Browsing by Author "Chauhan, Pankaj"

Now showing 1 - 5 of 5
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    Climate-driven acceleration in forest evapotranspiration fuelling extreme rainfall events in the Himalaya
    (IOP Publishing Ltd, 2021) Singh, Nilendu; Singh, Jayendra; Gupta, Anil K.; Brauning, Achim; Dimri, A.P.; Ramanathan, A.L.; Sharma, Vikram; Tiwari, Reet Kamal; Chakraborty, Joyeeta Singh; Chauhan, Pankaj; Shukla, Tanuj; Singhal, Mohit; Rawat, Suman; Agarwal, Shefali; Raja, P.
    Warming-induced expansion in vegetation coverage and activity can accelerate the montane hydrological regimes. However, the climate impacts on ecohydrology of forested valleys of the Himalaya are uncertain. In this study, utilizing results of about three centuries of cellulose isotope chronologies (? 13C and ? 18O) of dominant tree species, geo-chronological proxies, bio-geophysical dataset and simulations including satellite observations, we show an activation in the ecophysiological processes including evapotranspiration (ET) since the 1950s. Observation suggests rapid greening, while isotopic records indicate enhanced assimilation and transpiration in deciduous species vis-a-vis conifers post 1950s. Given strong vegetation-precipitation feedback and superimposed on the increasing trends of conducive atmospheric factors affecting valley-scale convective processes, intensification in forest ET is manifesting in a progressive enhancement in extreme rainfall events (EREs) since the last few decades. Results suggest that representation of ecophysiological processes and dynamics of seasonal moisture loading in observational and modelling framework is critical for understanding EREs under climate change. � 2021 The Author(s). Published by IOP Publishing Ltd.
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    Influence of Surface Nano-Structuring on Microstructure, Corrosion Behavior and Osteoblast Response of Commercially Pure Titanium Treated Through Ultrasonic Shot Peening
    (Springer, 2022) Chauhan, Pankaj; Shadangi, Yagnesh; Bhatnagar, Atul; Singh, Vakil; Chattopadhyay, Kausik
    Commercially pure titanium (Cp-Ti) is a widely accepted material for dental implants. Increases in surface roughness of these implants enhance osseointegration and result in better fixation with bone. Ultrasonic shot peening (USSP) is known to be an effective process of surface modification. This work is concerned with effect of USSP on modifications of the microstructure, surface topography, and corrosion behavior of Cp-Ti. The samples were USSPed for prolonged duration (30�min and 60�min) with both 1-mm and 3-mm diameter 100C6 steel balls. The thermal stability of the USSPed Cp-Ti was studied by stress relieving (SR) treatment at 500�C for 1 h. The surface roughness of the USSPed samples was increased and grains in the surface region were refined to nanoscale. The potentiodynamic polarization study showed significant increases in corrosion resistance of the USSPed samples. Further, enhanced osteoblast response was also observed for USSPed samples with and without SR. � 2022, The Minerals, Metals & Materials Society.
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    Spatial-diurnal variability of snow/glacier melt runoff in glacier regime river valley: Central Himalaya, India
    (Elsevier Ltd, 2021) Ahluwalia, Rajeev S.; Rai, S.P.; Meetei, Prikash N.; Kumar, Sushil; Sarangi, Shushanta; Chauhan, Pankaj; Karakoti, Indira
    Spatial-diurnal variability in the snow/glacier melt isotope signature and their influence on hydrograph separation based on mixing model received less attention. We present isotope data from a high elevation catchment (glacierized area: 286 km2) in the central Himalaya (India) and investigated the spatial-diurnal variability of snow/glacier meltwater along with inferences of groundwater dynamics. Isotope signature variation in streamflow was small during the study period. We applied a two-component mixing model using oxygen-18 and electrical conductivity. Hydrograph separation (snow/glacier meltwater and groundwater) was carried out for Bhagirathi River at three sites i.e., Gaumukh, Bhojbasa, and Gangotri, during the ablation period of 2014 (September). The Bhagirathi River is a major tributary of river Ganga, originate from Gangotri Glacier (~30 km), the largest glacier in central Himalaya. The electrical conductivity of the river is measured in-situ and varies from 10 ?S/cm to 100 ?S/cm. The river water isotope signature of oxygen was ranged from ?15.53� to ?14.32� from Gaumukh to Gangotri, snow samples were ranged from - 17.63� to - 15.86� collected at Gaumukh. Groundwater samples were varied from ?8.53� to-7.57� from Gaumukh to Gangotri. River water signature is close to snow/glacier melt runoff signature, reveal that the snow/glacier melt runoff contribution is higher in river flow. Average fractions of snow/glacier melt runoff were estimated ~82% to~76%, whereas groundwater was estimated ~18% to ~24%. The results of this study reveal the necessity of a multiple sampling approaches to characterize the snow/glacier melt and the importance of groundwater dynamics in catchments with snow/glacial runoff regime. � 2021
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    Temperature variability over Dokriani glacier region, Western Himalaya, India
    (Elsevier Ltd, 2023) Rastogi, Tanupriya; Singh, Jayendra; Singh, Nilendu; Chauhan, Pankaj; Yadav, Ram R.; Pandey, Bindhyachal
