20 search hits
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Perfluorooctanoic Acid (PFOA) and Perfluorooctane Sulfonate (PFOS) in an Aquatic Ecosystem - Distribution and Fate
(2009)
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Anna Maria Becker
- Perfluorooctanoate (PFOA) and perfluorooctane sulfonate (PFOS) belong to the family of perfluorinated surfactants (PFSs). They are widely distributed and persistent in the environment. For over 50 years, they have been used in numerous applications including paper and textile treatment, production of fluoropolymers, cosmetics and insecticides formulations, and fire fighting foams. They can enter the environment via direct and indirect emission sources such as manufacturing processes, use of commercial products containing PFSs, release of waste waters or degradation of precursor substances. Due to their physical-chemical properties, i.e. relatively good solubility, low volatility and stability under environmental conditions, water bodies are important sinks for these chemicals. The aim of this doctoral thesis was to elucidate sources, distribution and fate of PFOA and PFOS in an aquatic ecosystem that is not directly affected by fluorochemical production activity. The presented study was mainly focused on the river Roter Main, Bayreuth, Germany, and gives a good picture of the behaviour of PFOA and PFOS in such an aquatic ecosystem. In order to achieve the main goal, suitable analytical procedures for reliable quantification of trace amounts of the target analytes in different environmental matrices such as water, liquid and solid wastes, sediments, biological tissues, were developed or optimised. Each method included a solid-phase extraction step for analytes’ preconcentration and removal of interfering matrix, followed by quantitative determination via high performance liquid chromatography coupled to electrospray ionisation tandem mass spectrometery (HLPC-ESI-MS). Due to the possibility of ionisation suppression, isotope dilution or standard addition method was applied. Analysis of waste waters collected from four different waste water treatment plants (WWTPs) located in Upper Franconia, Bavaria, Germany, showed that the largest plant (Bayreuth) receiving waste waters of mostly commercial and industrial origin released the highest amount of PFOA and PFOS, whereas the smallest plant (Himmelkron) treating waste waters of only domestic source released the least. The monitoring of waste waters from the WWTP Bayreuth enabled to estimate the typical mass flows of PFOA and PFOS into river waters as about 1 and 5 g/day, respectively, showing that a plant of a medium-size, moderately industrialised city can be a major source of river pollution. Detailed investigation of PFOA and PFOS concentrations in liquid and solid wastes collected at different stages of the treatment process showed additional fluxes of these compounds inside the plant, likely due to the decomposition of their precursors. Analysis of sediments collected from the river receiving treated waste waters showed a significant increase in concentrations of both analytes downstream the outlet of the plant (up to 3- and 4-fold for PFOA and PFOS, respectively). PFOS concentrations were up to 40-fold higher in sediments than in river water, showing its higher adsorption potential in comparison to PFOA (max. sediment/water = 6). Once in the river, PFOS, and to a lower extent PFOA, can bioaccumulate in aquatic organisms. Although partially removed from water, they are still bioavailable for benthic organisms inhabiting the river thus entering the food chain. This was reflected in higher levels found in river goby - in comparison to chub - feeding on invertebrates living in the sediment. At the starting point of this doctoral thesis little information was available about environmental contamination with PFOA and PFOS in Germany, and it is the first study performed in Bavaria giving such a detailed picture of sources and fate of PFSs in a river ecosystem
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Les acteurs locaux du tourisme de désert au Maroc : Cas de lerg Chebbi et de Zagora-Mhamid
(2009)
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Asmae Bouaouinate
- Cette étude porte sur les acteurs locaux du tourisme de désert au Maroc, aussi bien à l erg Chebbi qu à Zagora-M hamid, dans un contexte où l Etat (le ministère du tourisme) est absent et où les initiatives locales, bonnes et moins bonnes foisonnent. Nous entendons par acteur l ensemble des prestataires de services au désert, tels : le chamelier, le (faux)guide, le chauffeur de Land-rover, le cuisinier, le propriétaire des unités d hébergement, le bivouaqueur, le caravanier, le bazariste, le « faux »-guide. L erg Chebbi est considéré comme une destination classique du tourisme de désert, car il s y est développé depuis les années 1970. Tandis que Zagora-M hamid s est développé une décennie plus tard à cause de la conjoncture politique défavorable de l époque qui ne favorisait pas l émergence d un tourisme de désert dans tout ce rayon géographique. La particularité de cette forme de tourisme qu´est le tourisme de désert réside dans son développement à partir d initiatives privées, locales, dans un milieu aride, refoulé au début dans l imaginaire marocain et considéré comme un espace marginalisé et enclavé mais qui s´est vu réapproprié la perception des touristes, notamment européens, et s est imposé comme espace mythique et comme produit touristique défini. Le désert marocain, proche du vieux continent et offrant la sécurité, s impose comme une destination du tourisme saharien quoique le Maroc ne dispose pas, en comparaison avec d autres pays sahariens, de très grands ergs, tels que l imaginaire du touriste européen se le représente. Et pourtant, en feuilletant les guides et les livres touristiques ainsi que les brochures des tour-opérateurs, il apparaît clair que cette partie du sud-est marocain est bien positionnée dans l échiquier touristique saharien. Le tourisme de désert au Maroc ou précisément à l erg Chebbi et à Zagora-M hamid est d une part considéré par l Etat, par le ministère du tourisme, comme une offre touristique qui étofferait les formules de voyages proposées dans le pays et par le ministère de l Intérieur, comme un moyen de fixer les populations locales dans leurs régions natales. Et d autre part il est considéré par les acteurs locaux essentiellement comme un moyen de gain économique et secondairement comme un moyen d´échange culturel et de valorisation de l identité locale mais dont ils deviennent, au fil des ans, fort dépendants. Tandis que pour les touristes, le tourisme de désert est cette forme de voyage sélective dont ils optent pour réaliser leur mythe : découvrir le Sahara. C est dans ce contexte qu on s est intéressé à approcher les efforts des acteurs locaux pour avoir de la clientèle, leurs réseaux et stratégies, leurs interactions avec les touristes et si derrière cette quête de gain la durabilité n est-elle pas mise en cause ?
