• Volume 122, Issue 2

      April 2013,   pages  271-558

    • Landscape level assessment of critically endangered vegetation of Lakshadweep islands using geo-spatial techniques

      C Sudhakar Reddy Bijan Debnath P Hari Krishna C S Jha

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      The conservation of biodiversity is essential for human survival and quality of the environment. Lakshadweep islands are vulnerable to global change and the representing remnant natural vegetation. Landscape fragmentation, disturbance regimes and biological richness have been studied using geo-spatial techniques. Littoral vegetation is the only natural vegetation type of Lakshadweep islands. Altogether 59 patches of the littoral vegetation occupying an area of 137.2 ha were identified. 58.06% of the littoral vegetation patches belongs to the patch-size class of > 5 ha. The remnant natural vegetation surviving with patches of less than 20 ha size indicates severe anthropogenic pressure. The fragmentation of littoral vegetation habitat into smaller isolated patches poses one of the key threats to biodiversity and coastal environment. Phytosociological observations revealed distinct plant communities and presence of invasive species in littoral vegetation. The high disturbance areas accounted for 59.11% area of the total vegetation. The overall spatial distribution of biological richness (BR) in Lakshadweep shows maximum BR at low level (78%), followed by medium (19%), high (2%) and very high (1%). The study emphasizes the importance of conserving the remnant natural vegetation, which is critically endangered.

    • Endemism due to climate change: Evidence from Poeciloneuron Bedd. (Clusiaceae) leaf fossil from Assam, India

      Gaurav Srivastava R C Mehrotra

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      A fossil leaf resembling Poeciloneuron indicum Bedd. (Clusiaceae) is described from the Late Oligocene (Chattian 28.4–23 Myr) sediments of Assam. The modern analogue is endemic to the Western Ghats which is situated in the same palaeolatitude. Its presence, along with other known fossil records, indicates that the seasonality in temperature was less pronounced and CMMT (cold month mean temperature) was not less than 18°C with plenty of rainfall, in the region during the period of deposition. The study also indicates that the plant phenology is sensitive towards climate change. The present study is in congruence with the global data.

    • Palynostratigraphy and depositional environment of Vastan Lignite Mine (Early Eocene), Gujarat, western India

      M R Rao Ashok Sahni R S Rana Poonam Verma

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      Early Eocene sedimentary successions of south Asia, are marked by the development of extensive fossilbearing, lignite-rich sediments prior to the collision of India with Asia and provide data on contemporary equatorial faunal and vegetational assemblages. One such productive locality in western India is the Vastan Lignite Mine representing approximately a 54–52 Ma sequence dated by the presence of benthic zone marker species, Nummulites burdigalensis burdigalensis. The present study on Vastan Lignite Mine succession is based on the spore-pollen and dinoflagellate cyst assemblages and documents contemporary vegetational changes. 86 genera and 105 species belonging to algal remains (including dinoflagellate cysts), fungal remains, pteridophytic spores and angiospermous pollen grains have been recorded. On the basis of first appearance, acme and decline of palynotaxa, three cenozones have been recognized and broadly reflect changing palaeodepositional environments. These are in ascending stratigraphic order (i) Proxapertites Spp. Cenozone, (ii) Operculodinium centrocarpum Cenozone and (iii) Spinizonocolpites Spp. Cenozone. The basal sequence is lagoonal, palm-dominated and overlain by more open marine conditions with dinoflagellate cysts and at the top, mangrove elements are dominant. The succession has also provided a unique record of fish, lizards, snakes, and mammals.

    • Nodular features from Proterozoic Sonia Sandstone, Jodhpur Group, Rajasthan: A litho-biotectonic perspective

