Bengal Basin Research Articles

Spatial distribution of manganese in groundwater and associated human health risk in the southern part of the Bengal Basin.

The scarcity of arsenic and iron-free safe drinking water is an alarming issue in the southern part of the Bengal Basin. The objectives of the present study were to investigate the spatial distribution of manganese (Mn) concentration in the shallow and deep groundwater and its associated health risks for the children and adults of entire southern Bengal Basin. The Mn concentration in the groundwater varied from 0 to 5.4 mg/L with an average value of 0.47 mg/L that exceeded the WHO's and Bangladesh drinking water guideline values of 0.4 and 0.1 mg/L, respectively. Mn concentration in the shallow wells overrode the deep ones. About 23% of the shallow wells and 11% of deep wells exceeded the WHO's safety limit of Mn concentration for human health. The human health risk related to Mn contamination was estimated by computing the average daily dosage (ADD) and hazard quotient (HQ) values for children and adults. The average computed HQ values found 0.108 and 0.099 for children and adults, respectively. The HQ values delimitated that children are posing a higher risk compared to the adults for the shallow wells. Deep wells were found risk-free for both children and adults. The areal coverage of shallow wells with HQ values > 1 was minimal compared to the total study area and covered only a small portion of Patuakhali and Barguna districts. The rest of the site does not pose any health risk due to Mn contamination for children and adults.

Living with arsenic in the environment: An examination of current awareness of farmers in the Bengal basin using hybrid feature selection and machine learning

High levels of arsenic in drinking water and food materials continue to pose a global health challenge. Over 127 million people alone in Bangladesh (BD) and West Bengal (WB) state of India are exposed to elevated levels of arsenic in drinking water. Despite decades of research and outreach, arsenic awareness in communities continue to be low. Specifically, very few studies reported arsenic awareness among low-income farming communities. A comprehensive approach to assess arsenic awareness is a key step in identifying research and development priorities so that appropriate stakeholder engagement may be designed to tackle arsenic menace. In this study, we developed a comprehensive arsenic awareness index (CAAI) and identified key awareness drivers (KADs) of arsenic to help evaluate farmers’ preferences in dealing with arsenic in the environment. The CAAI and KADs were developed using a questionnaire survey in conjunction with ten machine learning (ML) models coupled with a hybrid feature selection approach. Two questionnaire surveys comprising of 73 questions covering health, water and community, and food were conducted in arsenic-affected areas of WB and BD. Comparison of CAAIs showed that the BD farmers were generally more arsenic-aware (CAAI = 7.7) than WB farmers (CAAI = 6.8). Interestingly, the reverse was true for the awareness linked to arsenic in the food chain. Application of hybrid feature selection identified 15 KADs, which included factors related to stakeholder interventions and cropping practices instead of commonly perceived factors such as age, gender and income. Among ML algorithms, classification and regression trees and single C5.0 tree could estimate CAAIs with an average accuracy of 84%. Both communities agreed on policy changes on water testing and clean water supply. The CAAI and KADs combination revealed a contrasting arsenic awareness between the two farming communities, albeit their cultural similarities. Specifically, our study shows the need for increasing awareness of risks through the food chain in BD, whereas awareness campaigns should be strengthened to raise overall awareness in WB possibly through media channels as deemed effective in BD.

Prediction of groundwater trends for irrigation in Northern Bangladesh

Abstract Groundwater trends affect the domestic, agricultural, and industrial prospects of a region. The study area is Bogura, a northern region of Bangladesh, located on the Pleistocene terrace of the Bengal Basin. The aquifer consists of medium-to-coarse sand, located at a depth of 4.66–42.68 m; groundwater is scarce during dry seasons. The water table (WT) time-series data for 2007–2019 were used for forecasting and characterizing present and future groundwater conditions using existing numerical simulations. The annual groundwater budget for discharge and storage was 2,772 and 2,442 Mm3, respectively. Thus, the annual scarcity of groundwater was 330.4 Mm3 (13.5%), excluding the surface water contribution of 10 Mm3 (0.4%). The present spacing of deep tube wells (DTWs) and shallow tube wells (STWs) was 744 and 372 m, respectively. Currently, the DTW spacing ranged 744–800 m; however, the STW spacing of 250–372 m is higher than the set distance. Hence, further installations of STWs were strictly disallowed for irrigation. WT declined by 1.0 m in the last 13 years, i.e., 0.07 m or 1.2% decline rate per annum, causing water scarcity in the region during the peak period in the dry season (June–February), thus affecting irrigation and limiting agricultural production.

