Isotope fingerprinting of precipitation associated with western disturbances and Indian summer monsoons across the Himalayas

Precipitation samples were collected across the Himalayas from Kashmir (western Himalaya) to Assam (eastern Himalaya) to understand the variation of the stable isotopic content (delta O-18 and delta D) in precipitation associated with two dominant weather systems of the region: western disturbances (WDs) and Indian summer monsoon (ISM). Large spatial and temporal variations in isotopic values were noted with delta O-18 and delta D values ranging from -30.3 to +9.3 parts per thousand and -228 to +59 parts per thousand, respectively. The d-excess values also exhibit a large range of variation from -30 to +40 parts per thousand. In general, heavier isotopic values are observed in most of the samples in Jammu, whereas lighter values are observed in majority of the samples in Uttarakhand. Precipitation at Jammu seems to have undergone intense evaporation while that from Uttarakhand suggest normal Rayleigh fractionation/distillation of the air mass as it moves from the source region to the precipitation site and/or orographic lifting. The d-excess of rainfall in Kashmir has a distinctly higher median value of 18 parts per thousand compared to other precipitation sites with a median of 9-12 parts per thousand. Using distinct isotopic signatures, the regions receiving precipitation from two different weather systems have been identified.