      Arvind Singh Vikash Anand Prabhas Pandey Partha Pratim Chakraborty

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      The Sonia Sandstone of Proterozoic Jodhpur Group, Marwar Supergroup, exposed around the Sursagar dam area of Jodhpur town, Rajasthan exposes two varieties of nodular features, often spectacular in shape and size. On the basis of mode of occurrence (intra- or interbed) and stratal involvement (single or multiple) the features are classified as Type I and II. From granulometric and microscopic (optical and scanning electron) studies carried out on sandstones from the nodules and their host sandstones, geochemical analysis (SEM-EDAX) of intragranular cement present within Type I nodules, and appreciation of control of associated fracture system within Type II nodules, it is proposed that the two types of nodules vary in their formative mechanism and stage of formation. While Type I nodules are identified as product of processes operative at the early diagenetic, pre-lithification stage, the Type II nodules are undoubtedly the result of post-lithification origin triggered by formation of fracture system. Here we propose generation of vapour pressure (not exceeding the overlying hydrostatic pressure) by decay of thin, laterally impersistent organic mat as the causal factor for intrabed nodule (Type I) formation, which forced rarefication of local grain packing \tetit {vis-a-vis} early diagenetic silica cementation. The study warrants necessity of more studies on nodules to understand possible roles of organic matter and bedtransgressive fracture systems in their formation, going beyond the generalised secondary mineralization hypothesis.

    • Influence of epicentral distance on local seismic response in Kolkata City, India

      William K Mohanty Akhilesh K Verma Franco Vaccari Giuliano F Panza

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      The influence of source and epicentral distance on the local seismic response in the Kolkata city is investigated by computing the seismic ground motion along 2-D geological cross-sections in the Kolkata city for the earthquake that occurred on 12 June 1897 ($M_w$ = 8.1; focal mechanism: dip = 57°, strike = 110° and rake = 76°; focal depth = 9 km) in Shillong plateau. For the estimation of ground motion parameters, a hybrid technique is used, which is the combination of modal summation and finite difference method. This technique allows the estimation of site specific ground motion for various events located at different distances from Kolkata city, taking into account simultaneously the position and geometry of the seismic source, the mechanical properties of the propagation medium and the geotechnical properties of the site. The epicenter of the Shillong earthquake is about 460 km away from Kolkata. The estimated peak ground acceleration (PGA) varies in the range of 0.11–0.18 g and this range corresponds to the intensity of IX to X on the Mercalli-Cancani-Sieberg (MCS) scale and VIII on the Modified Mercalli (MM) scale. The maximum amplification in terms of response spectral ratio (RSR) varies from 10 to 12 in the frequency range 1.0–1.5 Hz. These amplifications occur in correspondence to low-velocity shallow, loose soil deposit. The comparison of these results with earlier ones obtained considering the Calcutta earthquake that occurred on 15 April 1964 ($M_w$ = 6.5; focal mechanism: dip = 32°, strike = 232° and rake = 56°; focal depth = 36 km) shows that the source parameters (magnitude and focal mechanism) and epicentral distance play an important role on site response but the variation in the frequency of the peak values (RSR) is negligible. The obtained results match with observed reported intensities in Kolkata region.

    • Comparative analysis of Vening-Meinesz Moritz isostatic models using the constant and variable crust-mantle density contrast – a case study of Zealandia

      Mohammad Bagherbandi Robert Tenzer

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      We compare three different numerical schemes of treating the Moho density contrast in gravimetric inverse problems for finding the Moho depths. The results are validated using the global crustal model CRUST2.0, which is determined based purely on seismic data. Firstly, the gravimetric recovery of the Moho depths is realized by solving Moritz’s generalization of the Vening-Meinesz inverse problem of isostasy while the constant Moho density contrast is adopted. The Pratt-Hayford isostatic model is then facilitated to estimate the variable Moho density contrast. This variable Moho density contrast is subsequently used to determine the Moho depths. Finally, the combined least-squares approach is applied to estimate jointly the Moho depths and density contract based on a priori error model. The EGM2008 global gravity model and the DTM2006.0 global topographic/bathymetric model are used to generate the isostatic gravity anomalies. The comparison of numerical results reveals that the optimal isostatic inverse scheme should take into consideration both the variable depth and density of compensation. This is achieved by applying the combined least-squares approach for a simultaneous estimation of both Moho parameters. We demonstrate that the result obtained using this method has the best agreement with the CRUST2.0 Moho depths. The numerical experiments are conducted at the regional study area of New Zealand’s continental shelf.

