Amazone (fleuve) | EPFL Graph Search

LAmazone (en espagnol Río Amazonas, en portugais Rio Amazonas) est un fleuve d'Amérique du Sud. C'est le plus puissant fleuve du monde : son débit moyen estimé à l'estuaire est de loin le plus élevé de celui de tous les fleuves de la planète et il équivaut au volume cumulé des six fleuves qui le suivent immédiatement dans l'ordre des débits. À elle seule, l'Amazone représente d'ailleurs environ un cinquième du débit fluvial du monde entier. Avec ses (dans sa branche Apurímac-Ucayali), c'est le plus long fleuve de la Terre devant le Nil. L'Amazone est aussi le plus grand fleuve par l'immensité de son bassin. Il draine une surface de (sans le rio Tocantins) soit 40 % de l'Amérique du Sud et l'équivalent d'une fois et demie la surface de l'Union européenne (le Congo, deuxième fleuve pour la superficie de son bassin, ne draine que de ). Le bassin de l'Amazone s'étend des latitudes 5° nord jusqu’à 20° sud. Le fleuve prend sa source dans la cordillère des Andes,

Improving Alpine Flood Prediction through Hydrological Process Characterization and Uncertainty Analysis

Cara Christine Tobin

Among the many challenges of Alpine flood prediction is describing complex, meteo-hydrological processes in a simplified, robust manner that can be easily integrated into operational forecasting. In this dissertation, improved methods to characterize these processes are developed and integrated into the hydrological modeling component of an operational flood forecasting system used in the Swiss Alps. Detailed studies are conducted to improve hydrological model inputs, processes and outputs. Improvements, detailed in four chapters of this thesis, address the overarching goal of this work – the reduction of flood forecasting uncertainty. The accuracy of flood predictions in Alpine areas is contingent upon adequate interpolation of meteorological forcings, which has significant impacts on discharge volumes and flood peaks. This thesis demonstrates an improvement in the interpolation of temperature and precipitation inputs using a robust variogram which considers anisotropy and using a geostatistical interpolation method to distribute inputs in space and time. Results show that using elevation as the external drift factor better describes orographically-induced precipitation and temperature gradients. Also, the consideration of anisotropy is integral in detailing spatial patterns of precipitation induced by storm advection. Hydrological flood forecasting in mountainous areas also requires accurate partitioning between rain and snowfall to properly estimate the extent of runoff contributing areas. Partitioning is improved in this work by using a new method to integrate Limited Area Model output. Unlike standard hydrological procedures in inferring snowfall limit estimates based on dry, ground temperature measurements, Limited Area Model output considers the vertical, humid, atmospheric structure in its snowfall limit calculations. In effect, this method provides good estimates of runoff contributing areas in the spring as evidenced by validation on discharge measurements and satellite images of snow coverage. Accurately describing snowmelt processes on a sub-daily scale is also of critical importance in Alpine flood forecasting. However, the complex topography of the study region has limited observations available for validation. This thesis presents the development of a new physically-based snowmelt method applicable to regions with limited data. This method uses only daily minimum and maximum temperatures to mimic the effects of radiation. A comparative analysis of snowmelt methods is validated with snow lysimeter data and with a unique, distributed meteorological dataset collected by a wireless weather station network. Results demonstrate that the new method is competitive with more complex snowmelt methods as shown by accurately reproducing diurnal snowmelt cycles. Conveying limits of certainty on flood prediction outputs to users is critical because of epistemic and aleatory errors inherent to environmental modeling. Due to the presence of these errors, the GLUE methodology and multi-criteria performance ideas have been adapted with a fit-for-purpose uncertainty estimation technique in the final part of this thesis. With this method, hydrological model parameters are constrained based on hydrograph behavior, with a particular focus on flood peak response. A key component of the technique is a visualization tool which shows acceptable ensembles of discharge with respect to individual and combined criteria. By integrating the aforementioned input and process improvements into the hydrological model, calibration achieves model outputs that capture observed river discharge. Also, the uncertainty associated with hydrological modeling output error is reduced. Findings of this thesis are applicable to operational flood forecasting in general and have proven utility in improving hydrological model predictions in mountainous regions. Due to the novelty of the developments in terms of new methods or the use of tools and data sources previously unexploited in flood forecasting, further testing of the improvements is recommended. Future research in quantifying the chain of uncertainty produced by combining probabilistic forecast inputs with the hydrological output ensembles is also critical when the improved flood forecasting model becomes fully operational.