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Evaluation of Modis Products Over Four European Ecological Study Sites
(2009)
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Quoc Dinh Nguyen
- Global vegetation is a key component of the climate system due to its key role in geosphere-biosphere-atmosphere interactions. Understanding these processes is of important for predicting future climate and the future state of terrestrial ecosystems. Land surface properties such as the land cover type and leaf area index (LAI) are used as essential inputs in many hydrological, ecological, and climate models. They are key parameters that describe the functioning of vegetation and are required for modeling vegetation productivity, land surface climatology, global carbon budgets and agricultural outputs as influenced by resource management. Successful modeling of these processes to quantitatively and accurately characterize global dynamics requires definition of these parameters periodically and globally with high accuracy. For this purpose the MODIS-based land surface products were designed and are now regularly available worldwide. Nevertheless, analyses based on MODIS inputs of land cover and LAI must be tested with respect to their reliability, in order that we can trust and use the outputs from simulation models quantifying water and carbon balances at large scale. The purpose of the research reported here is to determine the reliability of the MODIS spectral reflectance, land cover and LAI products for European landscapes which are highly fragmented and not necessarily homogeneous at the 1 km scale characteristic of MODIS products. A stepwise analysis has been carried out for reflectance, land cover and LAI products, comparing results from ground truth data and from high resolution remote sensing images (Landsat) to the coarser scale MODIS information. In this way, the influence of landscape fragmentation on the MODIS products should be clear and advice can be given about how they should be used in land surface modelling efforts. Four European locations were chosen for study; landscapes dominated by deciduous forest at Hesse, France; by coniferous forest at Tharandt, Germany, and by forest and grassland in mountainous terrain in the Berchtesgaden National Park, Germany and in Stubai Valley, Austria. All of these landscapes, however, have a mixture of land use. In order to compare measurements at intensive study plots with MODIS (1 km resolution), it was necessary to build a bridge via remote sensing data derived with Landsat TM (30 m resolution). It was demonstrated that for all study sites, the registration accuracy of Landsat TM images did not deviate by more than half of one pixel, and that the root mean square of error (RMSE) was less than 0.3 pixel when utilizing at least 40 ground control points and nearest-neighbor resampling technique. Comparing Landsat images with aerial photography clearly demonstrated that specific study sites on the ground could be identified and that the measured characteristics could be associated with Landsat pixel properties. The evaluation results showed that the MODIS reflectance product is reasonably accurate (less than 10 % absolute error). Certainly it is appropriate to utilize reflectance data from the two types of satellite images and to use these information in comparative examinations of land cover mapping and leaf area index estimation. The land cover comparison demonstrates that both the scale applied in classifications and the number and type of land use categories that are permitted lead to important shifts in the characterization of land cover when moving from 30 m to 1 km resolution of MODIS. Fragmentation in European landscapes is a fundamental problem encountered in the use of MODIS products. A true representation of the land surface cannot be obtained from the current MODIS land cover classifications at 1 km scale. The use of these descriptors in models describing land surface properties may potentially lead to large errors. Thus, exchange between the land surface and the atmosphere of water and CO2 as estimated by models using MODIS inputs will have a high level of uncertainty, and the results must be considered with caution. The problems in classification that are encountered lead to further difficulties in land surface characterization, since the retrieval of LAI uses land cover as an input variable. At the peak of vegetation development, MODIS LAI appears to strongly underestimate values of the Landsat based maps. During winter, the comparison is even worse, but is not consistent from grassland to deciduous forest and coniferous forest. The results cast doubt on the usefulness of MODIS LAI products as input to continental scale simulation models for carbon and water balances, at least in Europe where land cover is highly modified and fragmented due to centuries of human use and management. Use of the MODIS products in Europe requires that new techniques be considered to search for compatibility in averaging and aggregating information on land cover and reflectance that is used to estimate LAI for large areas. Keywords: Remote sensing; vegetation; MODIS; Landsat; LAI; Land cover classification; reflectance; evaluation.
