Hydrological change in response to climate change and permafrost dynamics in southern Siberia

Changes in the cryosphere caused by global warming are expected to alter the terrestrial hydrological cycle, with inevitable consequences for freshwater availability to humans and ecosystems. In particular, the presence of permafrost influences water fluxes and dynamics in the subsurface system, thus, complicating the hydrologic processes in the permafrost-affected region. However, due to the absence of long-term, large-scale permafrost observations, a systematic understanding of how the warming-related permafrost dynamics may influence the large-scale hydrological processes is not clear yet. This work aims to address this research gap by investigating the hydro-climatic dynamics during 1954-2013 over multiple temporal scales, i.e., 60-year, 30-year, decadal and event scales. The study area is focused on three southern Siberian catchments with diverse ecosystems and permafrost distributions, namely, the semi-arid Selenga featured with sporadic permafrost, the boreal Lena covered by discontinuous permafrost, and the boreal Aldan is underlain by continuous permafrost. Our investigations will help to gain insights into the consequences of permafrost degradation on the terrestrial water availability from a hydrology-permafrost-climate coupling perspective. Due to “seesaw-like” large-scale atmospheric circulations over the Siberian region, we found that precipitation and river runoff resemble periodical natural oscillations between dry and wet states: while Selenga oscillates in a wet-dry-wet-dry pattern from 1954 to 2013, Lena and Aldan oscillate in a dry-wet-dry-wet mode. On top of these oscillations, the precipitation in all the catchments shows negligible long-term (60-yr) trends. However, significant but divergent long-term trends are observed in the river runoff among these catchments that can be related to different permafrost degradation dynamics. The semi-arid Selenga, dominated by lateral permafrost degradation (i.e., decreasing permafrost extent), suffers from severe water loss due to enhanced infiltration and percolation, leading to a drier condition and decreasing runoff. However, in the boreal Lena and Aldan where vertical permafrost degradation prevails, the thickened active layer is still underlain by a frozen layer that sustains a more water-rich surface condition and increasing runoff. As a consequence, a significant regime shift exists between the dry and wet states. In Selenga which is oscillating from a wet to dry state during 1984-2013, lateral permafrost degradation has further enhanced the drying dynamics, with a reduced runoff efficiency and a long-lasting dry period. In Lena and Aldan which is shifting from a dry to wet regime during 1984-2013, vertical permafrost degradation has led to an even wetter hydro-climatic condition in the boreal region. These contrast behaviors reflect a “dry gets drier, wet becomes wetter” phenomenon observed by previous researches. Furthermore, we show that the enhanced surface-subsurface connections have also induced an unprecedented increase in the baseflow during cold seasons. These investigations highlight the remarkable imprint of warming-related permafrost dynamics on regional hydrological changes, and could provide profound implications for the management of terrestrial water resources in permafrost-affected regions