A steady state model of the Somali current     

P. K. Das  S. K. Dube  G. S. Rao 《Journal of Earth System Science》1987,96(3):279-290

A steady state model of the Somali current including forcing by both the curl and the divergence of the wind stress is discussed. The model equations are linear, but the results presented are for the one nonlinear case. The grid resolution was 12 km in the zonal and 24 km in the meridional direction. The streamfunction and velocity potential of the current are presented for forcing by divergence and compared with a situation when only the curl is present. The results indicate that a two-gyre system appears in July, a representative month for the summer monsoon, only when divergence is included. Computations with available data indicate that the divergence is comparable in magnitude to the curl near the location of the Somali current. The model produces three other important features: (i) strong upwelling off the east coast of Africa, (ii) downwelling over central Arabian sea and (iii) a strong eastward current in the upper layer towards the interior of the Arabian sea near 12°N.  相似文献   

Trace element characteristics,REE patterns and partition coefficients of zircons from different geological environments—A case study on Indian zircons   总被引：1，自引：0，他引：1  

A.V. Murali  R. Parthasarathy  T.M. Mahadevan  M.Sankar Das 《Geochimica et cosmochimica acta》1983,47(11):2047-2052

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Impacts of Present and Future Climate Variability and Change on Agriculture and Forestry in the Arid and Semi-Arid Tropics     

M. V. K. Sivakumar  H. P. Das  O. Brunini 《Climatic change》2005,70(1-2):31-72

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Impact of tsunami on texture and mineralogy of a major placer deposit in southwest coast of India   总被引：1，自引：0，他引：1  

N. Babu  D. S. Suresh Babu  P. N. Mohan Das 《Environmental Geology》2007,52(1):71-80

The great Indonesian earth quake (26 December 2004) triggered a tsunami wave across the Bay of Bengal and Indian Ocean basins and has brought a major havoc in several countries including India. The coastal segment between Thotapalli and Valiazhikal in Kerala state of southwest India, where considerably rich beach placer deposit with ilmenite percentage of more than 70% is concentrated, has been investigated to understand the impact of tsunami on coastal sediments. The grain size analysis flashes out the significant differences between the pre- and post-tsunami littoral environments. While the mineral grains collected during pre-tsunami period show well-sorted nature, the post-tsunami samples represent moderately to poorly sorted nature. Similarly, unimodal and bimodal distributions of the sediments have been recorded for pre- and post-tsunami sediments, respectively. Further, mineral assemblages corresponding to before and after this major wave activity clearly indicate the large-scale redistribution of sediments. The post-tsunami sediments register increasing trends of garnet, sillimanite and rutile. The total heavy mineral percentage of the post-tsunami sediment also shows an improved concentration, perhaps due to the large-scale transport of lighter fraction. Magnetite percentage of post-tsunami samples reflects higher concentration compared to the pre-tsunami samples, indicating the intensity of reworking process. X-ray diffraction patterns of ilmenite grains have confirmed the increased presence of pseduorutile, and pseudobrookite in post-tsunami samples, which could be due to the mixing of more altered grains. SEM examination of grains also confirms the significant alteration patterns on the ubiquitous mineral of placer body, the ilmenite. The reason for these textural, mineralogical and micromorphological changes in heavy minerals particularly in ilmenite, could be due to the churning action on the deeper sediments of onshore region or on the sediments entrapped in the near shelf region of the area, by the ∼ 6 m high tsunami waves.  相似文献   

Spatial assessment of groundwater use potential for irrigation in Teesta Barrage Project in Bangladesh     

Shahriar M. Wahid  Mukand S. Babel  Ashim Das Gupta  Roberto S. Clemente 《Hydrogeology Journal》2007,15(2):365-382

In Bangladesh, development of the groundwater resource for irrigation is a vital component of the government’s agricultural strategy to attain food self-sufficiency. Amidst reports of falling groundwater levels in many parts of Bangladesh, the potential of groundwater use has been investigated in the Teesta Barrage Project (TBP) in which large-scale groundwater development for dry-season irrigation has taken place in the recent past. Several techniques and tools have been applied such as the combination of analysis of groundwater hydrographs and mathematical modelling to derive key hydrogeological variables, calculation of net irrigation requirement and the use of geographical information systems. The results show that the economically attractive high-yielding variety (HYV) Boro (dry season) rice cultivation during the groundwater irrigation season may not be sustained in large parts of the project area if the current trends in abstraction are continued. However, due to spatial variation in abstraction, nine thanas (sub-districts)—out of a total of 21 in the project area—may still be able to expand groundwater-irrigated cropland and a groundwater-use potential of 40 mm/year may be created if deep-set shallow tubewells are used by the farmers to abstract groundwater. A structured approach, based on zoning of potential areas, is recommended for groundwater development and use.

