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An intimate glimpse into the microcosmic world of \"hybrid landscapes\" and their inhabitants. With this book, Kuntala Lahiri-Dutt and Gopa Samanta offer an intimate glimpse into the microcosmic world of \"hybrid environments.\" Focusing on chars -- the part-land, part-water, low-lying sandy masses that exist within the riverbeds in the floodplains of lower Bengal -- the authors show how, both as real-life examples and as metaphors, chars straddle the conventional categories of land and water, and how people who live on them fluctuate between legitimacy and illegitimacy. The result, a study of human habitation in the nebulous space between land and water, charts a new way of thinking about land, people, and people's ways of life. Kuntala Lahiri-Dutt is a senior fellow in resource management in the Asia-Pacific Program at the College of Asia and the Pacific at the Australian National University. She lives in Canberra. Gopa Samanta is an associate professor in geography at the University of Burdwan. She lives in Golapbag, Burdwan, India.

River confluences with a compound‐channel tributary are common in large river networks, for example, the Yangtze River basin and the Amazon basin. They affect the confluence hydrodynamics, nutrient depletion and fish migration in the network, due to the seasonal exposure and submergence of the tributary floodplain. The corresponding influencing mechanisms are critical but poorly understood. This study investigated the hydrodynamic response of channel flow confluence to the tributary floodplain, that is, various widths and heights of the floodplain. Two kinds of secondary circulations were identified: One was due to the tributary streamline curvature, and the other produced by flow separation in the floodplain step. An increase in the floodplain size enhanced the strength of the secondary circulation in the compound tributary channel, but it disappeared near the junction because of the effects of the main channel. The decreased tributary cross‐sectional area amplified the flow momentum, resulting in a larger separation zone near the tributary‐side wall. The strength of floodplain‐induced secondary circulation increased as the floodplain width increased, and it moved toward the tributary‐side bank destroying the separation zone. Moreover, strong upwelling in the secondary circulation caused the rising water surface in the separation zone, which was supposed to be a region of falling water surface caused by energy loss and negative pressure. A conceptual model was proposed to summarize the hydrodynamics of confluence with different tributary floodplain topography. Our results provided a comprehensive knowledge of the confluence hydrodynamics, which have important implication for the flood management and ecological restoration of river basin. Key Points Floodplain caused the secondary circulation, and its intensity and scale increased as the floodplain size increased Strong secondary circulation destroyed the separation zone near the bed and resulted in local rise of the water surface A conceptual model was proposed to summarize the hydrodynamic characteristics of confluence with tributary floodplain

By trapping sediment in reservoirs, dams interrupt the continuity of sediment transport through rivers, resulting in loss of reservoir storage and reduced usable life, and depriving downstream reaches of sediments essential for channel form and aquatic habitats. With the acceleration of new dam construction globally, these impacts are increasingly widespread. There are proven techniques to pass sediment through or around reservoirs, to preserve reservoir capacity and to minimize downstream impacts, but they are not applied in many situations where they would be effective. This paper summarizes collective experience from five continents in managing reservoir sediments and mitigating downstream sediment starvation. Where geometry is favorable it is often possible to bypass sediment around the reservoir, which avoids reservoir sedimentation and supplies sediment to downstream reaches with rates and timing similar to pre‐dam conditions. Sluicing (or drawdown routing) permits sediment to be transported through the reservoir rapidly to avoid sedimentation during high flows; it requires relatively large capacity outlets. Drawdown flushing involves scouring and re‐suspending sediment deposited in the reservoir and transporting it downstream through low‐level gates in the dam; it works best in narrow reservoirs with steep longitudinal gradients and with flow velocities maintained above the threshold to transport sediment. Turbidity currents can often be vented through the dam, with the advantage that the reservoir need not be drawn down to pass sediment. In planning dams, we recommend that these sediment management approaches be utilized where possible to sustain reservoir capacity and minimize environmental impacts of dams. Key Points Reservoirs trap sediment, losing storage capacity Downstream reaches can become sediment starved Many dams can be designed/operated to pass sediment

