Isotopic composition of daily precipitation along southern foothills of the Himalayas: impact of marine and continental sources of atmospheric moisture

摘要

The flow of the Himalayan rivers, a key source of fresh water to more than a billion people primarily depends upon the strength, behaviour and duration of Indian Summer Monsoon (ISM) and Western Disturbances (WD), two contrasting circulation regimes of the regional atmosphere. Analysis of H and O isotope composition of daily precipitation collected along the southern foothills of the Himalayas combined with extensive backward trajectory modelling was used to gain deeper insight into the mechanisms controlling isotopic composition of precipitation and the origin of atmospheric moisture and precipitation during ISM and WD periods. Daily precipitation samples have been collected during the period September 2008 – December 2011 at six stations, extending from Srinagar in the west (Kashmir state) to Dibrugarh in the east (Assam state). In total, 548 daily precipitation samples have been collected and analysed for their stable isotope composition. It is suggested that gradual reduction in H and O content of precipitation in the studied region, progressing from δO values close to zero down to ca. −10 ‰ in the course of ISM evolution, stems from regional, large-scale recycling of moisture-driven monsoonal circulation. Superimposed on this general trend are short-term fluctuations of the isotopic composition of rainfall, having their roots in the local effects such as enhanced convective activity and associated higher degree of rainout of moist air masses (local amount effect), cyclonic storms or impact of isotopically heavy moisture of continental origin. Seasonal footprint maps constructed for three stations representing western, central and eastern portion of the Himalayan region indicate that influence of monsoonal circulation reaches western edges of the Himalayan region. While characteristic imprint of monsoon air masses (increase of monthly rainfall amount) can be completely absent in the western Himalaya, the onset of ISM period in this region is still clearly visible in the isotopic composition of daily precipitation. Characteristic feature of daily precipitation collected during WD period is a gradual increase of H and O content, reaching positive δH and δO values towards the end of this period. This trend can be explained by growing importance of moisture of continental origin as a source of daily precipitation. High d-excess values of daily rainfall recorded at the monitoring stations (38 cases in total, range from 20.6 to 44.0 ‰) are attributed to moisture of continental origin released into the atmosphere during evaporation of surface water bodies and/or soil water evaporation.