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Article

Ecosystem Benefits of Large Dead Wood in Freshwater Environments  

Ellen Wohl

Large wood in freshwater environments is downed, dead wood pieces in river channels, floodplains, wetlands, and lakes. Large wood was historically much more abundant in freshwaters, but decades to centuries of deforestation and direct wood removal have decreased wood loads—volumes of large wood per unit area—in freshwaters around the world. The widespread public perception that large wood is undesirable in freshwater environments contrasts with scientific understanding of the beneficial effects of large wood. Large wood tends to increase the spatial heterogeneity of hydraulics, substrate, channel planform, and the floodplain and hyporheic zone in rivers. This equates to greater habitat diversity and refugia for organisms, as well as energy dissipation and storage of materials during floods, which can increase the resilience of the river to disturbances such as wildfire, drought, and flooding. Similarly, wood in lakes increases lakeshore and lakebed heterogeneity of hydraulics, substrate, habitat, nutrient uptake, and storage of particulate organic matter and sediment. Large wood in rivers and lakes provides an array of vital ecosystem functions, and both individual species and biotic communities are adversely affected by a lack of wood in rivers and lakes that have been managed in a way that reduces wood loads. River and lake management are now more likely to include protection of existing large wood and active reintroduction of large wood, but numerous questions remain regarding appropriate targets for wood loads in different environmental settings, including potential threshold wood loads necessary to create desired effects. Large wood can also directly and indirectly enhance carbon storage in freshwater environments, but this storage remains poorly quantified.

Article

Water Risks and Rural Development in Coastal Bangladesh  

Sonia Hoque and Mohammad Shamsudduha

Rural populations in river deltas experience multiple water risks, emerging from intersecting anthropogenic and hydroclimatic drivers of change. For more than 20 million inhabitants of coastal Bangladesh—living on the lower reaches of the Ganges–Brahmaputra–Meghna mega-delta—these water risks relate to access to safe drinking water, management of water resources for farm-based livelihoods, and protection from water-related hazards. To address these risks, water policies in the 20th century emphasized infrastructure development, ranging from embankments for flood protection to handpumps for rural water supply. However, interventions designed to promote aggregate economic growth often resulted in sociospatial inequalities in risk distribution, particularly when policy-makers and practitioners failed to recognize the complex dynamics of human–environment interactions in the world’s most hydromorphologically active delta. In Bangladesh’s southwestern region, construction of the polder system (embanked islands interlaced with tidal rivers) since the late 1960s has augmented agricultural production by protecting low-lying land from diurnal tidal action and frequent storm surges. However, anthropogenic modification of the natural hydrology, emulating the Dutch dyke system, has altered the sedimentation patterns and resulted in severe waterlogging since the 1980s. Contrary to their intended purpose of keeping saline water out, the polders also facilitated growth of export-oriented brackish water shrimp aquaculture, resulting in widespread environmental degradation and social inequalities from shifting power dynamics between large and small landholding farmers. Throughout the 1990s, there were several incidences of violent conflicts between the local communities and government authorities, as well as between different farmer groups. Waterlogged communities demanded to revert to indigenous practices of controlled flooding. Despite being formally adopted as a policy response, the implementation of tidal river management by the government has only been partially successful owing to bureaucratic delays, unfair compensation, and design flaws. Similarly, antishrimp movements gained momentum in several polders to ban the deliberate flooding of cropland with saline water. These narratives of conflict and cooperation demonstrate the complexities of policy outcomes, the unequal distribution of water risks, and the need to integrate local knowledge in decision-making. Social and spatial inequalities are also prevalent in access to safe drinking water owing to heterogeneity in groundwater salinity and infrastructure investments. Public investments are skewed toward low-salinity areas where tubewells are feasible, while high-salinity areas are often served by uncoordinated donor investments in alternative technologies, such as small piped schemes, reverse osmosis plants, and pond sand filters, and household self-supply through shallow tubewells and rainwater harvesting. These struggles to meet daily water needs from multiple sources pose uncertain and unequal water quality and affordability risks to coastal populations. The path-dependent sequences of infrastructure and institutional interventions that shaped the development trajectory of coastal Bangladesh exemplify the complexities of managing water risks and varied responses by public and private actors. While structural solutions still dominate the global water policy discourse, there is increased recognition of the nonlinearity of risks and responses, as well as the need to incorporate adaptive decision-making processes with room for social learning and uncertainties.

