1,721,097 research outputs found
Effects of earthquake and cyclone sequencing on landsliding and fluvial sediment transfer in a mountain catchment
No full textStatistical attribution of the influence of urban and tree cover change on streamflow: a comparison of large sample statistical approaches
Full text linkThe strengths and weaknesses of different statistical methodologies for attributing changes in streamflow to land cover are still poorly understood. We examine the relationships between high (Q99), mean (Qmean), and low (Q01) streamflow and urbanization or tree cover change in 729 catchments in the United States between 1992 and 2018. We apply two statistical modeling approaches and compare their performance. Panel regression models estimate the average effect of land cover changes on streamflow across all sites, and show that on average, a 1%-point increase in catchment urban area results in a small (0.6%–0.7%), but highly significant increase in mean and high flows. Meanwhile, a 1%-point increase in tree cover does not correspond to strongly significant changes in flow. We also fit a generalized linear model to each individual site, which results in highly varied model coefficients. The medians of the single-site coefficients show no significant relationships between either urbanization or tree cover change and any streamflow quantile (although at individual sites, the coefficients may be statistically significant and positive or negative). On the other hand, the GLM coefficients may provide greater nuance in catchments with specific attributes. This variation is not well represented through the panel model estimates of average effect, unless moderators are carefully considered. We highlight the value of statistical approaches for large-sample attribution of hydrological change, while cautioning that considerable variability exists
Surface water sensitivity to climate variability
Full text linkGlobal water security is known to depend on, among other things, the ability of societies to cope with hydrological risks. While there are several drivers that determine the severity of these risks, climatological mechanisms play an important role in describing their spatial and temporal characteristics. These mechanisms are often described as intra-annual and inter-annual sources of climate variability. Furthermore, anthropogenic climate change is understood to importantly perturb these mechanisms and in turn magnify hydrological risks. As such, understanding the way in which these mechanisms of climate variability influence hydrological processes has become a present and pressing scientific challenge. In particular, while existing methods look to explain the role of climate variability in hydrometeorological variables, namely precipitation and temperature, more research is required to explain how these mechanisms manifest in large-scale land surface hydrological processes and extremes. The objective of this thesis is to increase our understanding of the way that climate sources of variability influence the spatial and temporal heterogeneity of hydrological flows. This objective is addressed in a systematic way, by first exploring how hydrological flow characteristics are influenced by land surface hydrological processes in areas, with traditional rudimentary runoff representations. Building on this, this thesis secondly analyses the direct link between natural sources of climate variability and land surface hydrological processes and risks at the global scale. Lastly, the repercussions of anthropogenic climate change, in the context of current global climate agreements, in influencing hydrological extremes are explored. By examining the impacts of such extremes on global hydropower availability, a part of the ultimate consequences of hydrological risks on human systems are subsequently explored. In order to address these aims this thesis proposes a systemic framework that connects climate sources of variability and heterogeneity of flows, by combining various physical sub-models of a Land Surface Model (LSM) and other complementary tools. As such, this framework looks to link climate sources of variability, atmospheric responses, surface hydrological variables, hydrodynamics, hydrological extremes, and societal repercussions. To demonstrate the value of the framework, this thesis presents four case studies in which specific components and sub-models of this framework are utilized to address the mentioned objectives. The framework proposed here has helped to unveil and quantify new drivers that control river flows and hydrological risks. This includes explaining the snowpack characteristics that determine timing and magnitude of river flow peaks in snow-dominated regions. Also, by quantifying the inter-annual variability driven by Atmospheric Rivers, this thesis found that this form of moisture transport contributes to 22% of total global annual runoff and their variability importantly drives hydrological extremes in various global locations. Furthermore, by applying this framework, this thesis found that committing to a 1.5oC level of warming, instead of 2.0°C, as agreed in Paris in 2015, may importantly decrease high flow occurrences in regions such central Asia or western Europe. Similarly, this thesis found that the intensification of future low flow events, resulted from future climate targets, may lead to important global water losses which in turn would make almost a quarter of current global GHP vulnerable, importantly affecting the energy share in various Asian and Sub-Saharan countries
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Water Security, Droughts and the Quantification of their Risks to Agriculture: A Global Picture in Light of Climatic Change
