1,721,288 research outputs found
Modelling sediment transport in the Jonkershoek catchment Part 1: Model calibration and validation
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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
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
Impact van graanwortels op bodemerosie door geconcentreerde afvoer en implicaties voor bodembehoud
Full text linkImpact van graanwortels op bodemerosie door geconcentreerde afvoer en implicaties voor bodembehoud Vele akkergronden in heuvelachtige gebieden zijn onderhevig aan belangrijke bodemerosie door geconcentreerd afstromend water. Dit proefschrift onderzocht in hoeverre de wortels van graangewassen een reducerend effect uitoefenen op de vorming van geulen en ravijnen in akkers en hoe graanwortels kunnen aangewend worden om deze erosieprocessen te bestrijden. Een combinatie van laboratoriumexperimenten in een hydraulisch proefkanaal en metingen op akkers toonde aan dat graanwortels de kritische hydraulische sleepdruk voor sedimenttransport niet beïnvloeden, maar dat de erosiegevoeligheid van een bodem exponentieel afneemt met zowel toenemende graanworteldichtheid als toenemende bovengrondse biomassa. Waar de bovengrondse biomassa echter voornamelijk bescherming biedt tegen spat en intergeulerosie, verhogen wortels de weerstand van een bodem tegen geul- en ravijnerosie. Op het einde van het groeiseizoen is de weerstand van de bodem tussen de zaailijnen 34% kleiner dan deze onder de zaailijnen, omwille van de ruimtelijke en temporele variabiliteit in worteldichtheden. Een verdubbeling van de worteldichtheid kan bodemerosie als gevolg van geul- en ravijnerosie met maximaal 60% reduceren t.o.v. een bodem zonder wortels. In vergelijking met een enkelvoudig ingezaaide akker, ondervindt een dubbel ingezaaide graanakker onder ideale omstandigheden 40% minder erosie door geconcentreerde afvoer, zonder een verlies van graanopbrengst. Hiervan is 10% te wijten aan de toename in wortels en 30% als gevolg van de toename in bovengrondse biomassa. Het verdubbelen van de zaaidichtheid als methode om erosie te bestrijden in erosiegevoelige zones in graanakkers is geschikt voor zones met een toestroomgebied kleiner dan 0.75 ha en een lokale helling kleiner dan 15%. Onder deze omstandigheden vormt dubbel inzaaien van granen in erosiegevoelige zones een effectieve en efficiënte erosiebestrijdingstechniek die door landbouwers eenvoudig toe te passen is.status: Publishe
Assessment of sediment export from rural catchment areas using retention ponds in Central-Belgium.
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Sediment fluxen en koolstof dynamiek in het overstromingsgebied van een tropische rivier (Tana River, Kenia)
No full textRecent research has highlighted the importance of rivers in the global carbon cycle. Rivers transport considerable amounts of sediment and carbon (C) to the ocean and act as a major conduit between the continental and oceanic component of C cycle. However, a large fraction of the C in the fluvial system does not reach the ocean, either due to mineralization during transport or storage in transitory stores within the fluvial network. Our understanding of the role of these transitory stores under the different climatic regions remains limited. This research focuses on the temporal significance of a tropical floodplain (as transitory C store) and its role in the river C and sediment fluxes. The study is based in the Tana River, Kenya a tropical lowland semi-arid floodplain. Our research area was in the Garissa-Garsen (GSA-GSN) reach of the floodplain ~ 380 km river length and ~1,000 km2. We have used a unique combination of advanced biogeochemical and geophysical techniques to characterize the interaction between the floodplain and the main river, quantifying the fluxes of C and sediments transported in the river and deposited within the Tana River floodplain and analysing the temporal significance of Tana River floodplains as a C sink.