Spatiotemporal Analysis of Tropical Cyclone Landfalls in Northern Bay of Bengal, India and Bangladesh

In the Northern Bay of Bengal (NBoB), cyclonic storms (wind speed: ≥34 knots) inflict considerable losses in coastal Bangladesh and India. By building a georeferenced panel database of storm landfalls for this region, one can obtain spatial information on period-wise strike intensities. In this study, 168 cyclone strike locations along a 1259-km impact zone were analyzed for seven historical periods between 1877 and 2016. Cyclone landfall trends for this region denote a gradual reduction in storm frequencies since peaking in 1961–1980, which is generally in line with already established trends for the Northern Indian Ocean and Bay of Bengal basins. One notable exception for the NBoB in this is the unimodal seasonality of storm landfalls during 1877–1960, but that also conformed to the bimodal regional trends in the later periods. Over time, the median location of storm strikes in the NBoB has shifted eastward into westernmost Bangladesh, with a particularly marked variability in location after 1960. Northern Odisha and West Bengal, India, experienced more cyclone strikes over the past 140 years; but the highest mean wind speeds occurred along Bangladesh’s eastern coast. Also, storm impacts have varied considerably within and across zones; northern Odisha and the Sundarban region of West Bengal have withstood the greatest impacts. The pronounced spatial and temporal variations in cyclone impacts evinced in this study can provide valuable information for relief prioritization and community adaptation to cyclones in the region. The general method developed for this analysis can be applied to other coastal locations for management of storm impacts.

Plio-Pleistocene to Recent tectonostratigraphic evolution of the Lalmai anticline in the western Indo-Burman Range (Bangladesh): insights from lithofacies analysis and structural synthesis

The Lalmai anticline is one of the westernmost structures within the Eastern Fold Belt of the Bengal Basin that has surficial expression. The tectonic evolution of the Eastern Fold Belt is directly linked with the development of the Indo-Burman Range in response to the late Cenozoic convergence between the Indian and Burmese plates. However, very little is known about the Plio-Pleistocene to Recent tectonic evolution of the Lalmai anticline in the context of regional tectonics. Therefore, this research is aimed at lucid understand the Plio-Pleistocene to Recent tectonostratigraphic evolution of the Lalmai anticline based on detailed lithofacies and structural observations. Results show that Plio-Pleistocene Dupitila and Pleistocene Madhupur Clay formations are exposed in the Lalmai area. Based on sedimentary texture and structures, a total of six prominent lithofacies types have been identified. These are trough cross-stratified sandstone (St), planar cross-stratified sandstone (Sp), parallel laminated sandstone (Sl), massive sandstone (Sm), laminated shale (Fl), and mudstone (Fm) facies. The facies characteristics indicate a possible fluvial depositional environment for both the formations. Structural mapping shows that the bedding surfaces are nearly horizontal. Based on the missing of strata (both the Dupitila and Madhupur Clay formations) and presence of a fault escarpment, a N-S trending east-dipping reverse fault (namely, the Mainamati Fault) has been identified on the western margin of the Lalmai anticline. The tectonostratigraphic evolutionary models show that the last major structural activation along this Mainamati Fault can be linked to late Pleistocene to Recent time. This structural activation was possibly controlled by the latest ongoing E-W crustal shortening in response to the Indo-Burman subduction processes to the far east.