Understanding responses of stream discharge to precipitation in a watershed is important in gaining insights into watershed hydrology and estimating hydraulic parameters. Transfer functions in the spectral domain are commonly used to quantify the relationship between precipitation and discharge, and estimate watershed hydraulic parameters. However, previous models have not adequately accounted for the impact of the unsaturated zone. To address this, we have developed a novel analytical model that considers the effect of the unsaturated zone to obtain transfer functions within watersheds. These transfer functions are derived by the spectral method and verified through numerical simulations. The results indicate that the transfer functions are influenced significantly by the relative hydraulic conductivity exponent αk in the moisture characteristic curve. A higher αk results in a lower transfer function, indicating more robust filtering of hydrological signals. A thicker unsaturated zone results in lower transfer functions at higher frequencies. The traditional transfer functions, which neglect the retention capacity of the unsaturated zone, tend to overestimate hydrological responses at high frequencies. Our transfer functions agree well with integrated watershed‐scale flow models and are also applied to observed data from four watersheds in Iowa, providing reasonable estimates for the hydraulic parameters. This study contributes to a deeper understanding of watershed behavior and offers an enhanced tool for estimating hydraulic parameters with practical applications. Key Points Transfer functions are proposed to quantify hydrological signal filtration considering unsaturated flow described by Richards equation Thicker unsaturated zones or higher conductivity exponents enhance the filtration of hydrological signals Unsaturated zone generally possesses a stronger ability to filter watershed hydrological signals than that of saturated zone

The Lancang–Mekong River basin, as an important transboundary river in Southeast Asia, is challenged by rapid socio-economic development, especially the construction of hydropower dams. Furthermore, substantial factors, such as terrain, rainfall, soil properties and agricultural activity, affect and are highly susceptible to soil erosion and sediment yield. This study aimed to estimate average annual soil erosion in terms of spatial distribution and sediment deposition by using the revised universal soil loss equation (RUSLE) and GIS techniques. This study also applied remote sensing and available data sources for soil erosion analysis. Annual soil erosion in most parts of the study area range from 700 to 10,000 t/km2/y with a mean value of 5350 t/km2/y. Approximately 45% of the total area undergoes moderate erosion. Moreover, the assessments of sediment deposition and erosion using the modified RUSLE and the GIS techniques indicate high sediment erosion along the flow direction of the mainstream, from the upper Mekong River to the Mekong Delta. The northern part of the upper Mekong River and the central and southern parts of the lower Mekong River are the most vulnerable to the increase in soil erosion rates, indicating sediment deposition.

Understanding the impact of human‐made structures on groundwater levels is essential, with structures like dams or weirs presenting unique challenges and opportunities for study. The Baekje weir in South Korea presents an interesting case as the weir has undergone full gate opening, which is generally not the case for weirs and reservoirs, providing valuable opportunity for simulating weir removal conditions. The main objectives are investigation of groundwater level fluctuations under various weir operations, distances from the weir, and seasonal variations. The study utilizes observed data that simulates conditions with and without the weir, including scenarios of full gate opening. Multiple machine learning algorithms—Random Forest (RF), Artificial Neural Network, Support Vector Regression (SVR), Gradient Boosting, and Extreme Gradient Boosting (XGBoost)—are used to develop accurate groundwater level prediction models. The models' performance is assessed using coefficient of determination, Root mean square error (RMSE), Mean Absolute Error (MAE) indices, and visualized through Taylor diagrams. Results indicate that XGBoost outperforms other models in all three groups during both training and testing phases. Specifically, XGBoost surpasses RF by 2.09% (R2), 5.66% (RMSE), and 10.1% (MAE) in training, and outperforms SVR by 11.2% (R2), 42.0% (RMSE), and 129.2% (MAE) in testing. Additionally, the study generates groundwater level maps, providing a practical tool for managing groundwater systems and informing decision‐making in weir operations. This study not only sheds light on the dynamic relationship between weir operations and groundwater levels but also provides actionable insights for effective water management in similar hydrological settings. Key Points Predict daily groundwater level changes under different weir management policies, including the condition of fully opening the weir gates Apply machine learning algorithms to build the groundwater level prediction models and produce groundwater level map as the final product Conclude that weir management policies (full and partial openings, normal), distance from the weir, and seasons impact groundwater level

Water is essential for human activities and economic development, and the water environment significantly influences ecological balance and global climate. China and Southeast Asia are the most populous areas in the world, and their water resources are deteriorating day by day. We focus on five representative cities such as, Beijing, Jakarta, Hanoi, Kathmandu and Manila to investigate water-environmental problems with the ultimate goal of providing recommendations for sustainable urban water management. The study found that (1) the water environment of all cities has been polluted to varying levels, while the pollution has improved in Beijing and Jakarta, and the situation in other regions is severe. (2) The aquatic biodiversity has reduced, and its pollution is mainly caused by organic pollutants and decreasing river flow. In addition, numerous people live in megacities without access to clean surface water or piped drinking water, which greatly increases the use of groundwater. Further, frequent floods in the world leads to serious damage to urban infrastructure and further deterioration of water environment quality. To address these problems, countries and organizations have begun to construct wastewater treatment plants and develop water-saving technology to ensure healthy and sustainable development of water environment. The results and practical recommendations of this study can provide scientific insights for future research and management strategies to address water quality challenges during ongoing policy debates and decision-making processes.