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Experimental and theoretical examination of the chemical kinetics of a pollutant coating on porous particles
(2009)
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Radostin Gavrilov
- The persistence in the different compartments and in the atmospheric long-range transportation is important property of pesticides as representatives of the semivolatile substances. These compounds could be distributed dependent on the air pressure as well as the temperature - between gas and particle phase. In an aerosol smog chamber could be simulated the atmospheric degradation of airborne substances through hydroxyl radicals. The smog-chamber was cooled on 2 and -10°C and the degradation kinetic of semivolatile substance (Aldrin) coated on fine quartz particles (Aerosil 380) was researched. The coated Aerosil was mixed with water in ration 1:1000 and the suspension was sprayed into the chamber. Fine agglomerates were formed during the spaying with mean diameter approximately 1 µm. The precursors for the production of hydroxyl radicals were either reacting mixtures of hydrazine and ozone in absence of light source or photolysis of methylntrite. The concentration of hydroxyl radicals was varied over two powers of ten, from approximately 5•105 to 7•107 cm-3. The concentration of that OH–radicals was calculated over the degradation rate of hydrocarbons (n-octane, n-hexane, 2,2,3-trimthylbutane, 2,2-dimethylbutane and 2,2,3,3-tetramethylbutane). The hydrocarbons were cryofocussed in a glass-coated steel capillary at -110°C (using liquid nitrogen and a magnetic valve to control the flow) and analyzed gas chromatographically. Two products (Photoaldrin and Dieldrin) could be detected from the chemical reaction of Aldrin with hydroxyl radicals. A high concentration of the products were produced with additional experiments with coated Aerosil and glass balloons (d = 70 µm, unporous) and production of hydroxyl radicals from the methylnitrite photolysis in an irradiated rotating evaporator. The products were extracted from the carrier material and were identified with GC–MS. Photoaldrin was formed faster than Dieldrin and reacts also faster then Dieldrin. The temperature gradient between the top and the bottom of the smog-chamber was measured. The temperature difference is important for the air mixing of the chamber content. In the presence of a light-source (the fluorescence lamps are under the the smog camber) the temperature difference is 1.0 °C and ensures a fast mixing in the chamber. This difference of the not irradiated chamber is about 0.3 °C and causes a insufficient mixing, that is noticeable through strong fluctuations of the aerosol density. The structure of the aerosol agglomerates was imaged according to the ion etching method with FESEM (Field Emission Scanning Electron Microscopy) in the Fraunhofer-IKTS. The imagines were evaluated in this work with the program “Lince”. A maximum of the agglomerate diameter was obtained by 0,5 µm. The pore size distribution has a maximum by approximately 20 nm diameter. The life-time of Aldrin and respectively the rate constant of the reaction with hydroxyl radicals could be calculated directly from the experiments. The observed rate constant had a dependence on the OH–concentration in approaching form 3.5•10-5•[OH]-0.88 (the function yields a straight in double logarithmic scale). On the basis of the structure of the agglomerates, a mathematical model was applied from the literature in order to take the influence of the agglomerate structure into account. The observed concentration of Aldrin decreases because of the chemical reaction, the radial diffusion from the agglomerate center to the periphery and because of the evaporation of the substance from the agglomerate surface. It is considered also in the model that the concentration of the hydroxyl radicals alters with the penetration in the agglomerate. The penetration depth can not be determined experimentally. This value, as well as the life-time and the diffusion coefficient could be estimated from the experiments. If the experiment is made by low temperature, the evaporation could be neglect. The evaporated part of Aldrin decreases with the increasing OH concentration. The reciprocal life-time or the reaction rate of Aldrin increases linearly with the increase of the OH concentration. The rate constant of the reaction of Aldrin and OH radicals could be calculated from the reaction rate and the OH concentration. The OH rate constant was kOH = 6.2•10-11 ± 1.3•10-11 cm-3s-1. The effective diffusion coefficient was calculated by -10°C and yielded a value of Deff = 4.6•10-11 ± 2.2•10-11 cm2s-1.
