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. The flowof the Himalayan rivers, a key source of fresh water for more than a billionpeople primarily depends upon the strength, behaviour and duration of theIndian summer monsoon (ISM) and the western disturbances (WD), twocontrasting circulation regimes of the regional atmosphere. An analysis ofthe 2H and 18O isotope composition of daily precipitationcollected along the southern foothills of the Himalayas, combined withextensive backward trajectory modelling, was used to gain deeper insight intothe mechanisms controlling the isotopic composition of precipitation and theorigin of atmospheric moisture and precipitation during ISM and WD periods.Daily precipitation samples were collected during the period from September2008 to December 2011 at six stations, extending from Srinagar in the west(Kashmir state) to Dibrugarh in the east (Assam state). In total, 548 dailyprecipitation samples were collected and analysed for their stable isotopecomposition. It is suggested that the gradual reduction in the 2Hand 18O content of precipitation in the study region, progressingfrom δ18O values close to zero down to ca. −10 ‰ inthe course of ISM evolution, stems from regional, large-scale recycling ofmoisture-driven monsoonal circulation. Superimposed on this general trend areshort-term fluctuations of the isotopic composition of rainfall, which mighthave stem from local effects such as enhanced convective activity and theassociated higher degree of rainout of moist air masses (local amounteffect), the partial evaporation of raindrops, or the impact of isotopicallyheavy moisture generated in evapotranspiration processes taking place in thevicinity of rainfall sampling sites. Seasonal footprint maps constructed forthree stations representing the western, central and eastern portions of theHimalayan region indicate that the influence of monsoonal circulation reachesthe western edges of the Himalayan region. While the characteristic imprintof monsoonal air masses (increase of monthly rainfall amount) can becompletely absent in the western Himalayas, the onset of the ISM period inthis region is still clearly visible in the isotopic composition of dailyprecipitation. A characteristic feature of daily precipitation collectedduring the WD period is the gradual increase of 2H and18O content, reaching positive δ2H andδ18O values towards the end of the period. This trend can beexplained by the growing importance of moisture of continental origin as asource of daily precipitation. High deuterium-excess (d-excess) values ofdaily rainfall recorded at the monitoring stations (38 cases in total, rangefrom 20.6 to 44.0 ‰) are attributed to moisture of continentalorigin released into the atmosphere during the evaporation of surface waterbodies and/or soil water evaporation.

atmospheric chemistry and physics

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