Assessing key performance indicators in the shipbuilding industry; an MCDM approach

ABSTRACT A series of countries have focused on shipbuilding public policies, as it is commonly considered a deliberate industry and influences employment levels in many regions. Establishing key performance indicators can be a challenge to shipbuilding policymakers. This paper suggests an approach to this issue, based on three different MCDM techniques: the fuzzy DEMATEL, the Fuzzy ANP, and the MOORA. This approach makes it achievable to estimate performance and competitiveness, considering 25 different variables comprising four different viewpoints of a balanced scorecard approach. These dimensions include finance, customer, internal process, and learning and growth aspects. The methodology is applied to the analysis of the active shipyards of the Bay of Bengal Basin countries. Our main findings indicate that shipyards which are effective in ‘customer’ variables perform higher performance, while variables constituting the ‘learning and growth’ dimension demonstrate lower impact on it. Additionally, the dimensions of ‘finance’ and ‘customer’ show uttered impacts on the other dimensions, while ‘learning and growth’ has no impact on the other perspectives of the balanced scorecard. Through this methodology, researchers, practitioners, and policymakers might acquire a penetrating overview into the drivers of the global shift in shipbuilding industry.

Palaeoclimate estimates based on the late Miocene to early Pleistocene wood flora of the Bengal Basin: an insight into the climatic evolution of southern Asia

To assess the pattern of climatic evolution during the late Miocene to early Pleistocene in the largest fluvio-deltaic sedimentary system on the Earth, the Bengal Basin (BB), a quantitative palaeoclimatic reconstruction was made, based on 20 fossil wood floras. Those floras show that moisture-loving taxa have decreased considerably since the Miocene, especially at the western margin of the basin. A quantitative reconstruction of climate parameters reveals that the late Miocene−early Pliocene was warmer and wetter than now, yet with spatial variability. Seasonality of temperature was low in the basin and subsequently increased during the late Pliocene−early Pleistocene. Monsoon intensity was weaker during the interval from the late Miocene to early Pleistocene than the present day. A comparison of the retrieved data with some earlier records from sites either influenced by Indian summer monsoon (ISM) or East Asian summer monsoon (EASM) or both, the two branches of the Asian summer monsoon (AM) provide insights into the temporal and spatial patterns of climate evolution in southern Asia during the late Neogene–Quaternary transition. In general, a drop in temperature and a weakening in ISM strength since the early Pleistocene correlate with the global cooling trend, though with spatial differences.

Geomorphological Study of Jaflong Area near Dauki Fault Using Remote Sensing and Geographic Information System

Geomorphology is closely related to geology, soil science, hydrology and environmental science and is being increasingly applied in planning, mining and hydrological sectors, and within environmental consultancy and tourism. The study area lies in the north-eastern part of the country and tectonically this area belongs to Surma Basin situated into Bengal Fore deep zone of Bengal Basin also known as Sylhet Trough. Under this study an attempt was made to establish the relationship between geomorphic unit and existing landuse based on remote sensing data. In the study area nine landuse categories were identified through remote sensing and GIS techniques. Two different physiographic units which are North-Eastern Terrace Land and Surma-Kushiyara Flood Plain are belongs to the study are. Moreover two surface geological units include Young Gravelly Sand and Marsh Clay and Peat are also there. From analysis no relationship has been found between landuse classes with its physiography and surface geology.
 Environ. Sci. & Natural Resources, 12(1&2): 151-155, 2019

Geochemical, metagenomic, and physiological characterization of the multifaceted interaction between microbiome of an arsenic contaminated groundwater and aquifer sediment

