japanese river basin

Study region: The Banjo River basin on Kyushu Island in the southwest of Japan. Study focus: This study focuses on the impact of local and/or large scale climate change on the river flow in the small and steep river basin. The variability, trend, correlation and change of extreme and mean precipitation and river (flood) runoff are investigated. New hydrological insights: In the small and steep river basins of Japan, the maximum annual flood runoff which occurs in the wet season, is well correlated with maximum precipitation on a daily time scale. Daily flow and precipitation data recorded in the basin from 1960 to 2015, have a correlation coefficient R-2=0.72. A high variability of the maximum and mean annual flows, and significant rising trends in daily extreme precipitation and river (flood) flows were also found in the data analysis. The maximum annual flood has increased on average by 5.98 m(3)/s per year from 330 m(3)/s in 1960; this increase is fully accounted for by the trend in observed maximum annual precipitation which is 1.1 mm/day per year. On an annual timescale maximum precipitation and maximum flood discharges have become substantially higher in phase with the ENSO (El Nino-Southern Oscillation) in the Pacific Ocean. Time frequency analyses revealed that a periodic component of the annual maximum flood flow in the Banjo River basin with a period of about 6 to 8 years has become significant since the beginning of the 21 st century. The number of days with more than 100, 200, 300, 400 and 500 m(3)/s flows has increased, which is important for flood control planning.