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Transport Processes of Reactive Trace Gases in the Atmospheric Boundary Layer
(2009)
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Jens-Christopher Mayer
- Transport of trace gases within the atmospheric boundary layer plays a key role in feedback processes between the earth’s surface and the atmosphere and consequently in ecosystem budgets of carbon and nitrogen (among many more). For a correct quantification of the exchange between surface and atmosphere, it is crucial to understand the transport processes involved and to determine limitations of the presently available measurement techniques in order to apply the right technique with respect to the currently active transport processes. This dissertation focuses on three topics: (a) The analysis of effects of vertical transport mechanisms on surface measurements of trace gases, (b) the appropriate choice of an experimental setup to assess specific measurement errors of moving measurement systems and (c) the application of a series of measurement techniques for surface fluxes of reactive trace gases to determine their degree of agreement and to assess potential source of deviations. To study the impact of vertical transport mechanisms on surface trace gas measurements, this thesis presents a comprehensive set of measurements at the surface and within the atmospheric boundary layer (by tethered balloon). It enables the attribution of a recurrent negative excursion of ozone mixing ratios in the morning hours at a mountain summit to a very efficient vertical transport by free convection. It has been shown that, due to the rapid vertical transport, a layer of approximately 20 m thickness developed at the equilibrium height of the free convection, being located within the residual layer. It had a chemical composition similar to the air close to the ground while being surrounded by residual layer air masses. Hence, very strong gradients of the chemical composition were found within the residual layer. Evidence was found, that such a transport occurs rather frequently at this location, affecting at least 18 % of the days between April and September. To assess measurement errors introduced by the application of scanning methods as compared to gradient approaches, a higher temporal resolution of the vertical profiles was needed. Because of limitation inherent to a tethered balloon, an elevator based profiling system was installed, providing a temporal resolution of 10 minutes with a maximum ceiling of 100 m. Prior to the investigation of transport processes, the proper functioning of correction algorithms for the so-called dynamical error was investigated under real atmospheric conditions. This dynamical error is inherent to all moving measurement systems and arises from the non-zero response time of the deployed sensors. It has been shown that existing algorithms as well as one developed by the authors reliably balance the dynamical error. Furthermore it has been demonstrated, that the elevator data correlate with reference data at fixed levels with coefficients of determination being always greater than 0.992 at every level (10, 20, 40, 60, 80, 98 m). To evaluate the applicability of different flux measurement techniques for the determination of surface fluxes of reactive trace gases, three different approaches were compared. In order to determine surface fluxes of trace gases, a new modification of the modified Bowen ratio method was used. In this modification, the measurements of sensible heat flux and of the gradients were horizontally separated. This allowed the simultaneous measurement of the fluxes of various trace gases without creating errors due to flow distortion by bulky inlet systems. Surface emission fluxes of nitric oxide were found to be in the range 0.02 – 0.15 nmol m-2 s-1 (night/day), nitrogen dioxide fluxes varied around 0.1 nmol m-2 s-1 (deposition) with slightly positive values in the early afternoon, indicating emission. Ozone deposition fluxes ranged from close to zero to about 6 nmol m 2 s-1. A laboratory parameterization of biogenic soil emission fluxes of nitric oxide from incubated soil samples yielded values from 0.025 nmol m-2 s-1 to 0.12 nmol m-2 s-1 for environmental conditions encountered during the field campaign. This was in excellent agreement with the fluxes from field observations. Besides the comparison of field fluxes with laboratory data, a case study (1 night) comparison of carbon dioxide and ozone fluxes between two field methods was done. Results from the modified Bowen ratio method have been compared to fluxes derived from the integral boundary layer budget method. Both methods yielded similar mean carbon dioxide fluxes during the night. In contrast, ozone fluxes deviated between both methods. This deviation was attributed to chemical in-situ loss of ozone during night time within the profile being integrated by the budget method.