Geomicrobiological details of the interactions between groundwater microbiome (GWM) and arsenic (As)-rich aquifer sediment of Bengal basin was investigated through microcosm incubations. Role of key microorganisms and their specific interactions with As-bearing minerals was demarcated under organic carbon- amended and -unamended conditions. Acinetobacter (50.8 %), Brevundimonas (7.9 %), Sideroxydans (3.4 %), Alkanindiges (3.0 %) dominated the GWM. The microbiome catalysed considerable alterations in As-bearing mineral [Fe-(hydr)oxide and aluminosilicate] phases resulting in substantial changes in overall geochemistry and release of As (65 μg/L) and Fe (118 μg/L). Synergistic roles of autotrophic, NH4+-oxidizing Archaea (Thaumarchaeota) and chemoheterotrophic bacteria (Stenotrophomonas, Pseudomonas, Geobacter) of diverse metabolic abilities (NH4+-oxidizing, NO3−, As/Fe-reducing) were noted for observed changes. Organic carbon supported enhanced microbial growth and As mobilization (upto 403.2 μg As/L) from multiple mineral phases (hematite, magnetite, maghemite, biotite, etc.). In presence of high organic carbon, concerted actions of anaerobic, hydrocarbon-utilizing, As-, Fe-reducing Rhizobium, fermentative Escherichia, anaerobic Bacillales, metal-reducing and organic acid-utilizing Pseudomonas and Achromobacter were implicated in altering sediment mineralogy and biogeochemistry. Increase in abundance of arrA, arsC, bssA genes, and dissolution of Fe, Ca, Mg, Mn confirmed that dissimilatory-, cytosolic-As reduction, and mineral weathering fuelled by anaerobic (hydro)carbon metabolism are the predominant mechanisms of As release in aquifers of Bengal basin.

Petrological, geochemical, and microfacies analysis of the Sylhet limestone, Bengal Basin, Bangladesh: implication for depositional environment and diagenesis

The fossiliferous Sylhet limestone formation of the Eocene age in the Bengal Basin is important for scientific and geoheritage study. This formation is exposed in the two locations of Bengal Basin; however, it is least studied. In this study, petrological, geochemical, and microfacies analysis of the Sylhet limestone was carried out by analyzing twenty-eight (28) samples from the two exposed sections and two underground (boreholes) in the Bengal Basin. The microscopic and XRD analysis shows that the calcite is the most dominant mineral in the studied samples along with some amount of dolomite, siderite, ankerite, pyrite, and quartz. The presence of the crystal structure of calcite, dolomite, quartz, and pyrite was confirmed through SEM-EDX. Geochemical results show that the samples are composed of high CaO (14.296 to 53.326%) with a significant amount of Fe2O3 (0.095 to 18.482%), Al2O3 (0.664 to 7.357%), K20 (0.83 to 2.274%), MgO (0.157 to 8.109%), and SiO2 (0.08 to 0.451%). Beside the major oxides, some minor and trace elements including Sr, Mn, Zr, Zn, Sc, Co, V, Ba, Ni, and Cu were also measured. Among these, Sr, V, Mn, and Cu concentration within the Sylhet limestone directly indicates its depositional environment at a shallow continental shelf in open marine condition and its diagenetic change to convert as dolomitic limestone. Geochemical characteristics indicate that the Sylhet limestone is highly dolomitized with high Ca/Mg ratio and was deposited in a low-energy shallow continental shelf marine depositional environment. Higher values of the Ca/Mg ratio of the studied samples also confirm lower salinity during its deposition. The presence of silica in the limestone was observed with a positive/negative correlation with Ca which implies that the silica is derived either from a terrestrial source due to increases of sediment supply during the Middle Eocene for the consequences of the Indo-Asian plate Himalayan Collision or dissolution from the siliceous shells of fossils that replace the calcite. Eight (08) microfacies (MFT) types were identified based on abundance of foraminifera or calcareous algae (e.g., Nummulites sp., Discocyclina sp., Alveolina sp., Quinqueloculina sp., Cibicides sp., Lenticulina sp., Ovulites sp.) which are similar to grainstone/packstone carbonate of 18 standard microfacies types (SMF 18). The observed SMF was deposited under restricted to open marine carbonate platform of shallow water depth at different hydrodynamic conditions.

