Assessing characteristics and long-term trends in runoff and baseflow index in eastern Japan

Baseflow is the portion of streamflow derived from delayed subsurface pathways. The baseflow index (BFI) is a ratio of baseflow to total streamflow, and is an important hydrological variable when linking watershed characteris- tics to baseflow. The 'smoothed minima' procedure of baseflow separation was applied to streamflow data (29-67 years) from twenty-six gauges of watersheds in eastern Japan. The Mann-Kendall statistical test and Sen's slope estimator were used to identify trends and estimate the rate of change in annual and seasonal runoff and BFI per decade at 0.01 and 0.05 significance levels. To the best of our knowledge, this is the first study to investigate long-term trends in runoff and BFI for watersheds in the large-scale region of eastern Japan. Results showed significant trends in annual runoff and BFI, with a concentration of signifi- cant seasonal trends occurring in winter with five gauges showing trends in runoff and nine gauges showing trends in BFI. The results suggest that the response of annual and seasonal runoff and BFI to climate change can already be seen, which implies that policymakers need more informa- tion on the impacts of climate change and human activities to manage water resources sustainably.