During flood events, a significant amount of sediment is deposited within the GSA-GSN reach. Based on a combination of fallout radionuclide activities, we recorded a 50 yr mean sedimentation rates of 1.15-1.21 g cm-2 yr-1 and a 100 yr mean of 1.01 g cm-2 yr-1 (using 137Cs and 210Pbex, respectively). Event-based sediment deposition rates averaged between 2-15 mm vertical accretion, corresponding to an average of 0.58±0.42 g cm-2 (dry weight). Overbank flooding is common with a 73 yr flooding frequency of 1.05 flood yr-1 and an average of 1-2 Mt (15-30%) of the river sediment load being deposited in the GSA-GSN floodplain reach. Substantial amounts of organic carbon (OC) are deposited with this enormous amount of sediment. However, the deposited sediments are carbon poor at ~1.5% OC and the floodplain vegetation plays a big role in enrichment with additional OC inputs to levels above 3% OC at the (sub) surface layers. Sediment cores at floodplain locations therefore show relatively high OC concentrations (3-12% OC) in the (sub) surface layers, however, due to high mineralization rates, a sharp decrease of OC with depth is observed to less than 1% OC below ca. 60 cm depth.
Acknowledging that OC source is an important consideration in OC preservation, we investigated OC sources and storage with depth within the floodplain using d13C-OC as a proxy. Local disturbance by clearing of vegetation and subsistence agriculture caused homogenization of the d13C-OC depth profile, while fresh biomass inputs from the undisturbed floodplain locations appeared to accelerate mineralization of riverine sediment derived OC. Our data shows that despite the strong variability in 13C enrichment in deep layers at some sites, the OC in sediment deposits show no evidence of major shift in the dominant vegetation in the catchment. The post depositional dynamics of OC were dominated by a combination of isotope mixing, selective mineralization and kinetic fractionation upon OC mineralization making it difficult to constrain the principal mechanism responsible for the observed pattern in d13C-OC. Due to this intricate mechanism, it was difficult to properly model mineralization of below ground OC using the Introductory Carbon Balance Depth Explicit Model (ICBM-DE), a simplified two pool SOC mineralization model.
Finally when we made a budget of the fluxes both within the river and the floodplain, we found that during flooding, an intensive selective deposition in the different geomorphological featured within the floodplain occurred. This led to changes in grain size as different particle sizes were either remobilized or preferentially deposited in certain floodplain location. The sorting is also evidenced by high fluxes of fine grain size at both the upstream and downstream locations and at the same time deposition of fine materials within the floodplain. Remobilization of sediment mainly through bank incision and meander migrations was a major floodplain process. While about 6.7 Mt yr-1 sediment and 104 GgC yr-1 enters the floodplain at Garissa, 5.19 Mt yr-1 of sediment equivalent to 36 GgC yr-1 is mobilized and added to this pool annually through river incisions. Consequently, the majority of this sediment and C is lost, the main mechanisms include within river deposition (6.57 Mt yr-1 sediment or 66 GgC yr-1), overbank floodplain deposition (38 GgC yr-1, 2.45 Mt yr-1 or 27% of total sediment fluxes) and the rest is discharged as fluxes downstream of the floodplain in Garsen (34 GgC yr-1, 2.6 Mt yr-1, 24% of total sediment fluxes). Due to the intensive selective deposition and sorting of different grain sizes, the downstream riverine fluxes of suspended matter and particulate organic carbon pools are strongly influenced by these processes.status: Publishe
Meten en modelleren van hydrologische connectiviteit
No full textHydrological connectivity describes the internal linkages between runoff generation in upper parts of the catchment and the receiving water. It is quantified as the ratio of the runoff reaching the catchments outlet and the total internal runoff generation. It thus effectively bridges t he gap between the point-scale separation of rainfall into soil water st orage and (sub-)surface runoff as opposed to what we see as response at the hillslope or catchment scale. In between both, significant water red istribution from runoff source areas into sinks may occur such that isol ated active areas in upslope regions may become disconnected and may not contribute to the actual outflow. This pattern-process interaction is o ne of the main reasons why hydrological observations at laboratory or pl ot scale are inadequate to explain the phenomena witnessed on hillslopes and in catchments and is why hydrological connectivity has become, in t he last decade, a central concept in hydrology, particularly in semi-ari d environments. The overall purpose of this dissertation is to identify which factors control the connectivity of a hillslope and to d iscuss how this reflects on data collection and modelling.While traditionally hydrological variables