Understanding the characteristics of rapid intensity changes of Tropical Cyclones over North Indian Ocean

North Indian Ocean (NIO), which comprises of Bay of Bengal (BoB) and Arabian Sea (AS) basins, is one of the highly potential regions for Tropical Cyclones (TCs) in the world. Significant improvements have been achieved in the prediction of the movement of TCs, since the last decade. However, the prediction of sudden intensity changes becomes a challenging task for the research and operational meteorologists. Hence, the present study focuses on finding the climatological characteristics of such intensity changes over NIO regions. Rapid Intensification (RI) is defined as the 24-h maximum sustained surface wind speed rate equal to 30 knots (15.4 ms−1). The results suggest that the TCs formed over the NIO basin are both seasonal and basin sensitive. Since 2000, a significant trend is observed in RI TCs over the basin. At least one among three cyclones getting intensified is of RI category. More number of RI cases have been identified in the BoB basin than the AS. The post-monsoon season holds more RI and rapid decay cases, with 63% and 90% contribution. Most of the TCs are attaining RI onset in their initial stage. Further, India is receiving more landfalling RI TCs, followed by Bangladesh and Oman. The east coast of India, Tamil Nadu, and Andhra Pradesh are the most vulnerable states to these RI TCs. The cyclogenesis locations associated with RI cases hold higher moisture, and sea surface temperature as compared to the Non-RI cases.

Upper-ocean physical and biological features associated with Hudhud cyclone: A bio-physical modelling study

Abstract The east coast of India is highly prone towards the devastation by the cyclonic storms. The Bay of Bengal (BoB) basin in the northeastern Indian Ocean is characterized to provide adequate conditions to the cyclogenesis. The cyclogenesis is more active during pre- and post-monsoon seasons over the BoB. The post-monsoon season shows highest storm occurrence in the region due to presence of warm and freshwater in the upper-ocean which helps to maintain deep barrier layer and supports cyclogenesis. The very severe cyclonic storm, Hudhud was a post-monsoonal storm occurred during 07th–14th October 2014 over the BoB. Hudhud made landfall near Visakhapatnam coast at noon of 12th October 2014. The present study utilized a three dimensional coupled bio-physical model for the analysis of biogeochemical response of the upper-ocean to Hudhud cyclone. The different surface and subsurface physical and biological features along the track of Hudhud has depicted a cyclone-generated surface phytoplankton bloom and its associated decline of surface dissolved oxygen. The study aims at a better understanding of the spatial and temporal variability of the upper-ocean parameters in response to the passage of tropical cyclone (TC) Hudhud. It is found that the intense cyclonic wind stress caused upwelling in the wake of cyclone track about 1550 km from its genesis location. The excess surface freshwater flux due to precipitation establishes stronger stratification in the coastal region. This enhanced stratification found to restrict the supply of nutrients to the euphotic zone and, hence, limiting the surface primary productivity in the coastal region.

Analysis of spatio-temporal trend in groundwater elevation data from arsenic affected alluvial aquifers – Case study from Murshidabad district, West Bengal, Eastern India

Fluctuation in groundwater level is a time-dependent stochastic process. It is also a function of various inflow and outflow components to and from the hydrologic system concerned. Depth to water level data are measured through a network of observation wells or hydrograph stations to ascertain the degree of fluctuation in groundwater level at the desired scale, on a long-term basis. Basically, these depths to water level data are point measurements, which can be regarded as random variables furnishing changes in groundwater storage over time. The intrinsic in-homogeneity in aquifer materials introduces variations like jumps, trends, and periodicities in such hydrologic time series data. Thus, trending results from certain gradual, natural and/or anthropogenic interventions in the hydrologic environment and analyses of their trends are imperative for assessment of groundwater level scenario in the area of interest. This in turn, is essential for strategic planning and management for exploitation of the precious groundwater resource in the same area. The area of interest in this article, i.e., Murshidabad is one of the nine arsenic affected districts of West Bengal. Here, contamination persist within, shallow, arseniferous, alluvial aquifers, which are otherwise widely exploited for irrigation purposes. According to many researchers working in this area, over-exploitation of groundwater is the root cause for the plummeting water level and the widespread arsenic contamination as well. The present study intends to detect and analyze the trends persisting in the depth to water level data measured over a period from 1996 to 2016, in Murshidabad district of West Bengal, India, amidst its complex and contrasting hydrogeologic set-up and interpret the results in terms of the hydrologic attributes of the Bengal basin as a whole. The non-parametric Mann–Kendall test and the Sen’s slope estimator have been used to identify the linear trend persisting in the time series pre- and post-monsoon groundwater elevation values. The analysis indicates statistically significant decline in water level across the study area especially during the post-monsoon season. This can be attributed to the recharge–discharge disparity within the hydrologic regime; brought through intense pumping over the study area. Its ill effect being particularly observed in the western part of river Bhagirathi. Findings of such study are crucial for assessment of dynamic groundwater resources of the district and subsequently can be utilized as a decision support tool for groundwater management at micro-level.

