Regional and Seasonal Precipitation and Drought Trends in Ganga–Brahmaputra Basin

Satellite-based precipitation products can be a better alternative of rain gauges for hydro-meteorological studies in data-poor regions. This study aimed to evaluate how regional and seasonal precipitation and drought patterns had changed in the Ganga–Brahmaputra Basin between 1983 and 2020 with PERSIANN-CDR precipitation data. The spatial pattern of winter drought, monsoon drought, and Standardized Precipitation Index (SPI) calculated for different time scales were evaluated using principal component analysis. Ganga–Brahmaputra is one of the most populated river basins that flows through different geographical regions. Rain gauges are heterogeneously distributed in the basin due to its complex orography, highlighting the significance of gridded precipitation products over gauge observations for climate studies. Annual and monthly precipitation trends between 1983 and 2020 were evaluated using the original and modified Mann–Kendall trend test, and annual precipitation in the basin was found to be declining at a rate of 5.8 mm/year. An increasing trend was observed in pre-monsoon rainfall, whereas precipitation exhibited a decreasing trend for other months. Results of the Pettitt test showed precipitation time series was inhomogeneous and changepoint occurred around 2000. Decreasing trends of SPI indicated increasing frequency and intensity of drought events. Winter drought showed a clear spatial pattern in the basin; however, SPIs calculated for different time scales and monsoon drought had complex spatial patterns. This study demonstrates the applicability of satellite-based PERSIANN-CDR precipitation data in climate research in the Ganga–Brahmaputra Basin.

A new fish species of genus Garra (Teleostei: Cyprinidae) from Nagaland, India

A species of the genus Garra is described from the Langlung River, Brahmaputra basin, Nagaland, India. The new species is distinguished from its congeners in having weakly-developed unilobed proboscis, a distinct transverse lobe with 8–12 small sized unicuspid acanthoid tubercles, 30–32 lateral line scales, and 13–15 circumpeduncular scales.

Diversity of freshwater molluscs from the upper Brahmaputra Basin, Assam, India

A field survey was conducted for three consecutive years, 2015–17 to assess the diversity of freshwater molluscs (Gastropoda and Bivalvia) of the upper Brahmaputra Basin in Assam, India. Altogether, 18 gastropods and 27 bivalve species representing nine families were recorded from 17 sampling stations comprising small to large tributaries and wetlands in the flood-plains covering a total geographical area of approximately 3,500km2. A large fraction (15.55%) of the collected mollusc species are new records from the upper Brahmaputra Basin of Assam. Rarity in the occurrence of freshwater mollusc was confirmed with singleton and doubleton species accounting for 6.66% and unique species accounting for 35.55% of the total species recorded. It was observed that most of the mollusc species of the upper Brahmaputra Basin are either in the ‘Least Concern’ or ‘Data Deficient’ category of the IUCN Red List; except for Lymnaea ovalior (Annandale & Prashad, 1921) and Sphaerium austeni Prashad, 1921 assessed as ‘Vulnerable’ and ‘Near Threatened’, respectively. A significant trend in the diversity in terms of species richness and composition was observed across the sampling stations of the northern basin and southern basin of the river Brahmaputra.

Assessment of GloFAS ensemble flood forecast for the Brahmaputra basin: skilful lead-times to predict monsoon floods for early action in Bangladesh

<p>Flood is a frequent natural hazard in the Brahmaputra basin in Bangladesh during the South Asian summer monsoon between June to September. When will flooding start during monsoon and how long it will last are two important questions that forecasters need to answer. Predicting flood timing and duration with a sufficient lead-time is challenging for forecasters due to strong intraseasonal variation of floods within a monsoon.</p><p>The GloFAS forecasting system is run by ECMWF as part of the Copernicus Emergency Management Service and provides operational extended-range ensemble flood forecast with 30 days lead-time for the major river basins in the world. In this study, we evaluated GloFAS reforecasts for the Brahmaputra basin in Bangladesh for the period 1997–2019 at different lead-times against observed stream gauge and ECMWF ERA5 reanalysis river discharge data. We used various probabilistic forecast verification metrics, such as Relative Operating Characteristic (ROC), False Alarm Ratio (FAR), and Probability of Detection (POD), to study how forecast skill varies over different lead-times. We also assessed the skilful lead-times of the GloFAS forecast to predict flood timing and duration during the monsoon. These scores were calculated considering relevant flood threshold levels and action-based parameters, such as Action Lifetime, based on user needs in Bangladesh. The GloFAS forecast case study for the recent 2020 monsoon floods in the Brahmaputra basin shows that the onset of flood events was successfully predicted with a lead-time of 15 days. These forecasts were disseminated among the different stakeholders, including humanitarian agencies, flood and disaster management organisations, to inform forecast-based actions, such as evacuation of vulnerable people to safer places ahead of flood events. Our study demonstrates that GloFAS ability to predict monsoon floods in terms of timing and duration can improve national flood forecasting capabilities providing sufficient lead-time for early actions in Bangladesh. The study will help forecasters as well as users to understand forecast skill and associated uncertainty in probabilistic forecasts to predict flood events in Bangladesh.</p><p> </p><p> </p><p> </p>

