1,721,085 research outputs found
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
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Flooding duration and volume more important than peak discharge in explaining 18 years of gravel-cobble river change
Get PDFFloods play a critical role in geomorphic change, but whether peak magnitude, duration, volume, or frequency determines the resulting magnitude of erosion and deposition is a question often proposed in geomorphic effectiveness studies. This study investigated that question using digital elevation model differencing to compare and contrast three hydrologically distinct epochs of topographic change spanning 18 years in the 37-km gravel-cobble lower Yuba River (LYR) in northern California. Scour and fill were analyzed by volume at segment and geomorphic reach scales. Each epoch’s hydrology was characterized using 15-minute and daily averaged flow to obtain distinct peak and recurrence, duration, and volume metrics. Epochs 1 (1999-2008) and 3 (2014-2017) were wet with large floods reaching 3,206 and 2,466 m3/s, respectively, though of different flood durations. Epoch 2 (2008-2014) was a drought period with only four brief moderate floods (peak of 1,245 m3/s). Total volumetric changes showed that major geomorphic response occurred primarily during large flood events; however, total scour and net export of sediment varied greatly, with 20 times more export in epoch 3 compared to epoch 1. The key finding was that greater peak discharge was not correlated with greater net and total erosion; differences were better explained by duration and volume above floodway-filling stage. This finding highlights the importance of considering flood duration and volume, along with peak, to assess flood magnitude in the context of flood management, frequency analysis, and resulting geomorphic changes
Variations on the Author
Get PDF“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
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Predictability of cottonwood recruitment along a dynamic, regulated river
Get PDFRiparian vegetation planting and management are vital to river engineering projects. To inform these activities, a better understanding of what influences riparian vegetation recruitment and identifying where vegetation will most likely establish and survive is needed. This study investigated whether the recruitment of Populus fremontii (Fremont cottonwoods), a dominant riparian species in the western USA, could be predicted at 0.91-m2 resolution deterministically and statistically throughout a dynamic, alluvial regulated river. The testbed was a ~ 34-km stretch of the Yuba River in California, USA, mapped in 2017 after a large flood reset the terrain. Five years later from August through November 2022, a field campaign characterized juvenile cottonwoods recruitment. For the deterministic test, a riparian seedling recruitment model was used with expert-estimated parameters. Bioverification analysis, or comparison of model predictions to observed organism locations, found that the model did not accurately differentiate the predicted optimal locations from the lethal locations. Hydrophysical and topographic variables were then used as predictor rasters in a data-driven, supervised classification Random Forest (RF) model with cottonwood presence and absence data to statistically test whether recruitment locations could be predicted. The RF model performed well, having an overall accuracy of 87% and reached an AUC-ROC value of 94% with only a few predictors. When the statistical model was coupled with biophysical interpretations of model behavior, topographic variables were much more significant drivers for recruitment locations than hydrophysical variables. Further mechanistic developments to understand underlying governing equations and parameters are feasible drawing on the lessons from bioverification and the RF model. Both deterministic and statistical methods are recommended to advise stakeholders about suitable locations for riparian vegetation restoration or topographic characteristics needed to promote restoration efforts, as each yield unique insights
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe 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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Geomorphic analysis of 35 ephemeral river reaches with variable channel morphologies in the South Coast, California, USA.