Article

Machine Learning Tools for Water Resources Modeling and Management  

Giorgio Guariso and Matteo Sangiorgio

The pervasive diffusion of information and communication technologies that has characterized the end of the 20th and the beginning of the 21st centuries has profoundly impacted the way water management issues are studied. The possibility of collecting and storing large data sets has allowed the development of new classes of models that try to infer the relationships between the variables of interest directly from data rather than fit the classical physical and chemical laws to them. This approach, known as “data-driven,” belongs to the broader area of machine learning (ML) methods and can be applied to many water management problems. In hydrological modeling, ML tools can process diverse data sets, including satellite imagery, meteorological data, and historical records, to enhance predictions of streamflow, groundwater levels, and water availability and thus support water allocation, infrastructure planning, and operational decision-making. In water demand management, ML models can analyze historical water consumption patterns, weather data, and socioeconomic factors to predict future water demands. These models can support water utilities and policymakers in optimizing water allocation, planning infrastructure, and implementing effective conservation strategies. In reservoir management, advanced ML tools may be used to determine the operating rule of water structures by directly searching for the management policy or by mimicking a set of decisions with some desired properties. They may also be used to develop surrogate models that can be rapidly executed to determine the optimal course of action as a component of a decision-support system. ML methods have revolutionized water management studies by showing the power of data-driven insights. Thanks to their ability to make accurate forecasts, enhanced monitoring, and optimized resource allocation, adopting these tools is predicted to expand and consistently modify water management practices. Continued advancements in ML tools, data availability, and interdisciplinary collaborations will further propel the use of ML methods to address global water challenges and pave the way for a more resilient and sustainable water future.

Article

Infiltration of Water Into Soil  

John Nimmo and Rose Shillito

The infiltration of water into soil has profound importance as a central component of the hydrologic cycle and as the means of replenishing soil water that sustains terrestrial life. Systematic quantitative study of infiltration began in the 19th century and has continued through to the present as a central topic of soils, soil physics, and hydrology. Two forces drive infiltration: gravity, and capillarity, which results from the interaction of air-water surface tension with the solid components of soil. There are also two primary ways water moves into and within the soil. One is diffuse flow, through the pores between individual soil grains, moving from one to the next and so on. The other is preferential flow, through elongated channels such as those left by worms and roots. Diffuse flow is slow and continues as long as there is a net driving force. Preferential flow is fast and occurs only when water is supplied at high intensity, as during irrigation, major rainstorms, or floods. Both types are important in infiltration. Especially considering that preferential flow does not yet have a fully accepted theory, this means that infiltration entails multiple processes, some of them poorly understood. The soil at a given location has a limit to how much water it can absorb—the infiltration capacity. The interplay between the mode and rate of water supply, infiltration capacity, and characteristics of the soil and surrounding terrain determines infiltration into the soil. Much effort has gone into developing means of measuring and predicting both infiltration capacity and the actual infiltration rate. Various methods are available, and research is needed to improve their accuracy and ease of use.

Article

Groundwater Models  

Timothy M. Weigand, Matthew W. Farthing, and Casey T. Miller

Groundwater modeling is widely relied upon by environmental scientists and engineers to advanced understanding, make predictions, and design solutions to water resource problems of importance to society. Groundwater models are tools used to approximate subsurface behavior, including the movement of water, the chemical composition of the phases present, and the temperature distribution. As a model is a simplification of a real-world system, approximations and uncertainties are inherent to the modeling process. Due to this, special consideration must be given to the role of uncertainty quantification, as essentially all groundwater systems are stochastic in nature.