Sedimentology and basin-fill history of the Cenozoic succession of the Sylhet Trough, Bengal Basin, Bangladesh

The Sylhet Trough, a petroleum province of the Bengal Basin, accommodates a huge thickness of Eocene to Recent sedimentary successions. However, the basin-fill history of the trough is poorly understood; specifically, constrains on timing of deposition of the individual units are yet to be established. Therefore, we aimed at establishing sedimentation and basin-fill history of the Sylhet Trough based on detailed lithofacies analysis of the outcropping Cenozoic succession. We have divided the entire Cenozoic succession into three megasequences which can be further sub-divided into nine lithostratigraphic units based on bounding discontinuities, such as transgressive erosion surface, regressive erosion surface, transgressive surface, marine flooding surface, and incised valley floor. The oldest is the Megasequence 1, comprised of shallow marine shelfal deposits overlain by shallow marine to nearshore deposits. In the middle, the Megasequence 2 is representing tide-dominated marine to coastal (deltaic) depositional systems with evidence of cyclic marine regression and transgression. Repetitive occurrence of incised channel, tidal inlet, tidal ridge/shoal, tidal flat and other tidal deposits are separated by shelfal deposits. The top of the Megasequence 2 is marked by a pronounced erosion surface interpreted as an incised valley floor indicating the final phase of marine regression followed by the gradual establishment of the overlying continental-fluvial depositional systems (i.e., the Megasequence 3). This youngest megasequence is characterized by stacked braided river sand bars that pass up-sequence into meandering river deposits. Based on the Cenozoic eustatic sea level curve, we suggest that the upper boundaries of the Megasequence 1 and Megasequence 2 are approximately at 39.5 Ma and 5.0 Ma, respectively.

Geochemistry of Eocene to Pliocene strata of the Bengal Basin: Implications for provenance and erosion of the Himalaya

The earliest record of Himalayan detritus in the Himalayan foreland and the erosion sequence of different lithotectonic units of the Himalaya are critical for reconstructing collision and crustal deformation of the orogenic belt. In this study, bulk‐rock major, trace and rare earth elements of 19 fine‐grained clastic sedimentary rock samples from Eocene to Pliocene strata of the Bengal Basin were examined to study their provenance and erosion of the Himalaya. Combining with previous detrital zircon age data from the Bengal Basin, Nepal foreland basin, and South Tibet, we suggest that detritus from the uplifted Tethyan Himalayan Sequences and the Gangdese Arc had not been carried into the Himalayan foreland at least before Late Miocene, and that the Bengal Basin was in a passive continental margin setting from Eocene to Early Miocene, receiving detritus from a felsic sedimentary rock source. The earliest Himalayan detritus recorded in the Eocene strata of Bengal Basin were probably derived from the antecedent sedimentary cover of the Greater Himalayan crystalline rocks. We assume that the lack of Cretaceous Gangdese detritus in the Himalayan foreland from Eocene to Middle Miocene is due to a geomorphic barrier between the Tethyan Himalaya and the Himalayan foreland. Detritus from the Greater Himalayan crystalline rocks were first recorded in the Middle–Late Miocene Boka Bil Formation of the Bengal Basin, which is consistent with the uplift and exhumation time of the Greater Himalayan Sequences.

