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Dams are an integral component of water resources systems and management in the Upper Ganga basin. However, the impact of dam operations on the characteristics of hydrological extremes and their regional spatial pattern is less explored. Based on observational datasets, we studied the changes in the flood and drought characteristics at local and regional scales by comparing them during pre- and post-dam periods. It was observed that the annual frequency of streamflow is missing from the spectral representation of daily streamflow at the Tehri catchment during the post-dam period. The dam operation has reduced the flood peaks but increased their duration and volume, and the dam has influenced the variability in drought duration, intensity, and deficit. The operation of dams has changed the timing and strength of seasonality of these extremes. Ultimately, the dam has increased the risk of regional occurrences of droughts and minimized the risk of floods. [ABSTRACT FROM AUTHOR]

Recurring streamflow drought is a significant problem, and it is necessary to comprehend the spatiotemporal patterns and driving factors of historical droughts in order to manage future water supplies effectively. Drought has significant and often severe effects on semi-arid and arid zones due to the already limited availability of water in these regions. Therefore, this research examines streamflow drought fluctuations in India's second-largest basin, the Godavari. The Godavari Basin experiences diverse climatic conditions ranging from humid in the Western Ghats to semi-arid in central and western regions. Frequent hydrological droughts have plagued the Godavari basin. Still, the impact of human activities and climate change on the streamflow drought characteristics in the Godavari basin has not been thoroughly investigated. Further, assessing the regional control of streamflow drought deficit and intensity in a large river basin like Godavari is crucial. The findings show that streamflow drought is worst in central and eastern Godavari. Additionally, dammed and undammed locations have different streamflow drought characteristics. Droughts have lengthened in dam-affected areas. However, regions without dams have seen more deficit and intensity. Thus, different sustainable water management techniques are needed for the dammed and undammed regions of the Godavari basin. • Central and eastern Godavari experienced the worst streamflow droughts historically. • Dammed and undammed regions have different streamflow drought characteristics. • Drought periods have increased in the dammed regions. • Undammed regions have experienced higher intensity and deficit of streamflow droughts. [ABSTRACT FROM AUTHOR]

The Indian Summer Monsoon (ISM) characterizes the hydrometeorological variability across the north Indian region and contributes more than 75% of the annual rainfall during the monsoon (June–September) season. In the present study, we analysed the long-term monsoon rainfall for the Ganga River basin to investigate its spatio-temporal variability. A statistically increasing (10 to 17 mm/year; p < 0.05) trend has been observed in ISM rainfall for the mountainous region since 1980, accompanied by increased temperature. We further note that high, very high and extreme rainfall events are also increasing, enhancing the flash flood risk in the mountainous region. In contrast, the ISM rainfall in the alluvial region is observed to be statistically decreasing (−5 to −20 mm/year; p < 0.05) with the combined influence of reduced vegetation. These findings provide valuable insights into the variations in regional hydrology of the Ganga River basin caused by natural and anthropogenic factors. [ABSTRACT FROM AUTHOR]

• Morphometric analysis integrated with sediment connectivity and stream power index. • Variable sediment flux in upper Ganga and upper Kosi basins linked to morphometric characteristics. • Variable hillslope-channel and catchment connectivity contributes to sediment flux variations. • Slope variations manifested in stream power index also influence sediment transport. • Morphometric characterization has significant implications for sediment management. The Indo-Gangetic basin exhibits highly diverse hydro-geomorphic settings that influence the hydrology, sediment production, and transport rates of the rivers draining this region, and this, in turn, is manifested in morphometric diversity of river systems. The rivers draining the western Gangetic plains (WGP) show incised channels attributable to high stream power and low sediment yield. In contrast, the rivers draining the Eastern Gangetic Plains (EGP) have formed aggradational landforms such as megafans due to low stream power and high sediment yield. For a better process understanding of the causal factors of such spatial inhomogeneities in a river basin, a morphometric characterization of its hinterland is necessary. In this work, a set of morphometric parameters such as stream network, longitudinal profile, and hypsometric curve have been integrated with data on sediment connectivity and stream power index to establish the form-process relationships in two contrasting basins, the upper Ganga basin (UGB) and the upper Kosi basin (UKB). The results of this study suggest that high sediment flux in the UKB is attributed to high rainfall, steep topography, unstable channel longitudinal profiles, and high sediment connectivity. A large part of these sediments gets deposited within the channel belt in the alluvial reaches and is remobilized during high flow events. However, the UGB produces fewer sediments due to low rainfall, mild topography, stable channel longitudinal profile, and low sediment connectivity. Also, the presence of a major dam at Tehri further reduces the functional sediment connectivity in the Upper Ganga Basin. [ABSTRACT FROM AUTHOR]

