Arrival of monsoon, rainfall isotopic determination of the pre-monsoon - monsoon transition, tracing moisture sources across the Himalayas

Mountains are important water sources, storing water, releasing it in a buffered fashion and distributing it to the low-lying forelands. At the same time climate change is most pronounced in mountain regions, putting pressure on the water resources there. Understanding these changes requires better monitoring to robustly predict the changes in the mountain water cycle. In particular crucial is the seasonal supply of water by precipitation and the links with the evaporative source region, which again might depend on local changes there. To trace these pathways, automated water sampling devices are essential. To do so, we have developed an automated rain water sampler, that is robust to work under harsh conditions, can take 165 samples every 5min, is remotely accessible and provides samples of high analytical quality without atmospheric exchange.

From May to August 2022, we deployed 6 samplers across one of the most pronounced orographic precipitation gradients, the Himalayas in Nepal. The samplers were installed along the Kaligandaki River corridor, from ~ 100 m to 3700 m asl., from the border with India in the south to the dry region north of the mountain range. The sampling period was chosen to cover the sharp transition of the two contrasting seasons, pre-monsoon and monsoon. All samplers operated simultaneously for the full time with little technical downtime and we collected six unique high-resolution rainfall stable water isotope timeseries with roughly 1000 new samples. All measurements plot on the first order along the global meteoric water line. Temporally, the results show for all 6 stations a market trend with positive isotopic signatures during pre-monsoon and a very quick transition to negative signatures with the onset of monsoon. The pre-monsoon isotopic signatures are all in the range +30 to +40‰ in δ²H and all 6 stations at all elevations follow the same variability. Monsoon samples are in the range between -50 to -150‰ in δ²H. Unlike during pre-monsoon we observe a market separation of the isotopic signatures according to the elevation of the station. The lower station in the Gangetic Plains depicts signatures of around -50‰ δ²H, while the highest station features the lowest signatures of -150‰ δ²H. We attribute this market changes to the source signature of the evaporative source region. The pre-monsoon shows clear continental recycling signature and is sourced during the hot and moist pre-monsoon season from the Gangetic Foreland, Vapor during this season is transported in erratic and well mixed storm events toward the mountains. While the monsoon moisture is sourced far offshore in the Indian Ocean. The change between the two systems is very clear depicted in the isotopic signature. We accompany these analyses by lagrangian back trajectory analysis to determine the sources regions.

These findings show how variable the seasonal isotope input signatures in the Himalayan hydrological system are which has important consequences for tracing endmember signatures as well as the interpretation of climatological archives such as tree-rings or ice-cores and predicting future changes in the Himalayan water cycle with respect to the evaporative source regions.