A key question related to climate change projections is how will aquatic systems respond to changes in variables such as temperature and precipitation? This thesis uses GWLF, a simple catchment scale model to explore potential impacts of climate change on water quantity and quality. River discharge and nutrient loads were modelled for several warmer world scenarios. For one catchment in New York, USA changes in annual dissolved phosphorus loads decreased along with annual streamflow, and particulate phosphorus loads increased for a single future climate scenario. For the Galten catchment of Lake Mälaren, Sweden, the spring melt peak observed historically was reduced for six future scenarios. Peak runoff and dissolved phosphorus and nitrogen load maxima occurred in winter rather than early spring. A synthesis of model results for dissolved inorganic nitrogen (DIN) loadings for five European catchments showed changes in the timing and magnitude of peak DIN load for several future scenarios. In northern Europe, changes were largely due to increased winter streamflow and reduced snow pack and spring melt runoff. In western Europe, DIN loads increased in winter or early spring due to increased precipitation. The biological response for a warmer future scenario was modelled for the Galten basin of Lake Mälaren using GWLF coupled to a lake phytoplankton model and a physical lake model. An increase in cyanobacteria biomass accompanied by a decline in diatom biomass resulted from changes in the timing of nutrient export from the catchment. The projected increase in lake temperature favored an overall increase in total phytoplankton biomass. Lastly, a method based on hourly measurements of colored dissolved organic matter (CDOM) fluorescence provides the appropriate data for dissolved organic carbon (DOC) model parameterization and may also be used for surveillance of organic matter inputs to lakes.

No presente trabalho o modelo de qualidade de águas superficiais QUAL2E foi utilizado como apoio ao gerenciamento das águas da bacia do Córrego dos Bagres, localizada no Estado de São Paulo (20º30\' e 20º40\'35\'\'S e 47º35\'2\'\' e 47º20\'18\'\'W). A modelagem foi aplicada em toda a extensão do curso da água, cerca de 44 km, simulando o Oxigênio Dissolvido e a Demanda Bioquímica de Oxigênio. Procurou-se também determinar os mais importantes fatores que afetam a concentração de oxigênio dissolvido no curso d\'água. O modelo foi calibrado e validado de acordo com dados de monitoramento da agência ambiental estadual responsável pela UGRHI Sapucaí/Grande. Os coeficientes cinéticos no modelo foram definidos através da calibração. O desenvolvimento do estudo propiciou uma caracterização mais detalhada da bacia, e os resultados obtidos demonstram uma boa concordância entre os dados calculados pelo modelo QUAL2E e os observados, indicando que os valores dos coeficientes cinéticos adotados estavam adequados para a bacia em estudo. / ln this work, the water quality model QUAL2E was used as a tool for the water management of the Bagres stream basin, located in the State of São Paulo (20º30\' e 20°40\'35\'\'S e 47°35\'2\'\' e 47°20\'18\'\'W). The model was applied to the whole water body, about 44 km long, simulating Dissolved Oxygen and Biochemistry Oxygen Demand. One objective for this modeling was the investigation of the main factors affecting low DO concentrations in the Bagres basin. The model was calibrated and validated using data collected by the State environmental agency responsible for the Sapucaí/Grande UGRHI. Kinetic coefficients for the modeling were defined through calibration. This work has enabled a more detailed characterization of the basin, and the results obtained have shown a good agreement between the QUAL2E predicted results and the data, validating the values adopted for the kinetic coefficients as adequate for the basin under study.