Article

Wastewater Reclamation and Recycling  

Soyoon Kum and Lewis S. Rowles

Across the globe, freshwater scarcity is increasing due to overuse, climate change, and population growth. Increasing water security requires sufficient water from diverse water resources. Wastewater can be used as a valuable water resource to improve water security because it is ever-present and usually available throughout the year. However, wastewater is a convoluted solution because the sources of wastewater can vary greatly (e.g., domestic sewage, agricultural runoff, waste from livestock activity, and industrial effluent). Different sources of wastewater can have vastly different pollutants, and mainly times, it is a complex mixture. Therefore, wastewater treatment, unlike drinking water treatment, requires a different treatment strategy. Various wastewater sources can be reused through wastewater reclamation and recycling, and the required water quality varies depending on the targeted purpose (e.g., groundwater recharge, potable water usage, irrigation). One potential solution is employing tailored treatment schemes to fit the purpose. Assorted physical, chemical, and biological treatment technologies have been established or developed for wastewater reclamation and recycle. The advancement of wastewater reclamation technologies has focused on the reduction of energy consumption and the targeted removal of emerging contaminants. Beyond technological challenges, context can be important to consider for reuse due to public perception and local water rights. Since the early 1990s, several global wastewater reclamation examples have overcome challenges and proved the applicability of wastewater reclamation systems. These examples showed that wastewater reclamation can be a promising solution to alleviate water shortages. As water scarcity becomes more widespread, strong global initiatives are needed to make substantial progress for water reclamation and reuse.

Article

Review of the State of the Art in Analysis of the Economics of Water Resources Infrastructure  

Marc Jeuland

Water resources represent an essential input to most human activities, but harnessing them requires significant infrastructure. Such water control allows populations to cope with stochastic water availability, preserving uses during droughts while protecting against the ravages of floods. Economic analysis is particularly valuable for helping to guide infrastructure investment choices, and for comparing the relative value of so called hard and soft (noninfrastructure) approaches to water management. The historical evolution of the tools for conducting such economic analysis is considered. Given the multimillennial history of human reliance on water infrastructure, it may be surprising that economic assessments of its value are a relatively recent development. Owing to the need to justify the rapid deployment of major public-sector financing outlays for water infrastructure in the early 20th century, government agencies in the United States—the Army Corps of Engineers and the Bureau of Reclamation—were early pioneers in developing these applications. Their work faced numerous technical challenges, first addressed in the drafting of the cost-benefit norms of the “Green Book.” Subsequent methodological innovation then worked to address a suite of challenges related to nonmarket uses of water, stochastic hydrology, water systems interdependencies, the social opportunity cost of capital, and impacts on secondary markets, as well as endogenous sociocultural feedbacks. The improved methods that have emerged have now been applied extensively around the world, with applications increasingly focused on the Global South where the best infrastructure development opportunities remain today. The dominant tools for carrying out such economic analyses are simulation or optimization hydroeconomic models (HEM), but there are also other options: economy wide water-economy models (WEMs), sociohydrological models (SHMs), spreadsheet-based partial equilibrium cost-benefit models, and others. Each of these has different strengths and weaknesses. Notable innovations are also discussed. For HEMs, these include stochastic, fuzz, and robust optimization, respectively, as well as co-integration with models of other sectors (e.g., energy systems models). Recent cutting-edge work with WEMs and spreadsheet-based CBA models, meanwhile, has focused on linking these tools with spatially resolved HEMs. SHMs have only seen limited application to infrastructure valuation problems but have been useful for illuminating the paradox of flood management infrastructure increasing the incidence and severity of flood damages, and for explaining the co-evolution of water-based development and environmental concerns, which ironically then devalues the original infrastructure. Other notable innovations are apparent in multicriteria decision analysis, and in game-theoretic modeling of noncooperative water institutions. These advances notwithstanding, several issues continue to challenge accurate and helpful economic appraisal of water infrastructure and should be the subject of future investigations in this domain. These include better assessment of environmental and distributional impacts, incorporation of empirically based representations of costs and benefits, and greater attention to the opportunity costs of infrastructure. Existing tools are well evolved from those of a few decades ago, supported by enhancements in scientific understanding and computational power. Yet, they do appear to systematically produce inflated estimations of the net benefits of water infrastructure. Tackling existing shortcomings will require continued interdisciplinary collaboration between economists and scholars from other disciplines, to allow leveraging of new theoretical insights, empirical data analyses, and modeling innovations.

