Abstract:The observed retreat of several Himalayan glaciers and snow packs is a cause of concern for the huge population in southern Asia that is dependent on the glacial-fed rivers emanating from Himalayas. There is considerable uncertainty about how cryospheric recession in the Himalayan region will respond to climate change, and how the water resource availability will be affected. As a first step towards quantifying the contribution of glacier-melt water, hydrograph separation of River Ganga at Rishikesh into its constituent components, namely (i) surface runoff, (ii) glacial ice-melt and (iii) groundwater discharge has been done in this paper. A three-component mixing model has been employed using the values of υ 18 O and electrical conductivity (EC) of the river water, and its constituents, to estimate the time-varying relative fraction of each component. The relative fraction of the surface runoff peaks (70-90%) during winter, due to the near-zero contribution of glacial ice-melt, essentially represents the melting of surface snow from the catchment. The contribution of glacial ice-melt to the stream discharge peaks during summer and monsoon reaches a maximum value of ¾40% with an average of 32%. The fraction of groundwater discharge varies within a narrow range (15 š 5%) throughout the year. On the basis of the variation in the d-excess values of river water, it is also suggested that the snow-melt and ice-melt component has a significant fraction derived from winter precipitation with moisture source from mid-latitude westerlies (also known as western disturbances).

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The antiquity and decline of the Bronze Age Harappan civilization in the Indus-Ghaggar-Hakra river valleys is an enigma in archaeology. Weakening of the monsoon after ~5 ka BP (and droughts throughout the Asia) is a strong contender for the Harappan collapse, although controversy exists about the synchroneity of climate change and collapse of civilization. One reason for this controversy is lack of a continuous record of cultural levels and palaeomonsoon change in close proximity. We report a high resolution oxygen isotope (δ18O) record of animal teeth-bone phosphates from an archaeological trench itself at Bhirrana, NW India, preserving all cultural levels of this civilization. Bhirrana was part of a high concentration of settlements along the dried up mythical Vedic river valley ‘Saraswati’, an extension of Ghaggar river in the Thar desert. Isotope and archaeological data suggest that the pre-Harappans started inhabiting this area along the mighty Ghaggar-Hakra rivers fed by intensified monsoon from 9 to 7 ka BP. The monsoon monotonically declined after 7 ka yet the settlements continued to survive from early to mature Harappan time. Our study suggests that other cause like change in subsistence strategy by shifting crop patterns rather than climate change was responsible for Harappan collapse.

[1] Results of a 4 year (2005)(2006)(2007)(2008)) study of stable isotopic composition of daily rain and ground-level vapor (GLV) at a semiarid station in western India are reported. The GLV samples were collected by complete cryogenic trapping. The sampling was mostly limited to the rainy season (June, July, August, and September) and about a month before and after. The maximum number of samples was collected during the year 2007. The GLV has a steady baseline d18 O and dD composition without distinguishable seasonal differences. The d-excess of GLV indicates that its isotopic composition has a significant contribution from kinetic evaporation of nonlocal water sources. During a rain event, GLV rapidly interacts with raindrops and tends to move toward isotopic equilibrium. On cessation of rain, the d18 O and dD of GLV quickly return to the typical baseline values. Therefore, use of isotopic composition of monthly rainfall for estimating average monthly isotopic composition of GLV can lead to erroneous results. Within a rainy season, certain large rain events have depleted d18 O and dD values compared to other equally large rain events with significantly enriched d18 O and dD. These isotopic differences are apparently not related to amount of rainfall. Variable magnitude of evaporation from falling raindrops and/or cloud liquid water fraction cannot explain the observed differences. Instead, it is shown that varying source regions (Arabian Sea or Bay of Bengal) and cloud top temperature may be responsible for observed differences.Citation: Deshpande, R. D., A. S. Maurya, B. Kumar, A. Sarkar, and S. K. Gupta (2010), Rain-vapor interaction and vapor source identification using stable isotopes from semiarid western India,

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