Appropriately constructed pollutant export models can help set management priorities for catchments, identify critical pollutant source areas, and are important tools for developing and evaluating economically viable ways of minimising surface water pollution.¶ This thesis presents a comparison, an evaluation and an integration of models for predicting the export of environmental pollutants, in particular sediment, through river systems. A review of the capabilities and limitations of current water quality modelling approaches is made. Several water quality and quantity modelling approaches are applied and evaluated in the catchment of the upper Murrumbidgee River.¶ The IHACRES rainfall-runoff model and a simple hydrologic routing model are applied with the aim of developing a capacity to predict streamflow at various catchment scales and to enable integration with other pollutant load estimation techniques. Methods for calculating pollutant loads from observed pollutant concentration and modelled streamflow data are also investigated. Sediment export is estimated using these methods over a 10-year period for two case study subcatchments. Approaches for water quality sampling are discussed and a novel monitoring program using rising stage siphon samplers is presented. Results from a refinement of the Sediment River Network model in the upper Murrumbidgee catchment (SedNet-UM) are presented. The model provides a capacity to quantify sediment source, transport and to simulate the effects of management change in the catchment. The investigation of the model includes rigorous examination of the behaviour of the model through sensitivity assessment and comparison with other sediment modelling studies. The major conclusion reached through sensitivity assessment was that the outputs of the model are most sensitive to perturbation of the hydrologic parameters of the model.¶ The SedNet-UM application demonstrates that it is possible to construct stream pollutant models that assist in prioritising management across catchment scales. It can be concluded that SedNet and similar variants have much potential to address common resource management issues requiring the identification of the source, propagation and fate of environmental pollutants. In addition, incorporating the strengths of a conceptual rainfall-runoff model and the semi-distributed SedNet model has been identified as very useful for the future prediction of environmental pollutant export.

No presente trabalho o modelo de qualidade de águas superficiais QUAL2E foi utilizado como apoio ao gerenciamento das águas da bacia do Córrego dos Bagres, localizada no Estado de São Paulo (20º30\' e 20º40\'35\'\'S e 47º35\'2\'\' e 47º20\'18\'\'W). A modelagem foi aplicada em toda a extensão do curso da água, cerca de 44 km, simulando o Oxigênio Dissolvido e a Demanda Bioquímica de Oxigênio. Procurou-se também determinar os mais importantes fatores que afetam a concentração de oxigênio dissolvido no curso d\'água. O modelo foi calibrado e validado de acordo com dados de monitoramento da agência ambiental estadual responsável pela UGRHI Sapucaí/Grande. Os coeficientes cinéticos no modelo foram definidos através da calibração. O desenvolvimento do estudo propiciou uma caracterização mais detalhada da bacia, e os resultados obtidos demonstram uma boa concordância entre os dados calculados pelo modelo QUAL2E e os observados, indicando que os valores dos coeficientes cinéticos adotados estavam adequados para a bacia em estudo. / ln this work, the water quality model QUAL2E was used as a tool for the water management of the Bagres stream basin, located in the State of São Paulo (20º30\' e 20°40\'35\'\'S e 47°35\'2\'\' e 47°20\'18\'\'W). The model was applied to the whole water body, about 44 km long, simulating Dissolved Oxygen and Biochemistry Oxygen Demand. One objective for this modeling was the investigation of the main factors affecting low DO concentrations in the Bagres basin. The model was calibrated and validated using data collected by the State environmental agency responsible for the Sapucaí/Grande UGRHI. Kinetic coefficients for the modeling were defined through calibration. This work has enabled a more detailed characterization of the basin, and the results obtained have shown a good agreement between the QUAL2E predicted results and the data, validating the values adopted for the kinetic coefficients as adequate for the basin under study.

