1,720,987 research outputs found
Landscape-based hydrological modelling: Understanding the influence of climate, topography, and vegetation on catchment hydrology
No full textIn this thesis, a novel landscape-based hydrological model is presented that was developed and tested in numerous catchments around the world with various landscapes and climate conditions. A landscape is considered to consist of a topography and an ecosystem living on it. Firstly, the influence of climate on hydrological process was studied. It was assumed that an ecosystem organizes its root zone storage capacity to overcome droughts with a certain return period to survive, but not larger than needed, so as to save energy and nutrients. It was found that ecosystems organize their root zone storage capacity to overcome droughts with a return period of about 20 years. This indicates that the root zone storage capacity is the optimal result of an ecosystem adjusting to the climate. The size of the root zone storage capacity in turn has great influence on virtually all hydrological processes. Subsequently, in order to understand the influence of topography on hydrological behavior, we selected a cold-arid catchment in the upper Heihe River in China as the study site. The influence of topography was explicitly considered in the FLEX-Topo model. Firstly, topography data was applied to make a landscape classification. It is interesting to note that also the land cover map can be derived from topography information, indicating the great influence of topography on natural land cover. And then, within the framework of FLEX-Topo modelling approach, we applied a semi-distributed model structure to describe the different runoff generation mechanisms in different landscapes. We found that FLEX-Topo, allowing for catchment heterogeneity, performs much better than lumped models while transferring both model and optimized parameter sets to two nested catchments. To test the influence of vegetation and topography information on model transferability separately, a stepwise modelling approach was applied in 6 catchments in Thailand. Each model was calibrated on one catchment and then transferred with its optimized parameter sets to the other catchments. During calibration, all models exhibited similar skill to fit the hydrographs in all catchments. However, when transferred, the performance of the lumped model reduced dramatically, because it did not explicitly consider vegetation and topography. It was shown that individually, vegetation and topography helped to improve model transferability, especially in catchments with heterogeneous landscape composition. The likely reason is the co-evolution of topography, vegetation, soil and hydrology. Finally, a glacier subroutine was added to the FLEX-Topo modelling framework to improve its applicability in glacierized catchments, where glacier melt water is an essential water resource for downstream. The simulated results were not only validated by hydrographs as in other studies, but also by the long-term glacier mass balance and snow water equivalent data. Furthermore, after considering the different proportion of different landscapes, the glacier model could be successfully upscaled and transferred to a larger catchment. This further validates our proposed modelling concept, the process equations and parameterization.Water managementCivil Engineering and Geoscience
Using tracer data to calibrate a flexible, topography driven conceptual hydrological model
No full textThis study explores the possibility of calibrating a conceptual hydrological model to tracer (Oxygen-18) data measured in streamflow with the ultimate goal to predict discharge. The hypothesis is that (changes in) Oxygen-18 concentrations in the stream should tell something about the relative contribution of fast runoff compared to slow runoff (groundwater flow) inside the stream. A conceptual hydrological model incorporating both tracer dynamics and discharge should be able to model this. Using tracer previously measured in the Brugga catchment (South-West Germany) by people of the University of Freiburg, both lumped and FLEX-Topo inspired models are developed and calibrated to this data. Results showed that of all models considered, only a FLEX-Topo inspired model, containing a variable groundwater level that is able to occasionally (partially or wholly) submerge the wetland model structure, was able to predict a decent hydrograph when calibrating purely on tracer data. The results indicate that calibration of a FLEX-Topo inspired model to only tracer data is a real possibility, and opens the door to more sophisticated modelling attempts, for instance calibration on multiple tracers or multiple tracer sampling points.HydrologyWater ManagementCivil Engineering and Geoscience
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
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
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
MODIS snow cover data for calibration and evaluation of hydrological models in contrasting environments
No full textThis research studies different hydrological models in the catchments with various climates in the USA, and MODIS snow cover maps are introduced to assess and improve the models. In this research, assessment of the value of MODIS snow cover data in the hydrological modeling process is the main objective. Nine MOPEX catchments in the continental US are studied in this research and for each catchment three types of models are used: (1) a basic, lumped 3-box version of the HBV model, (2) a lumped version with snow reservoir, and (3) a semi-distributed formulation of HBV with snow reservoir. MODIS snow cover maps are used for assessing and improving the model snow component. In the models with snow reservoir, snow water storage is reduced to its binary information content: snow/no snow (i.e. 1 or 0), which is then directly linked with spatial pattern in MODIS snow cover data without the need for further assumptions, such as snow densities; the coincidence of snow water storage in the model and observed MODIS snow cover is used as additional calibration/evaluation metric to condition and test the realism of the respective models. In this research, daily meteorological and hydrological data (precipitation, evaporation, discharge, temperature) and maximum eight-day MODIS snow cover maps for the study catchments are used. In calibration, in each time step, the modelled maximum binary 8-day snow reservoir value is compared with MODIS snow cover data for the respective model or its sub-units (in the semi-distributed formulation). Model 03 is semi-distributed, by discretizing each study catchment into individual 200m elevation bands. The results show a large difference of model performance in the catchments with various climates. It is shown that MODIS snow cover data can be valuable to assess the degree to which spatial snow accumulation and ablation dynamics are represented in models, thereby increasing model realism, while not necessarily improving the calibrated performance of models.Civil Engineering and GeosciencesWater ManagementCIE5050-0
Probabilistic characterizations of flood hazards in deltas: Application to Hoek van Holland (Netherlands)
Get PDFCoastal flooding events pose a critical risk in delta areas, since they are characterized by population growth and urban expansion. A better understanding of Extreme Water Levels (EWLs), the mechanisms generating them, and their components, i.e., astronomical tide and storm surge is of great importance as they drive the maintenance and design of flood protection systems. Therefore, a statistical investigation of them can provide new insights for more reliable flood risk mitigation infrastructures. In this study, we analyse these components and compare different probabilistic methods i.e., univariate extreme value analysis, copula functions, and Joint Probability Method (JPM) for the better estimation of EWLs. We use Hoek van Holland (NL) as a representative case study, since the dynamic conditions of this deltaic environment with man-made infrastructures render the area of strategic importance. The results indicate that a more accurate estimate of the declustering time between extreme events can be achieved using correlation of high surges and high wind speeds, taking into consideration also the wind direction. In the Southwest Delta this time estimated to be around 4 days. Furthermore, the EWLs components, i.e., surge and astronomical tide, show negative dependence. From the comparison between statistical approaches to model EWLs, results show that EWLs estimated via EVA and JPM do not vary significantly, while copulas' seems to outperform the other methods. However, the selection of the proper copula to show the dependence is critical. As a conclusion, the analysis of the dependence between tides and storm surges can lead to more robust inferences of EWLs
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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