This paper investigates the historical and future trends in water balance components and their impacts on streamflow. The trend analyses were applied to the daily climatic and hydrologic variables from 109 subbasins in Korea during the historical period and future period obtained by a multimodel ensemble of 13 global circulation models (GCMs) of the Coupled Model Intercomparison Project, Phase 5 (CMIP5). A calibrated hydrologic model, the precipitation-streamflow modeling system (PRMS) model,

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... s applied to obtain hydrologic data. The results revealed apparent trends in streamflow, with increases in spring and decreases in the other seasons during the historical period. The reduction (or increase) in the amount of streamflow was counterbalanced by the reduction (or increase) in precipitation, groundwater, and soil moisture, which was mainly impacted by the increase (or reduction) in actual evapotranspiration. However, opposite trends are projected for the future period for streamflow and water cycle components, in which spring and winter are projected to have increasing trends mostly counterbalanced by the decreasing trends in precipitation and groundwater. The reasons for the reduction in streamflow include elevated evapotranspiration compared to precipitation, reduced soil moisture, and a significant decrease in groundwater recharge. In addition, the results of the seasonal variability among basins revealed higher variability in summer for the historical period and in winter for the future period, with maximum variability in the Sumjin River basin, indicating that streamflow fluctuated more strongly in the Sumjin River basin during the historical and future periods.