Population Exposure to Amphan-Scale Cyclones Under Future Climates

Background: Climate change impacts are felt disproportionately in developing countries, and in particular Southern Asia experiences the most damaging hydrometeorological events in the world, with loss of life from past cyclones in the hundreds of thousands. Despite this, the Bay of Bengal cyclone basin receives far less research attention than many of the others around the world. Here, we study the historical and future impacts of Super Cyclone Amphan, which made landfall in May 2020, bringing storm surges of 2-4 meters to coastlines of India and Bangladesh. Methods: In this modelling study, we combine projections of sea level rise from the Coupled Model Intercomparison Projection, phase 6 (CMIP6), with estimates of storm surge using a dynamic storm surge model. Sampling the spectrum of possible sea level rises, we consider a low, medium and high scenario, based on projections in 2100. We then feed these into a flood inundation model to simulate storm surge-induced flooding, had Cyclone Amphan occurred in these future worlds. Finally, we consider the change in future population growth and urbanisation, thereby calculating the change in population exposure to these future flooding events. Our approach is that of the extreme event attribution community, but projecting into the future rather than interrogating the past. Findings: We find that in 2100, the local sea level rise in the Bay of Bengal during the pre-monsoon cyclone season is between 0.32-0.84 m, depending on which emissions scenario is followed. If a Cyclone Amphan-scale storm surge occurred on top of that sea level rise, the future population of both India and Bangladesh will be more exposed, with India showing >200% increased exposure to extreme (>3 m) and moderate (>1 m) flooding under a high emissions scenario, and Bangladesh showing ~60-80% increased exposure to the same scenarios. The majority of this change in both countries comes from sea level rise rather than population changes, and in Bangladesh the future population change contributes negatively to the change in exposure, as more citizens migrate further inshore. However, if we follow an emissions scenario consistent with meeting the upper Paris Agreement climate goal, we project very little change in exposure. Interpretation: There is an urgent need to reduce carbon emissions to net zero, to prevent the negative impacts of climate change. By far the majority of cyclone research has been undertaken for countries such as America and Japan, with less resilient countries such as those in South Asia, which are more sensitive to changes in climate, seeing far less attention. With Cyclone Amphan occurring at the height of the COVID-19 crisis, we highlight how the risk was compounded and recommend that future climate risk assessments explicitly account for these potential non-linearities. Funding Statement: The main funding is from the Natural Environment Research Council.Declaration of Interests: The authors declare no competing interests.

Groundwater recharge processes in an Asian mega-delta: hydrometric evidence from Bangladesh

Groundwater is used intensively in Asian mega-deltas yet the processes by which groundwater is replenished in these deltaic systems remain inadequately understood. Drawing insight from hourly monitoring of groundwater levels and rainfall in two contrasting settings, comprising permeable surficial deposits of Holocene age and Plio-Pleistocene terrace deposits, together with longer-term, lower-frequency records of groundwater levels, river stage, and rainfall from the Bengal Basin, conceptual models of recharge processes in these two depositional environments are developed. The representivity of these conceptual models across the Bengal Basin in Bangladesh is explored by way of statistical cluster analysis of groundwater-level time series data. Observational records reveal that both diffuse and focused recharge processes occur in Holocene deposits, whereas recharge in Plio-Pleistocene deposits is dominated by indirect leakage from river channels where incision has enabled a direct hydraulic connection between river channels and the Plio-Pleistocene aquifer underlying surficial clays. Seasonal cycles of recharge and discharge including the onset of dry-season groundwater-fed irrigation are well characterised by compiled observational records. Groundwater depletion, evident from declining groundwater levels with a diminished seasonality, is pronounced in Plio-Pleistocene environments where direct recharge is inhibited by the surficial clays. In contrast, intensive shallow groundwater abstraction in Holocene environments can enhance direct and indirect recharge via a more permeable surface geology. The vital contributions of indirect recharge of shallow groundwater identified in both depositional settings in the Bengal Basin highlight the critical limitation of using models that exclude this process in the estimation of groundwater recharge in Asian mega-deltas.