are estimated through point observa tions dynamically varying in time, connectivity stresses the need to als o account for space. It is the spatiotemporal heterogeneity in infiltrat ion capacity and rainfall intensity that determines connectivity or disc onnectivity with complex and spatially varying thresholds governing whic h parts of the domain are active and which contributing and which domina nt processes affect storage, redistribution and connectivity. Once estab lished that it is the spatial pattern of heterogeneous runoff generation and abstraction and not merely the statistical distribution of this het erogeneity, we ask ourselves which aspects of pattern determine the conn ectivity and which are negligible. We test a number of hypotheses by opp osing landscape metrics with the modelled hydrograph of a virtual hillsl ope. Each metric describes a different characteristic of the pattern and their variable correlation with connectivity can thus supply an objecti ve criterion to ascertain what controls connectivity. The aggregation of runoff source areas in interaction with the flow distance to the outlet dominate in the presence of a mosaic of runoff sinks and sources. When the area reacts more homogeneous, e.g. due to high rainfall intensity, t he pattern disappears and it becomes mainly the travel time distribution that governs connectivity. While the used landscape metrics could only partially predict the connectivity, they are insightful tools for hypoth esis testing and, we believe, can reversely be used in basin classificat ion and the identification of dominant processes. If we establish a link between a certain metric and a particular basin functioning, we may als o decide upon a range of values of that metric that a basin should fulfil in order to be classified in a particular class.We propose ran domness of pattern as an important characteristic of heterogeneity to di stinguish two classes: when the heterogeneity is random, its particular configuration becomes superfluous and only the statistical distribution of its properties remains of interest. If, on the other hand, the hetero geneity expresses clear spatial configuration or gradients, accounting f or this configuration is indispensible. This has important implications for modelling as well as for data collection. If the particular spatial configuration has no influence on the outcome, it also does not need to be explicitly modelled. The degrees of freedom of the model can therefor e be reduced to a smaller number of parameters that set the statistical distribution rather than having one uncorrelated parameter for each spat ial element in the modelling space which would clearly lead to the probl em of equifinality. If we can only parameterise those spatial aspects th at matter, we effectively reduce the overparamaterisation that plagues m odern distributed modelling. As for data collection: since the exact spa tial configuration of random patterns is redundant knowledge, a sparse d ata collection that settles the statistical distribution of heterogeneit y suffices in that case; while in the presence of significant configurat ion, data sampling strategies should be tailored to capture the spatial patterns.If we want to gain a better understanding of proces ses, we need to build models that can test hypotheses and collect data t hat allows to falsify the hypotheses. Both the reconfirmation of space a s a crucial dimension beside time and the observation that non-random pa ttern matters, emphasises the need in connectivity related problems for data that can sample dense spatial variations in hydrological state vari ables and processes. We explore a combination of Electrical Resistivity Tomography (ERT) and Time Domain Reflectometry (TDR): ERT samples spatia l averages and can be translated to a dense 3D resistivity distribution. TDR, on the other hand, is a point-scale measurement and is used to sup ply a transformation between resistivity and water content, the so-calle d pedophysical relation. By collecting data both before and after a conc entrated flow experiment in a semi-arid gully, we map the total infiltra tion that occurred by taking the difference between the estimated water distributions before and after. The straightforward three-step scheme (E RT inversion, transformation to water content, subtraction) returns, how ever, an invalid solution. To improve the outcome, we developed an alter native inversion that constrains the outcome to comply to our knowledge of the experiment and that jointly inverses ERT and TDR while optimising the pedophysical parameters. Although these alterations effect signific ant improvements, the signal-to-noise ration in the data and the poor re solution of the outcome undermine the reliability of the obtained infiltration map. We were therefore unable to utilise it in further hypothesis testing or modelling. Nonetheless, we believe that ERT and TDR sampling in combination with our proposed inversion scheme can potentially give insightful results.status: Publishe
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