Abstract: Precipitation stable isotopes are integrated tracers of atmospheric processes worldwide. The precipitation stable isotopes (δ18O and δD) are influenced by numerous factors, including the condensation temperature, atmospheric humidity, air pressure, precipitation amounts, moisture sources and orographic terrain. In recent decades, many efforts have been dedicated to studying precipitation isotopic composition in various regions in the Tibetan Plateau,China,  located at the convergence between the westerlies and Indian monsoon. However, few studies are conducted in the Upper Indus Basin (UIB), which located in the western Tibetan Plateau. Based on the observation of precipitation stable isotopes (δ18O and δD) and local meteorological factors from August 2015 to July 2016, combining with the HYSPLIT model, this study investigated the characteristics of stable isotopes in precipitation, the relationship between isotopic variations and meteorological factors, Local Meteoric Water Line (LMWL), and the moisture sources of typical watershed in Bagrot Valley, Upper Indus Basin. The results show that precipitation δ18O and δD in Bagrot Valley displayed an obvious seasonal change. The δ18O and δD values are more depleted in autumn and winter but relatively enriched in spring and summer, indicating a significant temperature effec rather than precipitation amount effect. Moreover, the lower intercept and slope of Local Meteoric Water Line (LMWL) relative to GMWL indicated a strong sub-cloud evaporation owing to the relatively arid climate in the study area. Interestingly, different precipitation phases led to a difference in intercept and slope of meteoric water line. The lower intercept and slope of rainfall events result from strong sub-cloud evaporation. While meteoric water line of snowfall events has higher slope and intercept. It resulted from little impacts of recycled moisture and sub-cloud evaporation, due to the low temperature. Besides, compared with adjacent regions, the slopes of the LMWLs are nearly the same, however, the intercepts were increased with the latitude (except Shiquanhe), this may be caused by the strengthening sub-cloud evaporation and weakening effect of the marine moisture. In addition, the outputs of HYSPLIT model and results of precipitation stable isotopes demonstrated that Bagrot Valley is consistently dominated by the westerlies and local moisture recycling throughout the year and moisture source changes affect stable isotopes in precipitation. However, compared with Muztagata and Hotan, where marine moisture cannot reach, the study area of Bagrot Valley is affected by the marine moisture from the ocean occasionally as it lies southern to Muztagata and Hotan.

Key words: Upper Indus Basin, precipitation stable isotopes, meteoric water line, moisture sources