Evaluation of track length, residence time and translational speed for tropical cyclones in the North Indian ocean

ABSTRACT The present study performed a critical examination on the historical tracks of 185 tropical cyclonic events that developed over North Indian Ocean basin during the past 37 years (1982–2018). Analysis of cyclone tracks was carried out including all categories starting from Deep Depression (DD) to Super Cyclonic Storm (SuCS) stages. Results from the study indicate that the track length have decreased by about 13%, whereas the residence time increased by 7.3% for all categories considered during the study period. However, the high-intense cyclone categories showed an increase in both track lengths and residence time that was statistically significant. The net increase was about 10% and 43% for the track lengths and residence time, respectively. Segmented analysis clearly brings out the significant upward trend in track length and residence time during the recent decade (2009–2018). Further, the study examined the variability in translational speed on annual and inter-seasonal scales for cyclonic disturbances that formed over the Bay of Bengal basin. The overall trend is showing a decreasing pattern with a net reduction of 2.5 km/h over the past three decades. A detailed assessment was carried out to understand the trends in these parameters for the North Indian Ocean region. The study has practical implications in better understanding the characteristics of tropical cyclones and coastal vulnerability.

Ocean heat content and its role in tropical cyclogenesis for the Bay of Bengal basin

Anthropogenic factors continue to warm the oceans by storing sufficient memory of past accumulated effects resulting in higher Sea surface temperature (SST) and Ocean heat content (OHC) invigorating tropical cyclogenesis and tropical cyclone intensity. Prior studies indicate that an increase in accumulated tropical cyclone heat potential, a measure of OHC is directly related to high intense storms of longer durations. The present study is an attempt to understand the role of OHC in tropical cyclogenesis for the Bay of Bengal (BoB). Unlike the other global ocean basins, the BoB is a semi-marginal sea in the North Indian Ocean (NIO) having differential water mass characteristics attributed due to tremendous influx from riverine discharges, seasonal reversing monsoonal wind system and complex ocean circulation features. This study provides a detailed assessment on SST and OHC distributions during the pre- and post-monsoon seasons covering a period of 21 years (1991–2016) over four identified sub-domains, the potential locations for tropical cyclogenesis. The daily ERA-Interim, ORAS4, ORAS5 and 3-h RAMA buoy datasets were used in the present study. The spatial distribution and variations of SST and OHC anomalies during pre- and post-monsoon seasons in the BoB were also examined. Differential nature of inter-annual SST variability was noticed in the study region. Highest variability in SST is evidenced during both pre- and post-monsoon seasons for the northern Bay region showing a warming bias of ∼ 1.7 °C in the past 2 decades. Contrasting variability is also noticed for the eastern Bay during pre-monsoon that is almost three times higher as compared to the post-monsoon season. Maximum OHC variability is found for the southern and central-eastern Bay regions. It is seen that OHC variability has a strong tele-connection with Oceanic Nino Index associated with El Nino/La Nina events in the Pacific basin with their amplitudes found to increase from northern to southern regions of the Bay.

Representing heterogeneity of fluvio-deltaic aquifers in models of groundwater flow and solute transport: A multi-model investigation in the Bengal Basin

Groundwater in alluvial terrain, supporting much of the global requirement for irrigation and domestic water, is at risk of sustained water level decline and/or contamination over large areas. In southern Bangladesh excessive arsenic (As) in shallow groundwater has led to deeper groundwater becoming the preferred alternative source of potable water. The vulnerability of deeper tube-wells to As breakthrough from shallow levels can be assessed using groundwater modelling, but representation of alluvial aquifer heterogeneities in large-scale groundwater models presents a challenge; what level of complexity is required? To assess the optimum level of complexity necessary in models of groundwater flow and solute transport in the Bengal Aquifer System (BAS), we explore a range of representations of the lithological heterogeneity using a multi-modelling approach. We use an array of geological information including drillers’ logs (n = 589) and hydrocarbon exploration data (n = 11) across an area of 5000 km2 as a basis for alternative representations of upscaled aquifer heterogeneity, characterising hydrogeological structure in a series of five groundwater models at increasing levels of complexity. We rank the models by comparing model outcomes of travel time with available data on groundwater age based on 14C. The results demonstrate the importance of spatial heterogeneity and suggest the significance of incorporating vertical heterogeneity in model representations of the Bengal Aquifer System and similar spatially extensive fluvio-deltaic aquifers.