    • Landslide susceptibility mapping using support vector machine and GIS at the Golestan Province, Iran

      Hamid Reza Pourghasemi Abbas Goli Jirandeh Biswajeet Pradhan Chong Xu Candan Gokceoglu

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      The main goal of this study is to produce landslide susceptibility map using GIS-based support vector machine (SVM) at Kalaleh Township area of the Golestan province, Iran. In this paper, six different types of kernel classifiers such as linear, polynomial degree of 2, polynomial degree of 3, polynomial degree of 4, radial basis function (RBF) and sigmoid were used for landslide susceptibility mapping. At the first stage of the study, landslide locations were identified by aerial photographs and field surveys, and a total of 82 landslide locations were extracted from various sources. Of this, 75% of the landslides (61 landslide locations) are used as training dataset and the rest was used as (21 landslide locations) the validation dataset. Fourteen input data layers were employed as landslide conditioning factors in the landslide susceptibility modelling. These factors are slope degree, slope aspect, altitude, plan curvature, profile curvature, tangential curvature, surface area ratio (SAR), lithology, land use, distance from faults, distance from rivers, distance from roads, topographic wetness index (TWI) and stream power index (SPI). Using these conditioning factors, landslide susceptibility indices were calculated using support vector machine by employing six types of kernel function classifiers. Subsequently, the results were plotted in ArcGIS and six landslide susceptibility maps were produced. Then, using the success rate and the prediction rate methods, the validation process was performed by comparing the existing landslide data with the six landslide susceptibility maps. The validation results showed that success rates for six types of kernel models varied from 79% to 87%. Similarly, results of prediction rates showed that RBF (85%) and polynomial degree of 3 (83%) models performed slightly better than other types of kernel (polynomial degree of 2 = 78%, sigmoid = 78%, polynomial degree of 4 = 78%, and linear = 77%) models. Based on our results, the differences in the rates (success and prediction) of the six models are not really significant. So, the produced susceptibility maps will be useful for general land-use planning.

    • An assessment on the use of bivariate, multivariate and soft computing techniques for collapse susceptibility in GIS environ

      Işık Yilmaz Marian Marschalko Martin Bednarik

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      The paper presented herein compares and discusses the use of bivariate, multivariate and soft computing techniques for collapse susceptibility modelling. Conditional probability (CP), logistic regression (LR) and artificial neural networks (ANN) models representing the bivariate, multivariate and soft computing techniques were used in GIS based collapse susceptibility mapping in an area from Sivas basin (Turkey). Collapse-related factors, directly or indirectly related to the causes of collapse occurrence, such as distance from faults, slope angle and aspect, topographical elevation, distance from drainage, topographic wetness index (TWI), stream power index (SPI), Normalized Difference Vegetation Index (NDVI) by means of vegetation cover, distance from roads and settlements were used in the collapse susceptibility analyses. In the last stage of the analyses, collapse susceptibility maps were produced from the models, and they were then compared by means of their validations. However, Area Under Curve (AUC) values obtained from all three models showed that the map obtained from soft computing (ANN) model looks like more accurate than the other models, accuracies of all three models can be evaluated relatively similar. The results also showed that the conditional probability is an essential method in preparation of collapse susceptibility map and highly compatible with GIS operating features.

    • Modelling soil erosion risk based on RUSLE-3D using GIS in a Shivalik sub-watershed

      Suresh Kumar S P S Kushwaha

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      The RUSLE-3D (Revised Universal Soil Loss Equation-3D) model was implemented in geographic information system (GIS) for predicting the soil loss and the spatial patterns of soil erosion risk required for soil conservation planning. High resolution remote sensing data (IKONOS and IRS LISS-IV) were used to prepare land use/land cover and soil maps to derive the vegetation cover and the soil erodibility factor whereas Digital Elevation Model (DEM) was used to generate spatial topographic factor. Soil erodibility (K) factor in the sub-watershed ranged from 0.30 to 0.48. The sub-watershed is dominated by natural forest in the hilly landform and agricultural land in the piedmont and alluvial plains. Average soil loss was predicted to be lowest in very dense forest and highest in the open forest in the hilly landform. Agricultural land-1 and agriculture land-2 to have moderately high and low soil erosion risk, respectively. The study predicted that 15% area has ‘moderate’ to ‘moderately high’ and 26% area has high to very high risk of soil erosion in the sub-watershed.

