Impact of changing climatic conditions and permafrost degradation on discharge in southern Siberian catchments

Inspired by own field studies on permafrost and hydrological variability, a study was launched to investigate changes in runoff in southern Siberian catchments on the large scale. Until now, the reasons for the sometimes-drastic changes in river discharge remained more or less unanswered or were speculative. We combined data assimilation techniques, statistical analysis and hydrological simulation to investigate in detail the different climatic and physiographic impacts on the runoff behaviour of the Selenga (northern Mongolia / southern Siberia), the upper Lena and the Aldan (tributary to the Lena). Through this selection, we have included two different climatic and vegetation zones (semi-arid, transition zone between steppe and boreal forest, and humid, with exclusively boreal forest) as well as three areas with different permafrost extension (sporadic, discontinuous, continuous) in our investigations. We have selected a period of maximum data availability covering the years 1954-2013.

First, we were able to establish that due to the oscillating, large-scale atmospheric circulation over Siberia, precipitation and runoff are subject to periodic natural changes between rather dry and wet conditions. In addition to these oscillations, precipitation in all catchments shows negligible long-term trends over the 60 years period. Thus, we associate the long-term trends in runoff with the impacts of permafrost degradation. In the predominantly semi-arid Selenga catchment, lateral permafrost degradation prevails, i.e. decreasing permafrost extent. We found that there is a strong loss of water due to increased infiltration and seepage, resulting in drier conditions and ultimately lower runoff. In the boreal Lena and Aldan catchments, on the other hand, vertical permafrost degradation is prevalent. However, a frozen and thus impermeable layer remains below the growing active layer, which leads to overall wetter conditions and increased runoff. This contrasting behaviour mirrors the "dry becomes drier, wet becomes wetter" phenomenon described in earlier research. We also found that the warming-induced increases in permafrost thaw have led to major changes in the hydrological regimes of the three investigated basins. This integrated approach enabled new insights into the complex and highly dynamic hydrological changes that will provide impetus for subsequent studies.