The ongoing and proposed construction of large-scale hydropower dams in the Mekong river basin is a subject of intense debate and growing international concern due to the unprecedented and potentially irreversible impacts these dams are likely to have on the hydrological, agricultural, and ecological systems across the basin. Studies have shown that some of the dams built in the tributaries and the main stem of the upper Mekong have already caused basin-wide impacts by altering the magnitude and seasonality of flows, blocking sediment transport, affecting fisheries and livelihoods of downstream inhabitants, and changing the flood pulse to the Tonle Sap Lake. There are hundreds of additional dams planned for the near future that would result in further changes, potentially causing permanent damage to the highly productive agricultural systems and fisheries, as well as the riverine and floodplain ecosystems. Several studies have examined the potential impacts of existing and planned dams but the integrated effects of the dams when combined with the adverse hydrologic consequences of climate change remain largely unknown. Here, we provide a detailed review of the existing literature on the changes in climate, land use, and dam construction and the resulting impacts on hydrological, agricultural, and ecological systems across the Mekong. The review provides a basis to better understand the effects of climate change and accelerating human water management activities on the coupled hydrological-agricultural-ecological systems, and identifies existing challenges to study the region’s Water, Energy, and Food (WEF) nexus with emphasis on the influence of future dams and projected climate change. In the last section, we synthesize the results and highlight the urgent need to develop integrated models to holistically study the coupled natural-human systems across the basin that account for the impacts of climate change and water infrastructure development. This review provides a framework for future research in the Mekong, including studies that integrate hydrological, agricultural, and ecological modeling systems.

Hydrologic projections of current regional climate models are uncertain over mountainous regions because of the poor representation of complex terrains. Here, we use multiple AI models to generate observation-constrained projections of the occurrence probability of record events (REs) of annual maximum and minimum daily streamflow (Qmax and Qmin) in Pakistan. First, we trained and tested the AI models, using the occurrence probability of REs from simulated (input) data from five regional climate models and observed (output) river discharge data from 1962–2022. Then, we use the trained AI models to project the observation-constrained occurrence probability of REs until 2099 and conduct principal component analysis for the robust selection of AI model ensembles. The observation-constrained projections detect more REs in Qmax and Qmin in the late 21st century than expected under the stationary system does, highlighting intensifying hydroclimatic extremes. The upper Indus River shows that the return period of REs in Qmax and Qmin is approximately 15 years. The Chenab and Kabul Rivers (Jehlum River) are prone to more REs in Qmax (Qmin), with a return period of approximately 11 years. Uncertainties in the RE occurrence probability projections from regional climate models can be attributed to the heterogeneity in hydroclimatic and physiographic characteristics across the study basins. This study suggests the nonstationarity of four major river basins in Pakistan, where water resources management should be updated with basin-specific strategic plans.

The Lancang-Mekong River, the largest river in Southeast Asia, supports high biodiversity of fish but has also suffered from intensive human- and nature-induced stressors for decades, and it is feared that this trend will continue in coming decades. Although the river has received wide attention from researchers, spatiotemporal trends in fish diversity and production and of environmental stressors within the catchment remain unclear. Also, an overview of current knowledge for catchment-scale sustainable management is lacking. This study analysed research publications related to Mekong fishes and threats, and identified knowledge gaps and limitations in current research. We found an increasing trend in studies on fish and/or environmental stressors, but that some research themes, including dam construction, pollution, infectious disease, and salinization, vary across regions. In general, research foci were determined by geographical location, fish diversity, and environmental stressors in each region. For fish-related studies, research in the Upper and Middle Lancang-Mekong regions was mostly related to the conservation of wild fish populations and communities, while in the Mekong Delta, a large proportion of studies were related to fish farming. For studies related to environmental stressors, current research foci were mainly on persistent threats, including flow modification, pollution, habitat degradation, and overexploitation. However, coverage of some newly emerging threats, such as microplastic pollution and harmful algal blooms were lacking. This study helps provide a better understanding of factors impacting the Lancang-Mekong River and its fish communities, and provides a basis for future research, management, and conservation actions.