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Analysis of flow patterns and flow mechanisms in soils
(2009)
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Christina Bogner
- Matrix flow and preferential flow can occur concurrently in the same soil. Both flow regimes produce typical flow patterns that can be visualised in dye tracer experiments. To extract quantitative information from dye tracer studies a vast variability of approaches exists. One of them is to describe dye patterns by the so called dye coverage function, i.e. the percentage of stained area per soil depth. Based on extreme value statistics the dye coverage function can be reinterpreted as a probability function to find the tracer in a certain depth. Therefore, the two-parametric probability distribution 1 – H, H being the generalised Pareto distribution, can be fitted to the dye coverage function. The form parameter of this distribution serves as a risk index for vertical solute propagation. We did tracer experiments with Brilliant Blue FCF at three study sites: in a Norway spruce forest in southeast Germany, in a tropical mountain rainforest in southern Ecuador and on an agricultural field in southern France. We tested the ability of the risk index to summarise main information obtained in dye tracer studies and characterise flow patterns in different soils. Our results suggest that the risk index is to some degree invariant to changing experimental conditions (such as irrigation rate). The initial soil moisture, however, seems to have a large influence on the risk index. It is difficult to adjust the parameters of the generalised Pareto distribution when the dye coverage function fluctuates or does not decrease monotonically. This might be due to tortuosity of paths, varying flow mechanism or changing soil physical properties. Thus, we restricted the analysis to the lowest part of the profile. Since the theory of the risk index is based on extreme values of vertical solute propagation it is the lowest part of the profile that is the most interesting. We propose to combine the two parameters of the generalized Pareto distribution and to use the complete distribution 1 - H to estimate the risk of vertical solute propagation in soils. Despite a certain resistance to changes of experimental conditions, the risk index is not an intrinsic soil parameter. Since the flow regime in the same soil can be dominated either by preferential flow or by uniform matrix flow, the risk of vertical solute propagation will change. The adjusted parameters of the generalised Pareto distribution will capture the dominant flow regime as reflected by tracer flow patterns. Bearing in mind the boundary conditions of the tracer experiment like irrigation rate, the tracer employed, soil initial moisture or type of vegetation (permanent or seasonal, deep rooted or shallow rooted) it is possible to compare different study sites or to consider the same site at different boundary conditions and to access the risk of vertical solute propagation. Pattern analysis based on the risk index for vertical solute propagation revealed the occurrence of preferential flow at the German study site. To gain insight in flow mechanisms and possible impacts on soil chemistry we analysed soil texture, fine root density, soil bulk density, exchangeable cations, pH and total C and N contents in preferential flow paths and soil matrix. Results from linear mixed-effects models suggested that at this study site roots constituted main preferential flow paths and induced macropore flow, especially in the topsoil. In the subsoil root density decreased and inhomogeneous infiltration from preferential flow paths into the soil matrix caused non-uniform flow. There were no textural differences between the flow domains, but smaller bulk densities in preferential flow paths. This is probably due to a higher soil organic matter content in preferential flow paths. We found smaller pH values, more Ca, more Mg, more C and more N in preferential flow paths. Compared to the adjacent soil matrix, more Al and more Fe (but small absolute amounts) were found in the subsoil where macropore flow along root channels decreases and heterogeneous matrix flow dominates. These distinct chemical properties can be explained by root activity and translocation of solutes and DOC (dissolved organic carbon) via preferential flow paths. During transport along preferential flow paths contact time between DOC and soil is reduced so that DOC is transported to greater depth where it potentially forms organo-mineral associations. If this holds true, preferential flow is a mechanism that promotes C sequestration in subsoil and does not only influence its immediate environment around paths, but also underlying subsoil horizons.