EPFL2012

Development of a hyphenated analytical methodology for the fractionation and characterization of environmental colloids

Enrica Alasonati

Natural colloids are defined as macromolecules and nanoparticles with sizes ranging between 1 nm and 1 µm. Colloidal pool is complex "ill-defined" mixture of humic substances, exopolymeric substances (EPS), inorganic oxides, clays, etc. Aquatic colloids have been recognized as key mediators in the removal of trace elements, facilitating contaminant transport, organic carbon cycling and micronutrient bioavailability. The residence times of the colloidal pool (and associate contaminants) can be order of magnitude different from those of the particulate fraction, but unlike the "truly" dissolved fraction, can be subjected to different coagulation and sedimentation processes. Colloids may also influence metal bioavailability by affecting free metal ion concentrations, constraining diffusion and affecting the dissociation kinetics of biological processes. The important environmental role of natural colloids is largely influenced by their surface area and closely related colloidal size. Nonetheless, it is currently unclear to what extend colloidal size can influence their behaviour and contaminant (e.g. toxic metals) binding properties. In our view this is partially related to the difficulties in determination of the colloidal size distributions and their impact in metal behaviour, mainly due to the inherent heterogeneity of the environmental colloids and their low concentrations in the environment. Furthermore, in addition to average parameters, there is a lack of appropriate sample collection and processing procedures, as well as analytical techniques to measure simultaneously the size (or related parameter) distributions of the colloid associated metal. The objective of the present work was therefore to develop sensitive analytical procedures for the fractionation of natural colloids and to apply them to improve the current understanding on the fate of the colloids and associated toxic metals in the freshwaters. To achieve these goals, the capabilities of asymmetric flow-field flow fractionation (aFlFFF) coupled to several detectors was explored. aFlFFF is a versatile technique separating colloids according to their diffusion coefficient. The system was hyphenated with multi-angle laser light scattering (MALS), differential refractive index (DRI), UV detector and, when necessary, high resolution inductively coupled plasma mass spectrometry (ICP-MS). Correct interpretation of the experimental results and extrapolation of meaningful molecular parameters by using an analytical tool with such a level of complexity requires improvement of the knowledge of colloids behaviour in the aFlFFF channel and careful optimization of the separation conditions. Given the very complex and dynamic nature of environmental colloids, different experimental procedures was first optimized and validate under lab conditions by using "standardized" colloids (e.g. alginate and EPS isolate from lab bacterial cultures), then applied to characterize colloidal organic matter (COM) isolated from natural water systems. The influence of critical operating parameters, such as crossflow rate, carrier composition and concentration, and sample load, on the alginate retention was carefully evaluated. Combined information obtained simultaneously by DRI and MALS detectors, allowed to set the appropriate combination of optimal parameters for the characterization of alginate, as a model COM. The metal distributions of metal-alginate complexes were probed by aFlFFF - ICP-MS: average values and continuous distributions of molar masses, radius of gyration and hydrodynamic radius, critical for understanding their role as carriers of metal pollutants, were evaluated in presence of lead or cadmium and compared to those in metal-free solutions of alginate. Furthermore, the molecular characteristic of exopolymeric substances (EPS) secreted by bacterium Sinorhizobium meliloti wild type and exoY strains, which is deficient in the production of the succinoglycan, were studied. EPS were collected in the extracellular environment or desorbed from the cell surface using different extraction protocols. EPS fractions with different chemical characteristic and molar mass were found, depending on the strain and extraction protocol. Molecular parameters as the distributions and average values of molar mass and gyration radius, and the polydispersity index have been measured and compared with data obtained by the chemical characterization. Size exclusion chromatography coupled to UV detection was also employed to provide additional information on the small class of size. Moreover, the effect of the cadmium, lead and calcium addition to exopolymeric substances excreted in the extracellular environment was investigated, with the aim to study how does EPS composition, size and mass distributions, can affect the trace metal binding properties of EPS and how the metal addition affects the EPS conformation. Obtained results revealed that cadmium and calcium bound preferentially on the low-size species population and the amount of the bound metal increased with the increasing of the concentration of added metal. In contrast, lead bound on preferentially larger classes of EPS size. The aFlFFF separation parameters were also optimized for the fractionation and characterization of humic substances, as other COM component, and applied to probe the temporal and spatial variability in the size and mass characteristics of COM samples collected in the Amazon River basin. COM characterization demonstrated the existence of a clear decrease of size of the colloidal material when passing from small to large watercourses (spatial variation). No significant changes were found in size distributions of samples collected in different period of the year, during the low and high flow, respectively (temporal variation). The analytical methodology, adapted to determine not only mean characteristics, but the population distributions, developed in the present work, allowed to obtain new information about the colloidal organic matter size distribution, which play utmost role in colloids behaviour and fate in the environment, in particular as toxic metal carriers.