Get PDFDuring the last decade, 1-m resolution topo-bathymetric digital elevation models (DEMs) have become increasingly utilized within fluvial geomorphology, but most meter-scale geomorphic analyses have only been conducted on one to a small handful of river reaches. While such analyses have contributed greatly to our collective understanding of river discharge-topography interactions, which is applicable in both river restoration design and environmental flow regulation contexts, their generalizability across a range of river types remained largely unevaluated. This study assessed the dominance of a single hydro-morphodynamic mechanism, flow convergence routing, in 35 ephemeral rivers divided among 5 channel types in California's South Coast region. To do so, we conducted geomorphic covariance structure (GCS) analysis on longitudinal standardized width (Ws) and standardized, detrended bed elevation (Zs) spatial series from 1-m resolution DEMs to explore the stage-dependent nature of fluvial landforms. We compared and contrasted results among different river types and found that in all river types there are coherent, multi-scalar structures of longitudinal fluvial topography. We find that width undulation driven flow convergence routing is a broadly relevant channel altering mechanism, however, it’s relationship with water stage height differs between river types. For example, in partly confined riffle-pool rivers, resilient width undulations at a bankfull discharge appear to control the degree to which riffle-pool morphology is self-maintaining. But in mountainous confined river reaches, width’s relationship with inferred sedimentary dynamics increases somewhat linearly with water stage height, potentially as a function of the sediment size distribution
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Investigating Environmental Drivers of Migratory Habitat Selection in Adult Chinook Salmon with Implications for Homing and Straying Dynamics
Get PDFAnadromous salmonids utilize a combination of physical, chemical, and biological cues when migrating from ocean foraging grounds as mature adults to freshwater spawning grounds to complete their lifecycle. The biological processes behind homing behavior (migrating to their stream of natal origin to spawn) and straying behavior (migrating to non-natal habitats) have been well studied since the 1950’s. However, the abiotic environmental factors that partially drive these behaviors have received comparatively less attention. One aspect that is particularly understudied is the influence of localized channel hydraulics on habitat selection in migrating adults. The first chapter of this dissertation includes an investigation into migratory microhabitat selection at a major river confluence by California Central Valley fall-run Chinook salmon in response to localized hydraulic conditions, as well as temperature and turbidity. Conveyance (depth*velocity magnitude) was found to be the strongest hydraulic predictor of micro-scale habitat selection, and acts in conjunction with temperature to facilitate rheotactic swimming behavior. Chapter 2 includes modeled simulations of migratory movement under varying drought conditions, utilizing the same confluence used in Chapter 1 as a study system. These simulations indicate that both discharge magnitude in each river, as well as the ratio of discharge magnitudes between them, are significant drivers of the availability and distribution of preferred hydraulic microhabitats at the confluence, potentially influencing migratory routing. These findings support observed patterns in Chinook salmon escapement data in response to historical flows at this confluence. Chapter 3 includes a literature review and meta-analysis assessing the extent to which abiotic factors have been investigated in the existing literature on homing and straying behavior in anadromous salmonids. Only 70 out of 169 articles included at least one abiotic study component. This phenomenon was surprisingly consistent across study method types, study locations, and study species, with none of these attributes having statistically significant differences in frequency of abiotic components among attribute classes. The research presented in this dissertation provides valuable advancements in the field of fisheries ecology, particularly with respect to the topic of homing and straying in anadromous salmonids. Novel approaches to both field data collection and riverine habitat modeling were developed, and key results link open channel hydraulic processes to adult migratory behavior, filling an existing knowledge gap in the basic life history of salmonids
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Flow and sediment transport disturbance caused by dams
Get PDFA large number of dams and hydropower plants have been built in mountain regions due to the rich water resources and high potential energy. As a consequence, flow regulation associated with dams has led to profound changes to the intertwined hydrologic, ecologic, and geomorphic functioning of rivers. Understanding the effect of dams on the interaction between hydrologic, ecologic, and geomorphic is critical for river management that aims to maximize a range of potential benefits. Therefore, my Ph.D. research proposed to investigate basic and applied scientific questions about two important disturbances caused by dams on mountain rivers: hydropeaking and reservoir sedimentation.Hydropeaking is defined as rapid variations in power production by hydroelectric plants as a consequence of varying electricity generation and fluctuations in demand in the electricity market. In Chapter 1, public hourly flow data in the state of California was used to reveal the diversity of hydropeaking flow. To process a large amount of data and extract their hydrologic features, an open-source algorithm, Hydropeaking Events Detection Algorithm, was developed. Integratedclustering analysis was applied and identified four underlying hydropeaking patterns.Chapters 2 and 3 moved beyond pure statistical analysis to mechanistic observation and modeling of hydro-morphologic interactions in dam-regulated mountain rivers. Both studies investigated hydraulic and sediment transport regimes upstream of two concrete-arch dams. Chapter 2 exploratively studied hydrologic controls associated with current flow operations in a small dam. A new supplementary reservoir sedimentation management strategy based on water transfer was proposed for small dams. Chapter 3 applies flow convergence routing theory backward to redistribute sediment erosion upstream of the dam. Topographic steering and flow convergence routing was found to be able to redistribute sediment erosion pattern and keep sediment away from the key structure