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Instationarität und räumliche Variabilität in Abflusszeitreihen aus Süddeutschland
(2009)
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Birgit Thies
- Die meisten statistischen Methoden zur Auswertung hydrologischer Daten implizieren zumindest asymptotische Stationarität sowohl in den Wahrscheinlichkeitsdichten als auch in der dynamischen Struktur (ergodische Systeme). Vielfach werden außerdem lückenfreie Daten vorausgesetzt. In dieser Arbeit wurde untersucht, wie berechtigt diese Annahmen auf den in der Praxis zur Verfügung stehenden Zeitskalen von bis zu einem Jahrhundert sind. Dazu wurde ein Ensemble von langen, in täglicher Auflösung vorliegenden Abflusszeitreihen aus Süddeutschland analysiert. Die Spektralmethode des Lomb-Scargle-Periodogramms (LSP) – entwickelt für in unregelmäßigen Zeitabständen gemessene astronomische Daten – wurde auf ihre Anwendbarkeit für lückenbehaftete Abflusszeitreihen beurteilt. Die quantitative Fehlerabschätzung erfolgte in Abhängigkeit von Anteil, Zahl und Verteilung der Lücken. Die Methode liefert für einen Lückenanteil von 1 bis 10% verwertbare Ergebnisse und ist hier einfachen Interpolationsmethoden überlegen. Sind die Daten stark saisonal geprägt, sind auch höhere Lückenanteile unproblematisch, die ansonsten die Interpretierbarkeit der Peaks einschränken. Ist bei längeren Lücken eine Datenrekonstruktion aus geeigneten Nachbarpegeln möglich, liefern Fast-Fourier-Analysen der so vorbehandelten Daten bessere Ergebnisse als das LSP. Bei kurzen Lücken (<1%) sind einfache Interpolationsroutinen ausreichend. Der anhand der Modelleffizienz des LSP quantifizierbare maximal zu erwartende Qualitätsverlust kann mit einer für unkorrelierte Daten hergeleiteten Beziehung abgeschätzt und mittels weiterer Kriterien (Anzahl und Position der Lücken, Saisonalität der Daten) präzisiert werden. Das LSP ist damit für die Frequenzraumanalyse von Abflussdaten mit bis zu 10% Lücken eine sinnvolle Alternative zu aufwändigeren Interpolationsverfahren. Die Stationarität von 97 Pegeln im oberen Donauseinzugsgebiet wurde mittels Fenstertechnik auf Zeitskalen von 2 bis 30 Jahren mit verschiedenen Methoden untersucht. Exemplarisch durchgeführte sequentielle parametrische Verteilungsanpassungen waren methodisch unbefriedigend, nicht-parametrische Ansätze standen daher im Fokus. Herkömmliche Charakteristika von Verteilungen wie Perzentile und Momente und die Entwicklung in daraus abgeleiteten Phasenräumen wurden analysiert. Ausgehend von der Testgröße des Kolmogorow-Smirnow-Zweistichprobentests wurden verschiedene Stationaritätsmaße zum sequentiellen Vergleich von Verteilungen entwickelt, um statt einzelner Kennwerte die Veränderungen im gesamten Wertebereich zu berücksichtigen. Die neuen Maße eignen sich hervorragend zur Einschätzung der Stärke der Schwankungen im Pegelvergleich, ihre Darstellung in Matrizenform erlaubt eine detaillierte zeitliche Analyse der Einzelpegel. Der Zeitverlauf der integrativen Stationaritätsmaße ist weniger von einzelnen Episoden oder Extremereignissen geprägt und damit gleichmäßiger als die mit Hilfe einzelner Verteilungsmerkmale dargestellte Dynamik. Die Analyse des oberen Donaueinzugsgebiets zeigt einen fast alle Pegel betreffenden langfristigen Anstieg der Abflussmengen in den letzten drei Jahrzehnten. Hiervon betroffen sind insbesondere niedrige Abflüsse, abgeschwächt der mittlere Bereich der Werteverteilung und in nur geringem Maße die Hochwasserabflüsse. Streuung, Schiefe und Wölbung zeigen keine pegelübergreifenden langfristigen Tendenzen. Auf kürzeren Zeitskalen von 3 bis 6 Jahren ist bei sämtlichen Pegeln eine synchrone Mittelwerts-Schwankung zu beobachten, die sich abgeschwächt auch in Varianz und höheren Momenten zeigt. Aus dieser regional gleichmäßigen Abflussdynamik auf Zeitskalen von mehreren Jahren lassen sich starke räumliche Korrelationen ableiten, die sich über mehrere 100 km und damit über den Bereich des Untersuchungsgebiets hinaus erstrecken. Die Abflussschwankungen sind von Pegel zu Pegel unterschiedlich stark ausgeprägt, wobei die alpenbeeinflussten Abflüsse durch ihre vergleichsweise geringe Variabilität eine Sonderrolle einnehmen. Das pegelspezifische Instationaritätsniveau kann in der Praxis zur Beurteilung der Unsicherheit von aus Verteilungsanpassungen berechneten Bemessungsgrößen mit herangezogen werden.