Article

Biochar: An Emerging Carbon Abatement and Soil Management Strategy  

Holly Morgan, Saran Sohi, and Simon Shackley

Biochar is a charcoal that is used to improve land rather than as a fuel. Biochar is produced from biomass, usually through the process of pyrolysis. Due to the molecular structure and strength of the chemical bonds, the carbon in biochar is in a stable form and not readily mineralized to CO2 (as is the fate of most of the carbon in biomass). Because the carbon in biochar derives (via photosynthesis) from atmospheric CO2, biochar has the potential to be a net negative carbon technology/carbon dioxide removal option. Biochar is not a single homogeneous material. Its composition and properties (including longevity) differ according to feedstock (source biomass), pyrolysis (production) conditions, and its intended application. This variety and heterogeneity have so far eluded an agreed methodology for calculating biochar’s carbon abatement. Meta-analyses increasingly summarize the effects of biochar in pot and field trials. These results illuminate that biochar may have important agronomic benefits in poorer acidic tropical and subtropical soils, with one study indicating an average 25% yield increase across all trials. In temperate soils the impact is modest to trivial and the same study found no significant impact on crop yield arising from biochar amendment. There is much complexity in matching biochar to suitable soil-crop applications and this challenge has defied development of simple heuristics to enable implementation. Biochar has great potential as a carbon management technology and as a soil amendment. The lack of technically rigorous methodologies for measuring recalcitrant carbon limits development of the technology according to this specific purpose.

Article

Integrated Water Resource Management as an Organizing Concept  

Mohamed Ait-Kadi and Melvyn Kay

This is an immersive journey through different water management concepts. The conceptual attractiveness of concepts is not enough; they must be applicable in the real and fast-changing world. Thus, beyond the concepts, our long-standing challenge remains increasing water security. This is about stewardship of water resources for the greatest good of societies and the environment. It is a public responsibility requiring dynamic, adaptable, participatory, and balanced planning. It is all about coordination and sharing. Multi-sectoral approaches are needed to adequately address the threats and opportunities relating to water resources management in the context of climate change, rapid urbanization, and growing disparities. The processes involved are many and need consistency and long-term commitment to succeed. Climate change is closely related to the problems of water security, food security, energy security and environment sustainability. These interconnections are often ignored when policy-makers devise partial responses to individual problems. They call for broader public policy planning tools with the capacity to encourage legitimate public/collective clarification of the trade-offs and the assessment of the potential of multiple uses of water to facilitate development and growth. We need to avoid mental silos and to overcome the current piecemeal approach to solving the water problems. This requires a major shift in practice for organizations (governmental as well as donor organizations) accustomed to segregating water problems by subsectors. Our experience with integration tells us that (1) we need to invest in understanding the political economy of different sectors; (2) we need new institutional arrangements that function within increasing complexity, cutting across sectoral silos and sovereign boundaries; (3) top down approaches for resources management will not succeed without bottom-up efforts to help people improve their livelihoods and their capacity to adapt to increasing resource scarcity as well as to reduce unsustainable modes of production. Political will, as well as political skill, need visionary and strong leadership to bring opposing interests into balance to inform policy- making with scientific understanding, and to negotiate decisions that are socially accepted. Managing water effectively across a vast set of concerns requires equally vast coordination. Strong partnerships and knowledge creation and sharing are essential. Human civilization – we know- is a response to challenge. Certainly, water scarcity can be a source of conflict among competing users, particularly when combined with other factors of political or cultural tension. But it can also be an inducement to cooperation even in high tension areas. We believe that human civilization can find itself the resources to respond successfully to the many water challenges, and in the process make water a learning ground for building the expanded sense of community and sharing necessary to an increasingly interconnected world.

Article

Changes in Land Use Influenced by Anthropogenic Activity  

Lang Wang and Zong-Liang Yang

The terms “land cover” and “land use” are often used interchangeably, although they have different meanings. Land cover is the biophysical material at the surface of the Earth, whereas land use refers to how people use the land surface. Land use concerns the resources of the land, their products, and benefits, in addition to land management actions and activities. The history of changes in land use has passed through several major stages driven by developments in science and technology and demands for food, fiber, energy, and shelter. Modern changes in land use have been increasingly affected by anthropogenic activities at a scale and magnitude that have not been seen. These changes in land use are largely driven by population growth, urban expansion, increasing demands for energy and food, changes in diets and lifestyles, and changing socioeconomic conditions. About 70% of the Earth’s ice-free land surface has been altered by changes in land use, and these changes have had environmental impacts worldwide, ranging from effects on the composition of the Earth’s atmosphere and climate to the extensive modification of terrestrial ecosystems, habitats, and biodiversity. A number of different methods have been developed give a thorough understanding of these changes in land use and the multiple effects and feedbacks involved. Earth system observations and models are examples of two crucial technologies, although there are considerable uncertainties in both techniques. Cross-disciplinary collaborations are highly desirable in future studies of land use and management. The goals of mitigating climate change and maintaining sustainability should always be considered before implementing any new land management strategies.