    • Using artificial neural network approach for modelling rainfall–runoff due to typhoon

      S M Chen Y M Wang I Tsou

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      In Taiwan, owing to the nonuniform temporal and spatial distribution of rainfall and high mountains all over the country, hydrologic systems are very complex. Therefore, preventing and controlling flood disasters is imperative. Nevertheless, water level and flow records are essential in hydrological analysis for designing related water works of flood management. Due to the complexity of the hydrological process, reliable runoff is hardly predicted by applying linear and non-linear regression methods. Therefore, in this study, a model for estimating runoff by using rainfall data from a river basin is developed and a neural network technique is employed to recover missing data. For achieving the objectives, hourly rainfall and flow data from Nanhe, Taiwu, and Laii rainfall stations and Sinpi flow station in the Linbien basin are used. The data records were of 27 typhoons between the years 2005 and 2009. The feed forward back propagation network (FFBP) and conventional regression analysis (CRA) were employed to study their performances. From the statistical evaluation, it has been found that the performance of FFBP exceeded that of regression analysis as reflected by the determination coefficients $R^2$, which were 0.969 and 0.284 for FFBP and CRA, respectively.

    • A modified calculation model for groundwater flowing to horizontal seepage wells

      Wei Wang Peng Chen Qingqing Zheng Xinyu Zheng Kunming Lu

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      The simulation models for groundwater flowing to horizontal seepage wells proposed by Wang and Zhang (2007) are based on the theory of coupled seepage-pipe flow model which treats the well pipe as a highly permeable medium. However, the limitations of the existing model were found during applications. Specifically, a high-resolution grid is required to depict the complex structure of horizontal seepage wells; the permeability of the screen or wall material of radiating bores is usually neglected; and the irregularly distributed radiating bores cannot be accurately simulated. A modified calculation model of groundwater flowing to a horizontal seepage well is introduced in this paper. The exchange flow between well pipe and aquifer couples the turbulent flow inside the horizontal seepage well with laminar flow in the aquifer. The modified calculation model can reliably calculate the pumpage of a real horizontal seepage well. The characteristics of radiating bores, including the diameter, the permeability of screen material and irregular distribution of radiating bores, can be accurately depicted using the modified model that simulates the scenario in which several horizontal seepage wells work together.

    • Geochemical processes controlling the groundwater quality in lower Palar river basin, southern India

      M Senthilkumar L Elango

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      Hydrogeochemical study of groundwater was carried out in a part of the lower Palar river basin, southern India to determine the geochemical processes controlling the groundwater quality. Thirty-nine groundwater samples were collected from the study area and analysed for pH, Eh, EC, Ca, Mg, Na, K, HCO3, CO3, Cl and SO4. The analysed parameters of the groundwater in the study area were found to be well within the safe range in general with respect to the Bureau of Indian Standards for drinking water except for few locations. The results of these analyses were used to identify the geochemical processes that are taking place in this region. Cation exchange and silicate weathering are the important processes controlling the major ion distribution of the study area. Mass balance reaction model NETPATH was used to assess the ion exchange processes. High concentration of Ca in groundwater of the study area is due to the release of Ca by aquifer material and adsorption of Na due to ion exchange processes. Groundwater of the study area is suitable for drinking and irrigation purposes except for few locations.

    • Modelling catchment hydrological responses in a Himalayan Lake as a function of changing land use and land cover

      Bazigha Badar Shakil A Romshoo M A Khan

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      In this paper, we evaluate the impact of changing land use/land cover (LULC) on the hydrological processes in Dal lake catchment of Kashmir Himalayas by integrating remote sensing, simulation modeling and extensive field observations. Over the years, various anthropogenic pressures in the lake catchment have significantly altered the land system, impairing, \texttit {inter-alia}, sustained biotic communities and water quality of the lake. The primary objective of this paper was to help a better understanding of the LULC change, its driving forces and the overall impact on the hydrological response patterns. Multi-sensor and multi-temporal satellite data for 1992 and 2005 was used for determining the spatio-temporal dynamics of the lake catchment. Geographic Information System (GIS) based simulation model namely Generalized Watershed Loading Function (GWLF) was used to model the hydrological processes under the LULC conditions. We discuss spatio-temporal variations in LULC and identify factors contributing to these variations and analyze the corresponding impacts of the change on the hydrological processes like runoff, erosion and sedimentation. The simulated results on the hydrological responses reveal that depletion of the vegetation cover in the study area and increase in impervious and bare surface cover due to anthropogenic interventions are the primary reasons for the increased runoff, erosion and sediment discharges in the Dal lake catchment. This study concludes that LULC change in the catchment is a major concern that has disrupted the ecological stability and functioning of the Dal lake ecosystem.