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Vegetation ecology of springs: ecological, spatial and temporal patterns
(2009)
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Volker Audorff
- Acidification is a phenomenon, which affected the forested catchments of the northern hemisphere severely over recent decades. Acidic depositions depleted the buffering capacities of soil and groundwater, what lead to an impairment of forests, headwaters, and lakes. Even though the depositions were reduced considerably since the early 1990s, the recovery of catchments was found to occur time-delayed. The grade of recovery was found to vary significantly between regions. Biomonitoring is an appropriate tool to detect spatial and temporal patterns of ecosystem alterations, such as acidification and recovery. However, to know the interrelationships between organisms and their environment is an indispensable precondition for the identification of indicator species. The complexity of ecosystems and ecological processes hampers this quest oftentimes. Springs provide a natural setting that minimises such constraints. Compared to other habitat types, external factors are less relevant, which makes it easier to relate changes in species abundances to changes in their environment. Studying this species-environment relationship, here the response of plant species to the acidification of the spring waters was of particular interest. In a survey of five regions in Central Europe - taking spatial, hydrophysical as well as hydrochemical parameters of the springs into account - it was clearly shown that the species composition of springs is essentially determined by the spring water chemistry, and more precisely by the gradient of acidity and nutrient availability. This connection was reflected by spatial patterns within and between the regions. These patterns provide useful ecological information about spring water quality and in return about the acidity status of their forested catchments. Including catchment traits - like bedrock, climatic parameters, and forest vegetation - in the analyses, these emerged to be relevant for the species composition of springs, but less than the spring water chemistry. A path analysis showed that the catchments affect the vegetation of springs not directly, but indirectly via the determination of spring water quality. Hence, the catchments are a part of the functional chain, which is driven by the atmospheric depositions. The pH-value was found to represent the gradient of acidity and nutrient availability best. It can serve as a proxy measure that can be related to species occurrence and to species dynamics respectively, aiming to identify indicator species for assessing the status and alterations of spring water quality. With the aim to delineate niche optima and amplitudes, which in return can serve as indicator values, the realised niches of spring-inhabiting species were modelled with respect to pH. The niche attributes were found to be a matter of sampling scale. Larger plot sizes (grain) weakened the species-environment relationship, what consequently resulted in broader niche amplitudes. In contrast, the grain did not influence the species’ pH optima. Monitoring approaches that target to assess processes in time, such as acidification and recovery, are dependent on the response time of indicator species to changes in their environment. Investigating an interval of four consecutive years, inter-annual variability of the species composition could not be attributed to changes in the acidity of the spring waters. Looking at single species, bryophytes did not show a higher sensitivity to the inter-annual variability of the environment than vascular plants. Actually, only a minority of all species featured abundance changes which were significantly correlated to variations in spring water acidity. Our results suggest that the species inertia retards the vegetation dynamics of forest springs. A delayed or long-term integrating response of potential indicator species must be considered when evaluating their indicator suitability. In conclusion, the biomonitoring of spring water acidification or recovery is expedient only for longer time intervals. In a nutshell, the vegetation of springs is closely related to the hydrochemical traits of the spring waters, in particular to a gradient of acidity and nutrient availability. Individual species as well as whole plant communities are suitable indicators which allow for the monitoring of the acidity status of forested catchments. The results of this study contribute to a better understanding of the species-environment-relationships, and in return to an improvement of indicator systems.
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First principles phase diagram calculations in group IV carbides and Mg2SiO4 liquid from Molecular Dynamics
(2009)
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Omar Adjaoud
- Atomistic simulations on stability and physical properties of Earth materials are playing an increasingly important role in high pressure mineralogy. Such computations can provide guidance for experimental studies and insight into underlying causes of observations, or explore conditions and properties that are inaccessible to experiments at the current time. A variety of approaches have been applied in such research, with density functional theory based methods having become a reliable tool in computational mineral sciences. Despite this progress there are interesting problems which density functional theory based methods are not able to tackle on a routine basis. These include computations of phase diagrams and transport properties in liquids. The sub-solidus phase diagrams of the binary systems TiC-ZrC, TiC-HfC, ZrC-HfC at ambient pressure are computed based on electronic structure and energy calculations within density functional theory. Formation energies for a large number of supercells with compositions of (M1,M2)C, M1, M2 = Ti, Zr, or Hf, are computed by a plane-wave pseudopotential method. The energies serve as a basis for fitting cluster expansion Hamiltonians that are used to explore the sub-solidus phase diagram, i.e. stability of ordered intermediate compounds and the degree of miscibility in the systems by Monte Carlo simulations. Hamiltonians can be fit to the formation energies of the cells directly or after taking into account vibrational free energy. As it is prohibitive to compute vibrational free energy for all configurations they are approximated by the transferable force constant scheme: nearest neighbor force constants are computed for the end-member crystals with imposed but varying lattice parameters. The resulting bond stiffness versus bond length relationships are applied to the superstructures, using the relaxed bond lengths and their chemical identities as predictor. Significant miscibility gaps were predicted for the binaries TiC-ZrC and HfC-TiC, with consolute temperature in excess of 2000 K, in good agreement with experiments. The system HfC-ZrC shows complete miscibility at room temperature. Approximately symmetric phase diagram for HfC-TiC and asymmetric phase diagrams for HfC-ZrC