Reply on RC1

Abstract

Global warming has greatly impacted the hydrological processes and ecological environment in permafrost regions. Mean residence time (MRT) is a fundamental catchment descriptor that reveals hydrological information about storage, flow pathways, and water source within a particular catchment. However, water stable isotopes and MRT have scarcely been investigated due to limited data collection in the high-altitude permafrost regions. This study used the long-term stable isotopic observations to identify runoff components and applied the sine-wave exponential model to estimate water MRT in a high-altitude permafrost catchment (5,300 m a.s.l.) in the central Tibetan Plateau (TP). We found that the isotope composition in precipitation, stream, and supra-permafrost water exhibited obvious seasonal variability. Freeze-thaw cycles of permafrost active layer and direct input of precipitation significantly modified the stable isotope compositions in supra-permafrost and stream water. The hydrograph separation revealed that precipitation and supra-permafrost water accounted for 62 ± 13 % and 38 ± 13 % of the total discharge of stream water, respectively. We estimated that MRT for stream and supra-permafrost water was 100 and 255 days, respectively. Such shorter MRT of supra-permafrost and stream water (compared to the non-permafrost catchments) might reflect the unique characteristics of hydrological process in permafrost catchments. Moreover, the MRT of supra-permafrost water was found to be more sensitive to environmental change than that of stream water. Climate and vegetation factors affected the MRT of stream and supra-permafrost water mainly by changing the thickness of permafrost active layer. We conclude that global warming might retard the rate of water cycle in permafrost regions. Overall, our study deepened the understanding of hydrological processes in high-altitude permafrost catchments and provided a decision-making basis for ecological environmental protection and water resources safety in the source of rivers on the TP.