Determining the age and sources of stream water is critical for understanding the watershed hydrological processes and biogeochemical cycle. In this study, daily isotope data of rainfall and runoff, as well as continuously monitored conductivity data from June to October in 2019 in-Laoyeling(LYL) watershed located in permafrost region of northeastern China were used to separate streamflow components through the application of two independent methods: isotope-based hydrograph separations (IHS) and the conductivity mass balance (CMB) methods. The results showed that stream water in a boreal forest watershed with permafrost of the Daxing'an Mountains is mainly composed of pre-event water. Although the IHS method is more sensitive and provides more details than the CMB method, the results of both methods show a similar trend. The average value of the young water fractions (Fyw) for those aged less than 65 days is 5.6%, while the mean transit time (MTT) was calculated to be 3.33 years. These findings enhance our understanding of the fundamental characteristics of runoff generation mechanisms and changes in runoff components in permafrost regions. Such knowledge is crucial for effective regional water resource management under the context of climate change, such as construction of water conservancy facilities and prediction of flood and drought disasters.

Global warming has significantly impacted the hydrological processes in alpine cryosphere region. Water age is an essential descriptor of the hydrological function within a catchment. However, the mechanism of streamwater age variability remains unclear due to limited observational data and high altitudes of alpine catchment. In this study, long-term stable isotopic data on streamwater in a catchment in the central Tibetan Plateau (TP) were collected to assess the water age using the sine-wave approach and gamma distribution. Results showed that the mean streamwater age was 77 days, and that 30 % of streamwater was less than 41 days old on average. The streamwater age in this study was relatively younger than that in low-elevation natural catchments, indicating that the rapid drainage process occurs within the glacier and permafrost catchment. The fraction of young water (Fyw) of the streamwater decreased from 39 % at an upstream site to 28 % at the outlet, revealing the impact of permafrost (low Fyw: 25 %) on streamwater age. These variabilities were related to glacier and permafrost coverage, specifically in catchments with higher glacier coverage that are prone to have a lower water age. Temporally, the streamwater age was significantly influenced by precipitation, relative humidity, and glacier change and, to a lesser extent, permafrost change. Mechanically, glacier and permafrost changes influenced the water age by increasing the vertical flowpath length. This study provides new insights into the change in hy-drological processes in alpine headwater catchments under global warming.