    • Numerical modelling of seawater intrusion in Shenzhen (China) using a 3D densitydependent model including tidal effects

      Wei Lu Qingchun Yang Jordi D Martín Ricardo Juncosa

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      During the 1990s, groundwater overexploitation has resulted in seawater intrusion in the coastal aquifer of the Shenzhen city, China. Although water supply facilities have been improved and alleviated seawater intrusion in recent years, groundwater overexploitation is still of great concern in some local areas. In this work we present a three-dimensional density-dependent numerical model developed with the FEFLOW code, which is aimed at simulating the extent of seawater intrusion while including tidal effects and different groundwater pumping scenarios. Model calibration, using waterheads and reported chloride concentration, has been performed based on the data from 14 boreholes, which were monitored from May 2008 to December 2009. A fairly good fitness between the observed and computed values was obtained by a manual trial-and-error method. Model prediction has been carried out forward 3 years with the calibrated model taking into account high, medium and low tide levels and different groundwater exploitation schemes. The model results show that tide-induced seawater intrusion significantly affects the groundwater levels and concentrations near the estuarine of the Dasha river, which implies that an important hydraulic connection exists between this river and groundwater, even considering that some anti-seepage measures were taken in the river bed. Two pumping scenarios were considered in the calibrated model in order to predict the future changes in the water levels and chloride concentration. The numerical results reveal a decreased tendency of seawater intrusion if groundwater exploitation does not reach an upper bound of about 1.32 × 104 m3/d. The model results provide also insights for controlling seawater intrusion in such coastal aquifer systems.

    • 𝑛-Alkanes in surficial sediments of Visakhapatnam harbour, east coast of India

      V R Punyu R R Harji N B Bhosle S S Sawant K Venkat

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      Surface sediments collected from 19 stations along Visakhapatnam harbour were analysed for organic carbon (OC), 𝛿13CoC, total lipids (TL), total hydrocarbon (THC), 𝑛-alkane concentration and composition. OC, 𝛿13CoC, TL and THC ranged from 0.6% to 7.6%, -29.3 to -23.8‰, 300 to 14,948 𝜇 g g−1 dw, and 0.2 to 2,277 𝜇 g g−1 dw, respectively. Predominance of even carbon numbers 𝑛-alkanes C12–C21 with carbon preference index (CPI) of < 1 suggests major microbial influence. Fair abundance of odd carbon number 𝑛-alkanes in the range of C15–C22 and C23–C33 indicates some input from phytoplankton and terrestrial sources, respectively. Petrogenic input was evident from the presence of hopanes and steranes. The data suggest that organic matter (OM) sources varied spatially and were mostly derived from mixed source.

    • Validation of OCM-2 sensor performance in retrieving chlorophyll and TSM along the southwest Bay of Bengal coast

      R Shanthi D Poornima S Raja G Vijayabaskara Sethubathi T Thangaradjou T Balasubramanian K N Babu A K Shukla

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      The Chlorophyll and Total Suspended Matter (TSM) data retrieved from Ocean Colour Monitor (OCM-2) onboard Oceansat-2 were tested for the accuracy using in-situ measurements made along the southwest Bay of Bengal coast during cruises and monthly samplings synchronized with satellite overpass from January 2010 to May 2011. The observed range of in-situ chlorophyll 𝑎 and TSM concentrations were 0.10–4.60 𝜇 gl−1 and 12.70–34.56 mgl−1 respectively, while OCM-2 derived chlorophyll 𝑎 and TSM concentration ranged from 0.324 to 1.552 𝜇 gl−1 and 3.537 to 32.11 $mgl^{−1}$, respectively. The in-situ dataset was grouped into low (0.1-0.5 𝜇 gl−1), moderate (0.51-1.0 𝜇 gl−1) and high (< 1 𝜇 gl−1) chlorophyll concentration and low ($12.7–17.81 mgl^{−1}$), moderate ($18.1–29.0 \; mgl^{−1}$) and high (lt; 30 $mgl^{−1}$) TSM concentration for evaluating the performance of algorithms against different ranges of field measurements. The OCM-2 chlorophyll retrieval algorithm (OC4V4) showed a systematic and large overestimation of low chlorophyll values with $r^{2} = 0.607$, root mean square error (RMSE) = 0.33 𝜇 gl−1 and mean normalized bias (MNB) = 1.57 and consistent underestimation of high chlorophyll values with $r^2 = 0.497$, RMSE = 1.486 𝜇 gl−1 and MNB = 0.52 especially at nearshore waters due to the interference of suspended matter and coloured dissolved organic matter. However, moderate range of chlorophyll values showed better performance of OC4V4 algorithm in chlorophyll retrieval with $r^{2} = 0.676$, RMSE = 0.254 𝜇 gl−1 and MNB = 0.09 when compared to low and high chlorophyll values. The TSM algorithm (modified algorithm of Tassan 1994) showed large underestimation in TSM retrievals and this was proved by the statistical results which shown maximum $r^{2} = 0.551$ for low TSM values with less RMSE = 0.909 $mgl^{−1}$ and MNB = 0.616 error compared to moderate and high TSM values. OCM-2 retrieved TSM values were not well correlated with in-situ TSM concentration and constantly underestimates four times lesser than the in-situ measurements especially near the coast when TSM concentration was measured high. Though there was significant correlation exists between OCM-2 retrieved chlorophyll and TSM with in-situ measurements, the empirical algorithms employed did not give logical retrieval of both chlorophyll and TSM for the southwest Bay of Bengal (BoB). Thus, the present study revealed that the OCM-2 chlorophyll and TSM retrieval algorithms need to be tested further with extensive in-situ dataset around BoB to improve the regional algorithms for accurate measurements of chlorophyll and TSM in this region.

