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Baseflow is an all-important replenishment feature of the hydrologic cycle for maintaining surface water in basins across the world. Understanding its dynamics and the influence of climate variability and human activities will be beneficial for developing sustainable water-management strategies. In this study, we selected 11 basins within the Loess Plateau (a typical semiarid region in China) for assessment of the variations in baseflow and baseflow index (BFI; the ratio of baseflow volume to total streamflow volume) during the period 1961-2014. We compared eight baseflow separation methods and selected the median series in order to reduce the uncertainty. Results showed that values of the "Bflow" and Eckhardt filter methods were higher than values of the other six methods for the period of peak discharge. Annual baseflow exhibited a statistically significant downward trend (p < 0.01) in all 11 basins except Qingjian and Yanhe basins, while nine basins trended upward for BFI. All basins exhibited downward trends for seasonal baseflow in all seasons, with Yanhe and Qingjian showing the least significant trend. Variations in both baseflow and the BFI were more sensitive to variations in potential evapotranspiration than to variations in precipitation. Across these 11 basins for both the conservation engineering period (1971-1999) and the reforestation period (2000-2014), human activities (73% and 76%, respectively) contributed more than climate variability (27% and 24%, respectively) to average baseflow decline. However, climate variability was the main factor altering the BFI, with the contribution accounting for about 54% and 76%, respectively, for the two periods.