Hydrogeological typologies of the Indo-Gangetic basin alluvial aquifer,South Asia     

H. C. Bonsor  A. M. MacDonald  K. M. Ahmed  W. G. Burgess  M. Basharat  R. C. Calow  A. Dixit  S. S. D. Foster  K. Gopal  D. J. Lapworth  M. Moench  A. Mukherjee  M. S. Rao  M. Shamsudduha  L. Smith  R. G. Taylor  J. Tucker  F. van Steenbergen  S. K. Yadav  A. Zahid 《Hydrogeology Journal》2017,25(5):1377-1406

The Indo-Gangetic aquifer is one of the world’s most important transboundary water resources, and the most heavily exploited aquifer in the world. To better understand the aquifer system, typologies have been characterized for the aquifer, which integrate existing datasets across the Indo-Gangetic catchment basin at a transboundary scale for the first time, and provide an alternative conceptualization of this aquifer system. Traditionally considered and mapped as a single homogenous aquifer of comparable aquifer properties and groundwater resource at a transboundary scale, the typologies illuminate significant spatial differences in recharge, permeability, storage, and groundwater chemistry across the aquifer system at this transboundary scale. These changes are shown to be systematic, concurrent with large-scale changes in sedimentology of the Pleistocene and Holocene alluvial aquifer, climate, and recent irrigation practices. Seven typologies of the aquifer are presented, each having a distinct set of challenges and opportunities for groundwater development and a different resilience to abstraction and climate change. The seven typologies are: (1) the piedmont margin, (2) the Upper Indus and Upper-Mid Ganges, (3) the Lower Ganges and Mid Brahmaputra, (4) the fluvially influenced deltaic area of the Bengal Basin, (5) the Middle Indus and Upper Ganges, (6) the Lower Indus, and (7) the marine-influenced deltaic areas.  相似文献   

Mid-level clouds over the Sahara in a convection-permitting regional model     

Mantsis  Damianos F.  Sherwood  Steven  Dixit  Vishal  Morrison  Hugh  Thompson  Greg 《Climate Dynamics》2020,54(7):3425-3439

Climate Dynamics - The simulation of Saharan mid tropospheric clouds is investigated with the weather research and forecasting (WRF) regional atmospheric model at convection permitting...  相似文献   

Patterns of pH change inferred from chrysophyte microfossils in Adirondack and northern New England lakes     

John P. Smol  Sushil S. Dixit 《Journal of Paleolimnology》1990,4(1):31-41

The silica-scaled chrysophytes are being used increasingly in paleolimnological studies of lake acidification. This study compares the distribution of scaled chrysophytes in the surface sediment of a calibration set of lakes from the Adirondack Park and from northern New England. With the exception of two taxa, species distributions with respect to pH are similar in the two regions. The stratigraphic distribution of chrysophytes is then compared in five Adirondack and three New England lakes. All the presently acid lakes indicate recent lake acidification, with more acid tolerant chrysophytes increasing in relative frequency in the recent sediments. Because the timing of these species changes is unrelated to any watershed disturbances that may have occurred in the lakes' catchments, atmospheric deposition of acids is believed responsible for these recent pH changes.This is the seventh of a series of papers to be published by this journal which is a contribution of the Paleoecological Investigation of Recent Lake Acidification (PIRLA) project. Drs. D. F. Charles and D. R. Whitehead are guest editors for this series.  相似文献   

The effect of coriolis force on acceleration covariance in turbulent dusty flow with rotational symmetry     

T. Dixit 《Astrophysics and Space Science》1989,152(1):125-130

In this paper considering the turbulent dusty flow of an incompressible viscous fluid which is nearly isotropic and spatially homogeneous with an extra vector argument (rotation symmetry) the expression for acceleration covariance in the presence of the Coriolis force has been derived and solution has been obtained in terms of defining scalars.  相似文献   

Effect of solution saturation state and temperature on diopside dissolution     

Suvasis Dixit  Susan A Carroll 《Geochemical transactions》2007,8(1):1-14

Steady-state dissolution rates of diopside are measured as a function of solution saturation state using a titanium flow-through reactor at pH 7.5 and temperature ranging from 125 to 175°C. Diopside dissolved stoichiometrically under all experimental conditions and rates were not dependent on sample history. At each temperature, rates continuously decreased by two orders of magnitude as equilibrium was approached and did not exhibit a dissolution plateau of constant rates at high degrees of undersaturation. The variation of diopside dissolution rates with solution saturation can be described equally well with a ion exchange model based on transition state theory or pit nucleation model based on crystal growth/dissolution theory from 125 to 175°C. At 175°C, both models over predict dissolution rates by two orders of magnitude indicating that a secondary phase precipitated in the experiments. The ion exchange model assumes the formation of a Si-rich, Mg-deficient precursor complex. Lack of dependence of rates on steady-state aqueous calcium concentration supports the formation of such a complex, which is formed by exchange of protons for magnesium ions at the surface. Fit to the experimental data yields where the Mg-H exchange coefficient, n = 1.39, the apparent activation energy, E _a = 332 kJ mol^-1, and the apparent rate constant, k = 10^41.2 mol diopside cm^-2 s^-1. Fits to the data with the pit nucleation model suggest that diopside dissolution proceeds through retreat of steps developed by nucleation of pits created homogeneously at the mineral surface or at defect sites, where homogeneous nucleation occurs at lower degrees of saturation than defect-assisted nucleation. Rate expressions for each mechanism (i) were fit to where the step edge energy (α) for homogeneously nucleated pits were higher (275 to 65 mJ m^-2) than the pits nucleated at defects (39 to 65 mJ m^-2) and the activation energy associated with the temperature dependence of site density and the kinetic coefficient for homogeneously nucleated pits (E_{b-homogeneous} = 2.59 × 10^-16 mJ K^-1) were lower than the pits nucleated at defects (E_{b-defect assisted} = 8.44 × 10^-16 mJ K^-1).  相似文献   