    • Provenance and temporal variability of ice rafted debris in the Indian sector of the Southern Ocean during the last 22,000 years

      M C Manoj Meloth Thamban A Sahana Rahul Mohan Kotha Mahender

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      Ice rafted debris (IRD) records were studied in two sediment cores (SK200/22a and SK200/27) from the sub-Antarctic and Polar frontal regime of the Indian sector of Southern Ocean for their distribution and provenance during the last 22,000 years. The IRD fraction consists of quartz and lithic grains, with the lithic grains dominated by volcaniclastic materials. IRD content was high at marine isotope stage 2 but decreased dramatically to near absence at the Termination 1 and the Holocene. The concentration of IRD at glacial section of the core SK200/27 was nearly twice that of SK200/22a. Moreover, IRD were more abundant at the last glacial maxima (LGM) in SK200/27 with its peak abundance proceeding by nearly two millennia than at SK200/22a. It appears that an intensification of Antarctic glaciation combined with a northward migration of the Polar Front during LGM promoted high IRD flux at SK200/27 and subsequent deglacial warming have influenced the IRD supply at SK200/22a. Quartz and lithic grains may have derived from two different sources, the former transported from the Antarctic mainland, while the latter from the islands of volcanic origin from Southern Ocean. Sea-ice, influenced by the Antarctic Circumpolar Current is suggested to be a dominant mechanism for the distribution of lithic IRD in the region.

    • Characterizing spatial and seasonal variability of carbon dioxide and water vapour fluxes above a tropical mixed mangrove forest canopy, India

      Abhra Chanda Anirban Akhand Sudip Manna Sachinandan Dutta Sugata Hazra Indrani Das V K Dadhwal

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      The above canopy carbon dioxide and water vapour fluxes were measured by micrometeorological gradient technique at three distant stations, within the world’s largest mangrove ecosystem of Sundarban (Indian part), between April 2011 and March 2012. Quadrat analysis revealed that all the three study sites are characterized by a strong heterogeneity in the mangrove vegetation cover. At day time the forest was a sink for CO2, but its magnitude varied significantly from −0.39 to −1.33 mg m−2 s−1. The station named Jharkhali showed maximum annual fluxes followed by Henry Island and Bonnie Camp. Day time fluxes were higher during March and October, while in August and January the magnitudes were comparatively lower. The seasonal variation followed the same trend in all the sites. The spatial variation of CO2 flux above the canopy was mainly explained by the canopy density and photosynthetic efficiency of the mangrove species. The CO2 sink strength of the mangrove cover in different stations varied in the same way with the CO2 uptake potential of the species diversity in the respective sites. The relationship between the magnitude of day time CO2 uptake by the canopy and photosynthetic photon flux was defined by a non-linear exponential curve ($R^2$ ranging from 0.51 to 0.60). Water vapour fluxes varied between 1.4 and 69.5 mg m−2 s−1. There were significant differences in magnitude between day and night time water vapour fluxes, but no spatial variation was observed.