EPFL2009

Investigation of subglacial weathering under the Greenland Ice Sheet using silicon isotopes

Subglacial chemical weathering plays a key role in global silicate weathering budgets, contributing to the cycling of silicon (Si) in terrestrial and marine systems and the potential drawdown of carbon dioxide from the atmosphere. Here, we use data from two Greenland Ice Sheet (GrIS) catchments to demonstrate how Si isotopes from dissolved and amorphous particulate fractions (delta(DSi)-D-30 and delta(3)(0)ASi respectively) can be used together with major ion data to assess the degree of secondary silicate weathering product formation and redissolution in subglacial environments. We compare a time-series of summer melt seasons from the two study sites, which differ in catchment size (similar to 600 km(2) for Leverett Glacier (LG) and similar to 36 km(2) for Kiattuut Sermiat (KS)). Subglacial waters from LG have elevated Na+ and K+ ions in relation to Ca (2+) and Mg2+ ions, indicating a predominance of silicate weathering, whilst meltwaters from KS are characterised by carbonate weathering (hydrolysis and carbonation) throughout the melt season. Both catchments have mean delta(DSi)-D-30 values substantially lower than average riverine values (KS 0.41 parts per thousand, LG -0.25 parts per thousand, versus a global riverine mean of 1.25 parts per thousand) and display a seasonal decline, which is more pronounced at LG. The delta(30)ASi values (discharge weighted mean values KS -0.44 parts per thousand, LG -0.22 parts per thousand) are lighter than the bedrock (mean values KS -0.18 +/- 0.12 parts per thousand, LG 0.00 +/- 0.07 parts per thousand) in both catchments, indicating a secondary weathering product origin or leaching of lighter isotopes during initial weathering of crushed rock. When used in combination, the major ion and silicon isotope data reveal that the extent of silicate weathering and secondary phase redissolution are more pronounced at LG compared to KS. Contrasting weathering regimes and subglacial hydrology between catchments need to be considered when estimating the delta Si-30 composition of silica exported into polar oceans from the GrIS, with larger catchments likely to produce fluxes of lighter delta Si-30. As larger catchments dominate freshwater export to the ocean, GrIS meltwater is likely to be very light in isotopic composition, and the flux is likely to increase with ice melt as the climate warms. (C) 2019 Elsevier Ltd. All rights reserved.

2019