and TiC-ZrC were predicted. With the success of the method in the simple carbide systems similar computations can now be performed for geologically relevant mineral families. Mg2SiO4 liquid at high pressure is of central importance in our understanding of melts that occur in the deep Earth and in particular in the early history of our planet, when it was in a magma ocean stage. Due to high melting temperatures little is known experimentally about its high pressure thermodynamic and transport properties that govern magma ocean structure and dynamics. Molecular dynamics simulations now fill this gap. Currently, density functional theory based computations are restricted to a few hundred atoms and a few picoseconds. While such simulations allow for determination of thermodynamic properties, longer run durations and larger cells are necessary to obtain transport properties such as diffusivity and viscosity with sufficient precision. By contrast, semi-empirical pair potentials provide an efficient route to perform large-scale molecular dynamics simulations. They suffer, however, from the fact that the transferability of the potentials to different conditions is not guaranteed. The development of aspherical flexible potentials that are fit to density functional theory results bridge the gap between ab-initio methods and classical potentials. Comprehensive large-scale molecular dynamics simulations using the aspherical ionic model were performed on Mg2SiO4 melt to obtain thermodynamic properties as well as diffusivity and viscosity. The pressure-temperature range covered was 0-32 GPa and 2600-3200 K. The thermodynamic parameters agree well with density functional theory based results: the Grüneisen parameter $gamma$ was found to increase significantly with pressure. Diffusivity is predicted to decrease and viscosity to increase with pressure. Both transport properties were readily fit with closed Arrhenius expression. Independent estimates on diffusivity and viscosity allows an examination of their relation through the Eyring equation, often employed to compute viscosity from diffusivity data. The proportionality factor between them, the translation distance for a diffusion event $lambda$, is determined as $lambda$=18 AA at 0 GPa, and decreases with pressure. This is in good agreement with previous molecular dynamics simulations using classical potentials, but significantly larger than other estimates of $lambda$ based on experimental data that yield 2.8 AA $ < lambda < $ 5 AA. Combining the thermodynamic and viscosity fits a magma ocean adiabat and the associated viscosity profile were computed.
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Plant Species and Functional Diversity along Altitudinal Gradients, Southwest Ethiopian Highlands
(2009)
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Desalegn Wana Dalacho
- Understanding how biodiversity is organized across space and time has long been a central focus of ecologists and biogeographers. Altitudinal patterns of richness gradients are one of such striking patterns in the landscape. Despite its historical and ecological importance as a heuristic natural experimental site for development of ecological theories, the emergent patterns and mechanisms that structure them are poorly understood. This is partly because of the complex relationships of species to the environment and the choice of the response variable itself, i.e. using taxonomic richness as a metrics of diversity. This thesis, therefore, applies plant functional types (hereafter PFTs) approach to study the response of vegetation to environmental factors in the southwest Ethiopian highlands. It focuses on the classification of the vegetation into a few main plant functional response categories and relate them to environmental variables. For pattern identification and mechanistic explanations, a deconstructive approach of the taxonomic richness into its constituent components was used. Furthermore, the potential effects of land use/land cover change and global warming on the biodiversity of the study area was investigated. The results reveal that the application of plant functional types is a promising tool to understand vegetation-environment relationships. Local topographic attributes (altitude and slope) and soil properties found to structure the variance in the relative abundance of PFTs along environmental gradients. Moreover, specific response to drought favours the abundance of species with thorns/spines and tussocks in the lowlands as opposed to chilling which favours rosettes and rhizomes PFTs in the highlands. Concerning patterns of richness along altitudinal gradients, various structures of richness appear for total vascular plant species and growth forms. Woody plants, graminoids and climbers showed a uni-modal structure while ferns and herbs revealed an increasing pattern of richness along the altitudinal gradient. By contrast, total vascular plants species richness did not show any strong response to altitudinal gradients. Climate related water-energy dynamics, species area relationships due to the physical shape of the mountain, local topographic and soil conditions were found to be predominant factors structuring the observed richness in the study area. The threats to biodiversity loss due to land use/land cover change and global warming is eminent in the study area. Land conversion for agricultural purposes was a pervasive process that had a deleterious effect on the biodiversity of the study area. Population growth, socio-economic challenges (poverty) and government policy regimes drive land cover change processes. In addition, recent climate change poses a serious challenge to the biodiversity of the study area. The results of model predictions indicated that biodiversity of the study area will suffer severe consequences of lowland biotic attrition (i.e. the net loss of species richness in the tropical lowlands caused by altitudinal range shifts in the absence of new species arriving), range gap shifts and contraction, and extinction due to expected warming at the end of this century. The model also predicted that endangered and endemic species with restricted elevational ranges will disproportionately suffer from range contraction and extinction due to warming. In conclusion, the plant functional types approach was found to be an essential tool to reduce complexity of the vegetation of the study system and to elucidate vegetation-environment relationships. Moreover, the identification of emergent patterns and attributing them to mechanistic explanations are pre-requisites for conservation planning to save biodiversity of the study area. The study also evidenced that land use/land cover change and global warming will present strong threats to the loss of biodiversity in the study area. Salvaging biodiversity in the future requires the consideration of the effect of land use and climate change on vegetation responses. Consequently, nature conservation strategies and future reserve designs should take into account options of human assisted migration across fragmented landscapes and creating dispersal routes for species to track to new thermal niches.