High soil erosion is one of the severe problems in the Himalayan River basins. Significant regional variability in sediment production, transport and deposition in the Gangetic Rivers is a function of hydro-geomorphic diversity across the Ganga basin. For instance, the rivers draining the western Gangetic plains (WGP) are characterised by high stream power and low sediment yield whereas those in the eastern Gangetic plains (EGP) have low stream power and high sediment yield. Therefore, a systematic modelling framework is essential to comprehend the controls (i.e. climate, lithology, topography and human influence) on sediment production and to quantify soil erosion (SE) and sediment yield (SY) for individual river basins. In this work, we have used the Revised Universal Soil Loss Equation (RUSLE) and Sediment Delivery Ratio (SDR) equation to compute the SE and SY in a small basin (Garra River basin, WGP). We then assess the applicability of quantified results in this basin using a statistical framework for uncertainty estimation. We have then upscaled and quantified SE and SY variability for two river basins in different hydro-geomorphic settings viz. Upper Ganga basin (UGB) and the Kosi basin (KB) in the western and eastern parts of the Ganga basin respectively. This study predicts SE of 72 t/ha/year for KB and SY of 85 Mt/year and 75 Mt/year at upstream (Barakshetra) and downstream (Baltara) stations respectively with 26.8% associated uncertainty. Similarly, the UGB shows SE of 43 t/ha/year and SY of 36 Mt/year at Kanpur with 24.6% associated uncertainty. The overall estimated SY are validated using the observed data at gauge stations in both the basins. Further analysis suggests that rainfall erosivity, topographic steepness and agricultural practices have significant influence on SE and SY in both the basins. Finally, these modelled results are important to understand the linkage between river form and processes and to design sustainable river basin management strategies. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Sediment dynamics of a large tropical river in the Ganga basin. • Annual deposition of ∼20–50 MT of sediments in different reaches. • High sediment flux linked to channel instability and frequent flooding. • Human interventions have affected sediment dynamics significantly. • Sediment dynamics is the most important river management issue in the Kosi basin. Abstract Hydrological and sediment transport characteristics for the Kosi basin, which covers parts of Nepal and India, were analysed to understand the spatiotemporal variability of the hydrology and sediment dynamics of the Kosi basin and its implications for flood hazard and sediment dynamics. The study revealed that ∼56% of the discharge at Chatara (where all major tributaries of the Kosi meet) is contributed from the western part of the basin even though this constitutes only 34% of the total basin area. In contrast, the central and eastern parts of the basin constitute 57% and 8% of the basin area but contribute ∼38% and ∼16% of the discharge at Chatara, respectively. The contribution of sediment load at Chatara from the different tributaries of the Kosi River also shows a similar pattern. Of a total of ∼100 million tonnes of the annual sediment load at Chatara, ∼56% is transported from four tributaries: the Indrawati, Bhote Kosi, Tama Kosi (all draining from the west), and Tamor. The remaining ∼44% is transported by other tributaries upstream of Chatara, the most important being the Arun, Dudh Kosi, and Sun Kosi. Sediment budgeting in this study, based on annual sediment load data, suggested that ∼20 million tonnes of sediments are deposited between Chatara and Birpur annually. This study also found that ∼53 million tonnes of sediments are being accommodated between Birpur and Baltara annually. Sediment dynamics in the Kosi basin emerges as the most important river management issue, and this is closely linked to channel instability and frequent flooding in the alluvial plains. [ABSTRACT FROM AUTHOR]