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Resumen En Bangladesh, el desarrollo de las aguas subterráneas para riego es un componente vital del la estrategia agrícola del Gobierno para obtener la auto-suficiencia alimentaria. Un informe intermedio evidencia un descenso de los niveles de aguas subterráneas en muchas partes de Bangladesh, el uso potencial de aguas subterránea ha sido investigado en el Proyecto Tessta Barrage (TBP) en el cual ha tenido lugar el desarrollo de las aguas subterráneas a gran escala para el riego en la estación seca en un pasado reciente. Se ha aplicado algunas técnicas y herramientas, como la combinación del análisis de datos gráficos de aguas subterráneas y modelización matemática para obtener variables hidrogeológicas clave, el cálculo de la necesidad neta de riego y el uso de Sistemas de Información Geográfica. El resultado muestra que el cultivo, económicamente atractivo, de arroz HYV Boro (estación seca) durante la estación de riego con aguas subterráneas puede no ser sostenible en gran parte del área del proyecto si continúan las tendencias actuales en las extracciones. Sin embargo, debido a la variación espacial en la extracción, nueve thanas (subdistritos)—de un total de 21 en el área del Proyecto—pueden todavía sufrir un aumento de la tierra cultivable regada con aguas subterráneas y que se puede asumir un uso potencial de agua subterránea de 40 mm/estación si los granjeros utilizan sondeos someros instalados en fosas para la extracción. Se recomienda una aproximación estructurada para el desarrollo y uso de las aguas subterráneas, basada en la zonificación de áreas potenciales.

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Résumé Au Bangladesh, l’exploitation des ressources en eau souterraines pour l’irrigation est une composante vitale de la stratégie agricole gouvernementale pour atteindre l’auto-suffisance alimentaire. Parmi les baisses de niveaux piézométriques rapportées sur de nombreuses régions du Bangladesh, le potentiel d’utilisation des eaux souterraines a été étudié sur le Projet de Barrage de Teesta (TBP), où, dans un passé récent, les eaux souterraines ont été massivement exploitées pour l’irrigation en saison sèche. De nombreux outils et techniques ont été utilisés, comme l’analyse combinée des chroniques piézométriques et des modèles mathématiques pour dégager les variables hydrogéologiques clés, le calcul des besoins nets pour l’irrigation, et l’utilisation des systèmes d’information géographique. Les résultats démontrent que la culture en saison d’irrigation de la variété de riz HYV Boro (saison sèche), économiquement attrayante, n’est potentiellement pas viable sur de nombreux secteurs de la zone étudiée si les prélèvements perdurent selon la tendance actuelle. Cependant, du fait de la variation spatiale des prélèvements, neuf thanas (sous-districts)—sur un total de 21 sur le secteur d’étude—pourraient encore augmenter les surfaces irriguées, et une disponibilité en eau de 40 mm/saison pourrait être générée si les agriculteurs utilisaient des puits peu profonds pour exploiter les ressources souterraines. Une approche structurée, basée sur le zonage des secteurs potentiels, est recommandée pour l’exploitation et l’utilisation des eaux souterraines.

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Impact of Doppler Radar Wind in Simulating the Intensity and Propagation of Rainbands Associated with Mesoscale Convective Complexes Using MM5-3DVAR System     

S. Abhilash  Someshwar Das  S. R. Kalsi  M. Das Gupta  K. Mohankumar  John P. George  S. K. Banerjee  S. B. Thampi  D. Pradhan 《Pure and Applied Geophysics》2007,164(8-9):1491-1509

Pre-monsoon rainfall around Kolkata (northeastern part of India) is mostly of convective origin as 80% of the seasonal rainfall is produced by Mesoscale Convective Systems (MCS). Accurate prediction of the intensity and structure of these convective cloud clusters becomes challenging, mostly because the convective clouds within these clusters are short lived and the inaccuracy in the models initial state to represent the mesoscale details of the true atmospheric state. Besides the role in observing the internal structure of the precipitating systems, Doppler Weather Radar (DWR) provides an important data source for mesoscale and microscale weather analysis and forecasting. An attempt has been made to initialize the storm-scale numerical model using retrieved wind fields from single Doppler radar. In the present study, Doppler wind velocities from the Kolkata Doppler weather radar are assimilated into a mesoscale model, MM5 model using the three-dimensional variational data assimilation (3DVAR) system for the prediction of intense convective events that occurred during 0600 UTC on 5 May and 0000 UTC on 7 May, 2005. In order to evaluate the impact of the DWR wind data in simulating these severe storms, three experiments were carried out. The results show that assimilation of Doppler radar wind data has a positive impact on the prediction of intensity, organization and propagation of rain bands associated with these mesoscale convective systems. The assimilation system has to be modified further to incorporate the radar reflectivity data so that simulation of the microphysical and thermodynamic structure of these convective storms can be improved.  相似文献   

High iridium concentration of alkaline rocks of Deccan and implications to K/T boundary     