Surface chemistry and reactivity of biogenic silica   总被引：2，自引：0，他引：2  

Suvasis Dixit  Philippe Van Cappellen 《Geochimica et cosmochimica acta》2002,66(14):2559-2568

The surface chemistry of cultured diatoms was compared to that of biosiliceous material in Southern Ocean sediments, using potentiometric titrations and aluminum adsorption experiments. Aerosil 200, a well-studied synthetic amorphous silica, served as reference solid. Surface charge development and aluminum adsorption on cultured diatom shells were comparable to Aerosil. The surface chemical properties of biosiliceous material buried to depths of 15-25 cm in Southern Ocean sediments, however, deviated markedly from those of the cultured diatoms. In pH range 4-8.5, the surface charge density was systematically lower for biogenic silica from the sediments. In addition, the aluminum adsorption edge on the biosiliceous sediments was shifted to higher pHs by about 0.4 units. The results indicate that ionizable surface silanol groups on diagenetically altered diatom shells are less abundant and, possibly, less acidic than on freshly cultured diatoms. The observed differences in surface chemical structure are consistent with the progressive loss of reactivity, or aging, of biogenic silica which promotes its preservation in sediments.  相似文献   

Use of traveltime skips in refraction analysis to delineate velocity inversion1     

H.C. Tewari  M.M. Dixit  P.R.K. Murty 《Geophysical Prospecting》1995,43(6):793-804

First arrival refraction data does not normally provide any indication of the velocity inversion problem. However, under certain favourable circumstances, when the low-velocity layer (LVL) is considerably thicker than the overlying higher-velocity layer (HVL), the velocity inversion can be seen in the form of a traveltime skip. Model Studies show that in such cases the length of the HVL traveltime branch can be used to determine the thickness of the HVL and the magnitude of the traveltime skip in order to determine the thickness of the LVL. This is also applicable in the case of field data.  相似文献   

Electron cyclotron waves in the presence of parallel electric fields in the Earth’s auroral plasma     

S. Kumar  S. K. Dixit  A. K. Gwal 《Annales Geophysicae》1997,15(1):24-28

The electron cyclotron waves that originate at low altitudes (<0.5 R_E) and observed by ground facilities have been studied in the presence of a weak parallel electric field in auroral magnetoplasma consisting of trapped energetic auroral electrons and cold background electrons of ionospheric origin. The model distribution for auroral trapped electrons is taken as Maxwellian ring distribution. An expression for the growth rate has been obtained in the presence of parallel electric field assuming that the real frequency in the whistler mode is not affected by the presence of the electric field. The results show that waves grow (or damp) in amplitude for a parallel (or antiparallel) electric field. The influence of the electric field is more pronounced at a shorter wavelength spectrum. An increase in population of energetic electrons increases the growth rate and thus, plays a significant role in the wave excitation process in the auroral regions.  相似文献   

Paleolimnological reconstructions of Rouyn-Noranda lakes within the zone of influence of the Horne Smelter,Québec,Canada     

Aruna S. Dixit  S. Alpay  Sushil S. Dixit  John P. Smol 《Journal of Paleolimnology》2007,38(2):209-226

Diatoms were identified and enumerated from a surface sediment calibration set of 50 lakes in northwestern Québec. The relationship between species composition and environmental variables was examined using canonical correspondence analysis (CCA). Forward selection and Monte Carlo permutation tests in CCA indicated that diatom species distributions in the data set are most strongly correlated to lakewater pH. A strong (r ² _boot = 0.83) weighted averaging calibration model, that includes bootstrapped error estimates, was developed for inferring past lakewater pH. Using this model, temporal changes in pH were reconstructed for two kettle lakes, Lac de la Pépinière and Lac Perron. Based on limnological data, both the study lakes were expected to have recently acidified due to increased acidic precipitation and increases in anthropogenic metal loading. However, our long-term pH inference data indicate that these lakes were naturally acidic during pre-industrial times. Nonetheless, the rate of acidification, particularly in Lac de la Pépinière, has accelerated in the last ∼75 years. These long-term pH records developed for the dilute lakes in northwestern Québec suggest that the region has received increased atmospheric pollutants from the nearby Horne smelter in Rouyn-Noranda. The pH inference profiles are markedly different from many other paleolimnological studies in acid-sensitive regions of Canada that have become acidic primarily as a result of industrial activities. Electronic supplementary material The online version of this article (doi: ) contains supplementary material, which is available to authorized users.  相似文献   

Preparation of volume table of SAL (Shorea robusta) - an approach using satellite data     

V. K. Srivastava  A. M. Rai  R. K. Dixit  M. P. Oza  A. Narayana 《International Journal of Applied Earth Observation and Geoinformation》1999,1(3-4)

Sal (Shorea robusta) is an important forest tree species in north and north-eastern India. Large-scale plantations of this species have been raised there under taungya and coppice system of management. The conventional volume table prepared for high sal forest is referred to infer the volume of production of this species. Earlier workers have used aerial remote sensing data to develop volume tables of this species. In the present study a volume table for sal is developed based on remotely sensed satellite data using a regression technique. A two-step method was developed to estimate mean tree volume from satellite data. In step 1, mean crown diameter — an intermediate variable - was estimated from satellite data. In step 2, the estimated mean crown diameter was used to estimate the mean tree volume. Addition of age of the crop as an independent variable improved the predictive ability of the regression equation.  相似文献