    Long-term climate records which help decipher past climate variability and its impact are scarce in the tough terrain of the Himalayan region. Therefore, in order to fill the climate data gap and understand the glacier climate linkage, we developed a 231 year long (1785�2015 CE) March�June temperature record using ring-width chronology of Himalayan fir (Abies pindrow (Royle ex D.Don) Royle) for the Din Gad valley, Dokriani glacier region, Uttarkashi, Uttarakhand, in the Western Himalaya. The Din Gad, originating from the Dokriani glacier, is a meltwater river contributing to Bhagirathi catchment in the headwaters of the socio-economically vital Ganga River. The 21-year running mean of the temperature record showed 1978�1998 CE as the coldest period followed by 1925�1945 CE, and 1890�1910 CE as the warmest period followed by 1946�1966 CE over the entire time series. The reconstruction matches well with tree-ring based temperature records available from the Garhwal Himalaya. It also shows similarity to tree-ring based temperature reconstructions from the Western Himalaya, Nepal, Tibetan Plateau and Bhutan, thus displaying a regional scale climate signal. The low frequency fluctuation patterns of the March�June temperature also matches with Asia and Northern hemisphere temperature records. Reconstructed March�June temperature record showed a statistically negligible warming temperature trend during 1901�1989 CE in the 20th century. It, however, captured a warming spike from 1990s CE which continues rising into the 21st century, which is also evident in the Northern hemisphere temperature record. Moreover, temperature rise is not anomalous in the past 231 years and well within range of the rest of the series. The present temperature record exclusively from the glacier region revealed a strong linkage with the benchmark Dokriani glacier's winter mass balance (November�April) revealing mass loss (gain) episodes occurred in warm (cool) phases. This first such record from the glacier valleys in Ganga headwaters would be of great value at providing insight into past climate variability and glacier behaviour with respect to climate change in long term perspective, and thus would provide valuable information for water resource management in light of climate change. � 2023 Elsevier Ltd and INQUA
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    Tree-Ring Isotopic Records Suggest Seasonal Importance of Moisture Dynamics Over Glacial Valleys of the Central Himalaya
    (Frontiers Media S.A., 2022) Singh, Nilendu; Shekhar, Mayank; Parida, Bikash Ranjan; Gupta, Anil K.; Sain, Kalachand; Rai, Santosh K.; Br�uning, Achim; Singh Charkaborty, Joyeeta; Sharma, Vikram; Kamal Tiwari, Reet; Chauhan, Pankaj; Montagnani, Leonardo
    Accelerated glacier mass loss is primarily attributed to greenhouse-induced global warming. Land�climate interactions have increasingly been recognized as an important forcing at the regional-local scale, but the related effects on the Himalayan glaciers are less explored and thought to be an important factor regulating spatial heterogeneity. The aim of the present study is a multi-decadal approximation of glacier�hydroclimate interaction over the western region of the central Himalaya (WCH). Multi-species, highly coherent, tree-ring cellulose ?18O chronologies from three sites across the WCH were used to derive atmospheric humidity (Atmospheric Moisture Content: AMC) record of the last four centuries. Annual-scale AMC reconstruction implies a decreasing regional atmospheric moisture since the mid-19th century and a sharp decline in recent decades (1960s). Coherency analyses between regional AMC and glacier mass balance (GMB) indicate an abrupt phase-shift in the relationship after the 1960s within a common record of the last 273�years. To ascertain the cause of this phase-shift, annual AMC was disintegrated into seasonal-scale, utilizing ?200�years of ?18O record of a deciduous tree species. Seasonal (winter: October�March; summer: April�September) AMC reconstructions and disaggregation results indicate higher sensitivity of regional ice-mass variability to winter moisture dynamics than summer.Winter season AMC reconstruction confirms a revival of winter westerlies-driven moisture influx in the region since the 1970�s. Meanwhile, the record for the summer season AMC indicates a gradual decline in moisture influx from the beginning of the 20th century. Interestingly, despite a prominent decline in Indian summer monsoon (ISM) precipitation after the mid-20th century, the summer season AMC�GMB relation remained stable. We hypothesize that decadal-scale greening, and consequently increased evapotranspiration and pre-monsoon precipitation might have been recycled through the summer season, to compensate for the ISM part of precipitation. However, isotope-enabled ecophysiological models and measurements would strengthen this hypothesis. In addition, high-resolution radiative forcing and long-term vegetation greening trends point towards a probable influence of valley greening on GMB. Our results indicate that attribution of ice mass to large-scale dynamics is likely to be modulated by local vegetation changes. This study contributes to the understanding of long-term hydroclimate�ice mass variability in the central Himalaya, where predictions are crucial for managing water resources and ecosystems. Copyright � 2022 Singh, Shekhar, Parida, Gupta, Sain, Rai, Br�uning, Singh Charkaborty, Sharma, Kamal Tiwari, Chauhan and Montagnani.