Contribution of summer net precipitation to winter river discharge in permafrost zone of the Lena River basin

Winter discharge of the Lena River (Russia) has increased over the previous several decades. However, the impact of permafrost thawing and of changing hydrological processes induced by climate change on the river’s winter discharge is not well-quantified. Here, using a coupled land surface model and a distributed discharge model, we conducted trend analyses to examine the sensitivity of winter discharge to permafrost thawing and water budget change in the Lena River basin during 1979–2016. An increasing trend of winter baseflow was found in upper parts of both the Lena River basin and the Aldan River basin, where summer net precipitation showed a statistically significant increase. The increased summer net precipitation resulted in higher soil moisture in the deepened active layer in late summer and early autumn, which was linked to autumn and winter baseflow. These implications were examined from the perspective of interrelations among the trends of active layer thickness, soil moisture, and baseflow in the cold season by identifying regions in which all the variables exhibited positive trends. The identified source regions were primarily in the lower Lena River basin and upper basins of the Lena and Aldan rivers, although winter baseflow was more dominant in the latter regions owing to the freezing effect of the active layer. Thinning of river ice induced by warming temperatures also contributed to the increase of winter river discharge. These results suggest that the increased winter discharge was strongly associated with climate-change-related enhancement of permafrost thawing and increase in net precipitation that affected soil hydrological processes, which will be strengthened further in the context of global warming.