Increased Socio-economic Vulnerability in the Floodplains of Brahmaputra Basin, India

<p>Floods are a recurrent natural phenomenon in the Brahmaputra basin, India’s one of the major river basin. The government employed flood mitigation strategies that have mainly been focused on structural measures that involve the construction of dams, embankments, dykes, and anti-erosion structures. Consequently, the economic investments in Flood Protection Structures (FPS) have significantly increased over time. However, despite significant FPS investments, the socio-economic losses due to floods are reportedly intensifying. In this study on coupled human flood systems, our aim is to resolve this paradox through a stepwise methodology. Firstly, land use land cover (LULC) changes are investigated before and after the completion of 74 FPS units, constructed over the past two decades. The proximity to settlement used as a proxy measure of socio-economic characteristics shows a consistent land-use growth pattern. Secondly, a socio-vulnerability index (SoVI) is developed based on the population density, distance from the river, vegetation, soil type, and LULC changes. Furthermore, SoVI maps are developed by integrating the weighted maps of the above socio-economic factors. A significant increase in the socio-economic vulnerability is observed in the floodplains of Brahmaputra, possibly due to the provision of a false sense of security. This study, hence, establishes a causal relationship between the increase in social vulnerability index and the increased FPS investments. These results are expected to help the stakeholders to identify the critical characteristics that escalate the social vulnerability in the flood plains and thereby to strategize the investments in FPS.</p>

Estimation of Source Parameters and their scaling relationship of small to moderate magnitude earthquakes for northeast India

<p>We investigate the source parameters of 87 local earthquakes (3.5 ≤ M<sub>L</sub> ≤ 5.0) that occurred in West Brahmaputra basin and its neighbouring area, using body wave displacement spectra. Seismic moment, corner frequency, source dimension and static stress drop are estimated using a grid search method based on the model of circular source. The measured seismic moments, corner frequency and moment magnitude ranges from   to  N-m, 0.7 to 12.1 and 3.0 to 4.8, respectively. The average ratio of corner frequency of P - and S - waves is 2.21.<strong> </strong>The scaling relationship of seismic moment against corner frequency is also studied for various tectonics regimes separately. Median stress drop values of individual earthquake vary from ~ 0.1 to 38.5 MPa, with an average value of about ~ 6 MPa. Spatial variation of stress drop observed for different tectonic unit reveals a higher stress drop values associated with West Brahmaputra basin, Shillong-Mikir plateau and Indo-Myanmar subduction zone suggesting a higher stress accumulation that may increase the probability of higher magnitude earthquake. The empirical relationship between M<sub>L</sub> and M<sub>W</sub> scale is also derived for hazard assessment.</p>

Aborichthys uniobarensis, a new species of river loach (Cypriniformes: Nemacheilidae) from Arunachal Pradesh, India

A new species of nemacheilid loach, Aborichthys uniobarensis, is described from the Senkhi stream, upper Brahmaputra basin in Arunachal Pradesh, northeastern India. Aborichthys uniobarensis is distinguished from all congeners by the presence of 6–14 fused oblique bars along the dorso-lateral margin of the body, 21–28 oblique bars along the flank, vent closer to the snout tip than to the caudal fin base and caudal fin oval shaped with upper half more extended than lower.

The composition and heterogeneity of the rich and diverse rotifers (Rotifera: Eurotatoria) of the floodplain lakes (beels) of the Majuli River Island, Assam State (Northeast India)

The tropical and subtropical floodplain lakes are hypothesized as one of the globally interesting rotifer rich ecosystems. Our study indicating the biodiverse Rotifera of ten ‘slightly acidic to circum-neutral, moderately hard-water and de-mineralized’ tropical floodplain lakes (beels) of the Majuli River Island of the Brahmaputra river basin of northeast India mendorses this hypothesis. Total richness, the richness of important families and community similarities affirm heterogeneity mof the rotifer species composition amongst the beels. The species richness registers significant spatio-temporal variations with seasonal differences amongst the beels and individually in each beel, and lacks significant influence of the recorded abiotic factors. The constellations of 76-81 species in three beels during winter and 76 species in one beel during pre-monsoon are noteworthy instances of ‘Rotifera paradox’. The rotifer fauna of the Majuli beels registers affinity with Southeast Asian and Oriental faunas, records several species of the regional distribution interest and exhibits the littoral-periphytic nature, while application of Q B/T and Q L/B quotients depicts limitations. This study merits ecological diversity interest for Rotifera vis-avis the floodplain lakes of India and elsewhere from the tropics and subtropics, and assumes biodiversity conservation importance due to threat of extinction of the Majuli – an alluvial floodplain of the Brahmaputra basin.