Full text linkAs a consequence of climatic change, climate variability is expected to increase and climate extremes to become more frequent. Rising water and food demand are further exacerbating the risks to global water and food security. The variability but also the spatial inter-connectedness in our globalized world make our systems more vulnerable to shocks and disasters. To sustain the global water and food security, more knowledge about risks, especially risks of simultaneous shocks is needed. This thesis maps and quantifies risks to global water and food security from a water-food-climate perspective. It starts on a global scale looking at water security in major river basins and then concentrates on major food producing regions of three important crops. The thesis explores how storage can buffer inter- and intra-regional hydrological variability. A water balance model is developed and used to find hotspots of water shortages and to identify river basins where more investment in infrastructure is needed to improve and sustain water security. Looking at food security, global wheat, maize and soybean breadbaskets are identified and used to estimate risks of simultaneous production shocks. Focusing on wheat, I apply different copula approaches to model joint risks of low yields. It is shown quantitatively that (i) it is important to include spatial dependencies in risks studies and that (ii) inter-regional risk pooling could decrease post-disaster liabilities of governments and international organizations. The last part of the thesis focuses on climate impacts on food production. Relevant climate variables for crop growth in the breadbaskets are identified and joint climate risks are estimated using regular vine copulas. It is shown that so far, only wheat has experienced an increase in simultaneous climate risks. In maize and soybean production regions, positive and negative climate risk changes are offsetting each other on a global scale. Looking at future projections, however, it is shown that under a 1.5 and 2 °C global mean warming, simultaneous climate risks increase for all three crops, especially for maize where the return periods of all five breadbaskets experiencing climate risks decrease from 16 to every second year. The findings of this thesis can inform policy makers, businesses and international organizations about risks to global water and food security resulting from climate variability and extremes. It indicates where policies and infrastructure investments are needed to maintain water security, it can assist in building inter-governmental risk pooling schemes and contribute to current climate policy discussions.</p
Negotiated risk management of transboundary rivers
Full text linkReaching agreements over water management on transboundary rivers is a complex yet necessary endeavour to assure that humans can live within the limits of available resources. The myriad of challenges is both physical and social in nature; the uncertainty of water availability due to natural hydrologic variability is often increased by the involvement of multiple management institutions. Jurisdictions of control are typically defined by political borders, and thus they represent distinct geographic domains and interests. Increasing scarcity, driven by rapidly expanding populations and our growing awareness of climatic non-stationary, increases the urgency to find agreements among these institutions. Although the need is significant and growing, a lack of available approaches exist that considers the physical, technical and political dynamics to address these complex challenges. This thesis describes novel analytical methods to engage in the complex political realm of transboundary river management. Building from an engineering systems analysis approach to engage this topic, the main hypotheses of this thesis are: (1) Existing analytical approaches for water resource development are useful but often constrained in a transboundary negotiation context, and (2) cooperation among co-riparian water management institutions can be significantly increased with strategic implementation of analytical tools to jointly manage current and future risks. To test this hypothesis, this thesis presents an analytical approach that (1) examines previous applications of water resource models to identify their perceived contribution to managing transboundary rivers, (2) develops a new modelling framework that engages with transboundary negotiations, and (3) incorporates methods for risk-based decision making to evaluate the benefits, opportunities and trade-offs of cooperation among co-riparian states. A retrospective analysis is conducted on the Colorado and Murray-Darling River Basins to understand lessons learned from recent applications of analytical modelling tools. New methods are then developed and applied to the rapidly changing Eastern Nile River Basin. The ongoing construction of the Grand Ethiopian Renaissance Dam (GERD) and the implications on downstream countries of Sudan and Egypt provides the context and a relevant case for testing the methods and evaluating the hypotheses. Results from this thesis demonstrate the distinct advantages of an early development of system-wide analytical tools within a transboundary context, which is made available to all parties. Conversely, the challenges of reconciling multiple models used by different institutions after full allocation is reached in a basin is a significant barrier to cooperative management. Results also demonstrate the advantages of developing an analytical tool that is sufficiently accurate, transparent and flexible to seek creative solutions, and the need to select an appropriate breadth and depth of model design that conveys its credibility, saliency and legitimacy to support a decision-making process. The appropriate design of tools to consider multiple future hydrologic scenarios can shift a discourse from rigid water allocations to considering the effects of new developments in terms of changes to risks, and to allow stakeholders to decide whether these changes are tolerable when juxtaposed with the benefits that new infrastructure provides. Finally, the results show how risks among multiple stakeholders can be evaluated under expanding uncertainties, and cooperative solutions can be sought to minimise or balance these risks. The application of the proposed methods to the Eastern Nile Basin indicates that solutions are indeed possible that benefit all three countries. A number of cooperative solutions are identified that suggest operational rules for the new and existing infrastructure. These operations can be responsive to variable climatic conditions and thus encourage dynamic cooperation. In this light, the developments in Ethiopia need not be a risk, but can result in substantial benefits to the downstream countries if agreements can be reached. Embedding highly adaptable analytical tools within a negotiation process can help to overcome the challenges faced at this historic point on the Nile River
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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