Abstract: Water cycling in Qinghai-Tibet Plateau is a hot topic. However, the research on water transport at the watershed scale is relatively weak. In this study, stable isotopes of hydrogen and oxygen (δ²H and δ¹⁸O) were used to determine the temporal and spatial variation of precipitation, shallow groundwater, middle-layer groundwater, marsh water, spring water, creek water, lake water and soil water in a typical watershed of Haibei National Field Scientific Observation and Research Station (Haibei Station) in Qinghai Province. The MixSIAR model was used to quantitatively analyze water source of each water body and their exchange. The results showed that there were significant differences in the characteristics of hydrogen and oxygen stable isotopes of different water sources. The variations of isotope values in lake water and precipitation were greater than that of other waters, and the isotope enrichment in lake water was higher than that of other water bodies. Shallow groundwater (3 m) was an important regulating reservoir for water conservation in small watershed, while spring water was the surface manifestation of shallow groundwater. Soil water was important in water conservation of small watershed, as well as the transition in vertical exchange of precipitation-shallow groundwater (3 m)-middle groundwater (20 m). Soil water and shallow groundwater replenished water to creeks and marshes through lateral runoff. This study provides a basis for the optimization of hydrological model and the utilization of water resources in small alpine watersheds.

Key words: hydrogen and oxygen isotopes tracing, Mix SIAR model, small watershed, water exchange