P. N. Shukla  N. Bhandari  Anirban Das  A. D. Shukla  J. S. Ray 《Journal of Earth System Science》2001,110(2):103-110

We report here an unusually high concentration of iridium in some alkali basalts and alkaline rocks of Deccan region having an age of about 65Ma, similar to the age of the Cretaceous-Tertiary boundary. The alkali basalts of Anjar, in the western periphery of Deccan province, have iridium concentration as high as 178pg/g whereas the alkaline rocks and basalts associated with the Amba Dongar carbonatite complex have concentrations ranging between 8 and 80 pg/g. Some of these values are more than an order of magnitude higher than the concentration in the tholeiitic basalts of Deccan, indicating the significance of alkaline magmatism in the iridium inventory at the Cretaceous-Tertiary boundary. Despite higher concentration, their contribution to the global inventory of iridium in the Cretaceous-Tertiary boundary clays remains small. The concentration of iridium in fluorites from Amba Dongar was found to be <30 pg/g indicating that iridium is not incorporated during their formation in hydrothermal activity.  相似文献   

Mapping of tropical dry deciduous forest and landuse in part of Vindhyan range using satellite remote sensing     

Pant  D N  Das  K K  Roy  P S 《Journal of the Indian Society of Remote Sensing》1992,20(1):9-20

Forest vegetation of Vindhyan range located in the north of G.B. Pant Sagar (dam) has been subjected to degradation due to high biotic pressure caused by the installation of thermal power plants, coal mining, heavy cattle grazing etc. In the present study Landsat TM FCC of 1∶250,000 scale was visually analysed with respect to forest vegetation types, crown density and structure along with other landuse/land cover classes. ExceptShorea robusta (Sal) andLagerstroemia parviflora (Lendia) all forest vegetation types show higher percentage of degradation and under-stocked condition with respect to their areal extent under study. Overall classification accuracy of the forest types has been found to be 88.94%. This indicates that for obtaining reliable mapping accuracy in dry deciduous areas, satellite remote sensing data of appropriate season is essential.

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Simulation Of Salt-Water Encroachment In A Multi-Layer Groundwater System,Bangkok, Thailand     

S. Gangopadhyay  A. Das Gupta 《Hydrogeology Journal》1995,3(4):74-88

Salt-water encroachment in the multi-layer groundwater system underlying the Bangkok metropolitan area was simulated with a quasi-three-dimensional flow and solute-transport model. The quasi-three-dimensional model used in this study is based on the model SUTRA. Accurate conceptualization of the initial state of the system with regard to the distribution of salt-water concentration is very important for modeling, especially in areas where localized zones of high salt concentration exist in the groundwater. Data adequacy and model results were evaluated by a geostatistical analysis. The model is capable of simulating the regional trend of potentiometric levels and salt concentration. However, lack of monitoring data in areas where localized zones of high salt concentration exist resulted in large model residuals. RÉSUMÉ: L'empiétement des eaux salées dans le système hydrologique multi-couche sous-jacent la zone métropolitaine de Bangkok a été simulé à l'aide d'un modèle quasi-tridimensionnel représentant l'écoulement des eaux et le transport d'éléments en solution. Le modèle quasi-tridimensionnel utilisé dans cette étude est basé sur le modèle SUTRA. Une conceptualisation exacte de l'état initial du système en ce qui concerne la répartition de la concentration des eaux salines est très important, particulièrement dans les endroits localisés où l'eau souterraine presente une forte concentration en sel. La qualité des donnés et des résultats a été estimée à l'aide d'une analyse géostatistique. Le modèle est capable de simuler la tendance régionale des niveaux potentiométriques et de la concentration en sel. Cependant, le manque de données de contrôle dans les endroits à forte concentration a donné lieu à des résiduels importants. RESUMEN: La intrusión de agua salina en el sistema multicapa bajo el área metropolitana de Bangkok se simuló mediante un modelo de flujo y transporte de solutos casi-tridimensional, basado en el modelo SUTRA. La conceptualizacián precisa de la distribución inicial de concentraciones de agua salina es muy importante para el modelo, especialmente en áreas donde existen zonas locales de alta concentración salina en el agua subterránea. La exactitud de los datos y del modelo se evaluaron mediante un análisis geoestadístico. El modelo permite simular la tendencia regional de los niveles piezométricos y la concentración de sales. Sin embargo, la falta de datos de muestreo en las zonas locales de alta salinidad resultaron en valores altos de los residuos calculados por el modelo.  相似文献   

On coda waves of earthquakes     

Sushil Chandra Das Gupta 《Pure and Applied Geophysics》1959,43(1):45-74

Summary Coda waves viz. the tail portion of an earthquake record have been observed and analysed byCarder, Macelwane and others. They showed that the periods of such waves increase with the increase of epicentral distances.Carder observed that these waves have very little transverse component so that these may be considered as of the type of Rayleigh waves. RecentlyOmote showed that the Coda waves contain three periodsT ₁,T ₂,T ₃ of whichT ₁ increases with epi-central distances as observed by previous observers. ButT ₂,T ₃ remain constant for all earthquakes from different epicentral distances.Omote tried to explain this phenomenon by considering that the surface of the earth consists of several layers andT ₂,T ₃ are free oscillation periods of the surface layers.T ₁ period has been explained bySezawa and also byJeffreys which has been shown byGutenberg. The author has attempted to explain the periodsT ₂,T ₃ by considering passage of cracks at the focal region. The Rayleigh wave character of Coda waves and low velocity of such waves have been explained.  相似文献