    • Global distribution of pauses observed with satellite measurements

      M Venkat Ratnam P Kishore Isabella Velicogna

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      Several studies have been carried out on the tropopause, stratopause, and mesopause (collectively termed as ‘pauses’) independently; however, all the pauses have not been studied together. We present global distribution of altitudes and temperatures of these pauses observed with long-term space borne high resolution measurements of Global Positioning System (GPS) Radio Occultation (RO) and Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) aboard Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED) satellite. Here we study the commonality and differences observed in the variability of all the pauses. We also examined how good other datasets will represent these features among (and in between) different satellite measurements, re-analysis, and model data. Hemispheric differences observed in all the pauses are also reported. In addition, we show that asymmetries between northern and southern hemispheres continue up to the mesopause. We analyze inter and intra-seasonal variations and long-term trends of these pauses at different latitudes. Finally, a new reference temperature profile is shown from the ground to 110 km for tropical, mid-latitudes, and polar latitudes for both northern and southern hemispheres.

    • Recovery curves of the lightning discharges occurring in the dissipation stage of thunderstorms

      S D Pawar A K Kamra

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      Measurements of atmospheric electric field made below two thunderstorms show that all lightning discharges occurring in the dissipating stage of a thunderstorm occur at almost the same value of the predischarge electric field at the ground surface. The observation is explained on the basis of the shielding of the electric fields generated by the positive charge in the downdrafts by the negative charge in the screening layers formed around them in the subcloud layer. Our observations suggest that in the dissipating stage of the thunderstorm, the charge generating mechanisms in cloud have ceased to operate and the charge being transported from the upper to lower regions of cloud by downdrafts is the only in-cloud process affecting the surface electric field and/or enhancing the electric field stress in and below the cloud base to cause yet another lightning discharge.

    • Ventilation coefficient trends in the recent decades over four major Indian metropolitan cities

      U S Iyer P Ernest Raj

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      Thirty years radiosonde data (1971–2000) at 00 UTC for winter months over four major Indian metros, viz., Mumbai, Delhi, Kolkata and Chennai is analysed to study the trends and long term variations in ventilation coefficients and the consequences on the air quality due to these variations in the four metros. A decreasing trend in ventilation coefficient is observed in all the four metros during the 30 years period indicating increasing pollution potential and a degradation in the air quality over these urban centers. In Delhi, the ventilation coefficient decreased at the rate of 49 and 32 m2/s/year in the months of December and February, respectively during the 30-year period. In Mumbai, the average decrease in ventilation coefficient in winter months is about 15 m2/s/year whereas for Kolkata it is 14 and 17 m2/s/year in December and February, respectively. A decreasing trend in ventilation coefficient is observed in Chennai too although it is not significant. The decreasing ventilation coefficient increased the ground level pollution thereby deteriorating the air quality for the urban population. For Mumbai and Kolkata, decreasing mixing depths and decreasing wind speed contributed to the decreasing ventilation coefficient whereas for Delhi and Chennai decreasing wind speed was responsible for the decrease in ventilation coefficient. Further, the pollution potential was much higher in Delhi which is an inland station as compared to Mumbai, Kolkata and Chennai which are coastal stations under the influence of marine environment. Compared to Delhi, the pollution potential over these three metros was lower as the prevailing sea-breeze helped in the dispersal of pollutants thereby reducing their ground level concentration.

    • Seasonal forecasting of Bangladesh summer monsoon rainfall using simple multiple regression model

      Md Mizanur Rahman M Rafiuddin Md Mahbub Alam

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      In this paper, the development of a statistical forecasting method for summer monsoon rainfall over Bangladesh is described. Predictors for Bangladesh summer monsoon (June–September) rainfall were identified from the large scale ocean–atmospheric circulation variables (i.e., sea-surface temperature, surface air temperature and sea level pressure). The predictors exhibited a significant relationship with Bangladesh summer monsoon rainfall during the period 1961–2007. After carrying out a detailed analysis of various global climate datasets; three predictors were selected. The model performance was evaluated during the period 1977–2007. The model showed better performance in their hindcast seasonal monsoon rainfall over Bangladesh. The RMSE and Heidke skill score for 31 years was 8.13 and 0.37, respectively, and the correlation between the predicted and observed rainfall was 0.74. The BIAS of the forecasts (% of long period average, LPA) was −0.85 and Hit score was 58%. The experimental forecasts for the year 2008 summer monsoon rainfall based on the model were also found to be in good agreement with the observation.

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