Key drivers controlling the stable isotopes in precipitation on the leeward side of the central Himalayas

The Himalayas, as a barrier to the monsoonal moisture transport, cause an arid climate on the leeward side. This study investigates the mechanisms of isotopic variations in precipitation on the leeward side of the central Himalayas. Data of Xigaze, which is just north of the central Himalayas, suggest precipitation falls overwhelmingly in the summer monsoon season and subcloud evaporation plays an important role in altering the stable isotopes in precipitation due to the arid climate. Precipitation at Xigaze demonstrates a significant isotopic amount effect, whereby there is a significantly negative correlation between rainfall amount and δ18O. Such a result is found to partly arise from the effect of subcloud evaporation, as reflected by increasing evaporative signatures (higher δ18O but lower d-excess (d-excess = δD-8δ18O)) in rainfall with decreasing rainfall amount. Spatially, a pseud-altitude effect on precipitation δ18O and d-excess is observed on the leeward side of the central Himalayas. Lower d-excess but higher δ18O in precipitation are found at Xigaze than at Tingri, and this is likely caused by the stronger subcloud evaporation at the lower-altitude site of Xigaze due to its longer descent distance from the cloud base to the ground rather than the rainout process. Additionally, a synoptic survey of streamwaters across the central Himalayas helps to confirm that the spatial variation in isotopic values of precipitation is closely related to the difference in moisture source between windward and leeward sides and the degree of subcloud evaporation on the leeward side. These findings augment our understanding of local- to regional-scale hydroclimate in the central Himalayas.