Assessment of streamwater age using water stable isotopes in a headwater catchment of the central Tibetan Plateau

Abstract

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 hydrological processes in alpine headwater catchments under global warming.