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Morphology of mountain river channels: autogenetic organization and deterministic controls
Get PDFIn alluvial rivers, whose bed and banks are made up of mobile sediment, river geometry generally adjusts to just mobilize the most resistant material lining the channel. In essence, such rivers are freely formed by sediment transport processes (e.g. erosion, deposition, transport, storage). In laterally confined coarse-bedded rivers where adjustment of channel planform and gradient are more restricted this is not always the case, resulting in channel morphologies characterized by resistant boundaries and large substrates, and predominance of other modes of adjustment such as changes in bed roughness. While study of mountain river geomorphology and hydraulics has increased in last 30 years, there remains room for methodological advancement and basic science exploration, especially given availability of technological innovations in data collection, modeling, and data analysis. Thus, the goals of this dissertation were: (i) to advance methods for geomorphic and hydraulic assessment suited to mountain river settings; and (ii) improve foundational understanding of mountain river channel morphology, geomorphic processes, and process-morphology linkages. Using a 13.2-km segment of the mountainous Yuba River (Northern California) as a test bed river study site, three chapters present this work:In Chapter 1, a procedure was developed to map sub-meter resolution large bed elements (LBEs) from a 3D point-cloud and test the hypothesis that element configurations were organized to maximize flow resistance. The procedure, which involved applying a ground classification algorithm to produce a roughness surface model and extracting LBEs with a marker controlled watershed algorithm, resulted in mapping 42,176 LBEs in the study site. Scale and discharge-dependent LBE concentration and spacing metrics quantified for multiple laterally and/or hierarchically nested spatial domains and classified using three flow-resistance based hydrodynamic regimes confirmed nearly all segment- and reach-scale LBE concentrations corresponded to a state of maximum resistance. However, disparities between concentration and spacing metrics left open questions about resistance maximization as an extremal model of geomorphic adjustment. Finer scale analyses demonstrated spatial variability of LBE configurations, but identified maximum resistance act as an attractor state toward which conditions converge. Lastly, lateral variability of LBE metrics had implications for discharge-dependent resistance.Chapter 2 couples sub-meter resolution 2D hydrodynamic modeling with LBE mapping from Chapter 1 to present novel distributions of LBE relative submergences at multiple spatial scales and explores the dynamism of this hydraulic property across discharges and spatial domains (e.g. segment and reaches). Analysis confirmed the rate at which statistical and parametric properties changed between discharge-dependent LBE relative submergence datasets were statistically equivalent between study reaches, which we term ‘process-based similarity’. One interpretation of this consistent scaling is that it represents a dynamic equilibrium in channel adjustment toward a critical state that minimizes the variance of how resistance changes with discharge between reaches. The presented ability to account for more complete representation of bed-surface heterogeneity and the joint-distribution of local flow depths has far reaching implications. For instance, accounting for relative submergence distributions in resistance equations could improve prediction and address the limitation that a 1:1 relation exists between mean depth and mean velocity present in most resistance equations. Observation of discharge-dependent dynamism of LBE height distributions also calls into question the practice of holding roughness coefficients constant, and the ability to map individual LBEs provides a sensible method for parameterizing spatially variable roughness lengths scales.Finally, Chapter 3 presents an investigation into local topographic controls and morphodynamic processes involved in formation and/or persistence of morphological unit scale fluvial landforms (MU) and addresses five scientific questions about mountain river MUs and their hydro-morphological (HM) variables. The study applies a rigorous top-down classification followed by bottom-up analysis experimental design whereby a standard method involving meter-scale 2D hydrodynamic modeling and a baseflow hydraulics decision tree were used to classify and map nine spatially explicit baseflow in-channel MU types in the study site. Discretizing the study site into cross-sectional polygons a total of 2539 cross-sections were identified as being dominated by a single MU type and a diverse set of 18 HM variables, representing an array of possible hydraulic and geomorphic controls on MU formation and/or maintenance, were measured at these cross-sections. Cumulatively, study results develop holistic descriptions of HM variable conditions where certain MUs and/or groups of MUs occurred, interpret processes involved in the formation/persistence of these MU types, and provide inference on how HM variables exert deterministic control
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