Climate change, global warming and anthropogenic disturbances are threatening freshwater ecosystems globally. The protection and preservation of freshwater environments, its biodiversity and all of its services for human well-being requires comprehensive knowledge of the impacts that climate change and anthropogenic disturbances have on freshwaters and freshwater species. In-depth knowledge needed for conservation strategies can be established through versatile assessments. Quantitative assessments and the investigation of prevailing environmental relationships within ecosystems constitute the basis for sustaining freshwater systems. However, it is a great challenge to quantify the multifaceted effects of climate change and to broaden the understanding of complex environmental relationships. This thesis aims at contributing to an extension of the understanding of climate change impacts on freshwater ecosystems and environmental relationships, which implies the provision of useful guidelines for the protection and preservation of freshwaters. For this, various statistical approaches based on comprehensive data sets are applied at different scales, ranging from local to global assessments. In particular, five research studies investigating the (1) water quality-nutrient and temperature relationships in European lakes, (2) drivers of freshwater fish species distributions across varying scales in the Danube River delta, (3) globally derived thermal response curves and thermal properties of native European freshwater species, (4) differences between thermal properties derived from native and global range data, and (5) thermal performances of freshwater fish species for different life stages and different global future dispersal scenarios are presented to address the effects of environmental change. Main results of this thesis comprise various aspects of conservation implications and planning. (i) The first study outlines drivers influencing water quality through studying multi-dimensional relationships and compares different modelling techniques in order to outline models that are suitable for the identification of complex driver interactions. (ii) The second study addresses scale effects on the performance of species distribution models, which are commonly used for assessments of climate change impacts, and identifies key predictors driving distributions for the varying scales and studied species. (iii) The third study parameterizes thermal responses of species from different taxonomic groups and assesses the potential resilience in terms of warming tolerance and additional thermal properties as well as the influence of future rising temperatures on current distributions. (iv) The fourth study quantifies the differences in thermal response curves and thermal properties for freshwater fishes derived from global and continental data in order to clarify the need for using global range data in studies making suggestions for conservation planning. (v) The last study estimates the impact of changing climatic conditions on species distribution ranges of two fish species for different time periods by including biotic information about thermal performances for various life stages. Overall, this thesis contributes to the broad field of studying consequences and impacts of climate change on freshwater ecosystems. By applying statistical methods tailored to the underlying investigations, useful implications for conservation planning are derived.

The modelling of the processes of advection and dispersion-diffusion is the most crucial factor in solute transport simulations. It is generally appreciated that the first order upwind difference scheme gives rise to excessive numerical diffusion, whereas the conventional second order central difference scheme exhibits severe oscillations for advection dominated transport, especially in regions of high solute gradients or discontinuities. Higher order schemes have therefore become increasingly used for improved accuracy and for reducing grid scale oscillations. Two such schemes are the QUICK (Quadratic Upwind Interpolation for Convective Kinematics) and TOASOD (Third Order Advection Second Order Diffusion) schemes, which are similar in formulation but different in accuracy, with the two schemes being second and third order accurate in space respectively for finite difference models. These two schemes can be written in various finite difference forms for transient solute transport models, with the different representations having different numerical properties and computational efficiencies. Although these two schemes are advectively (or convectively) stable, it has been shown that the originally proposed explicit QUICK and TOASOD schemes become numerically unstable for the case of pure advection. The stability constraints have been established for each scheme representation based upon the von Neumann stability analysis. All the derived schemes have been tested for various initial solute distributions and for a number of continuous discharge cases, with both constant and time varying velocity fields. The 1-D QUICKEST (QUICK with Estimated Streaming Term) scheme is third order accurate both in time and space. It has been shown analytically and numerically that a previously derived quasi 2-D explicit QUICKEST scheme, with a reduced accuracy in time, is unstable for the case of pure advection. The modified 2-D explicit QUICKEST, ADI-TOASOD and ADI-QUICK schemes have been developed herein and proved to be numerically stable, with the bility sta- region of each derived 2-D scheme having also been established. All these derived 2-D schemesh ave been tested in a 2-D domain for various initial solute distributions with both uniform and rotational flow fields. They were further tested for a number of 2-D continuous discharge cases, with the corresponding exact solutions having also been derived herein. All the numerical tests in both the 1-D and 2-D cases were compared with the corresponding exact solutions and the results obtained using various other difference schemes, with the higher order schemes generally producing more accurate predictions, except for the characteristic based schemes which failed to conserve mass for the 2-D rotational flow tests. The ADI-TOASOD scheme has also been applied to two water quality studies in the U. K., simulating nitrate and faecal coliform distributions respectively, with the results showing a marked improvement in comparison with the results obtained by the second order central difference scheme. Details are also given of a refined numerical representation of flooding and drying of tidal flood plains for hydrodynamic modelling, with the results showing considerable improvements in comparison with a number of existing models and in good agreement with the field measured data in a natural harbour study.

Climate change and socio-economic development are greatly affecting the quality of freshwater, especially the excessive accumulation of nutrients (N and P), which eventually leads to the occurrence of eutrophication. Lake Vomb is one of the main sources of drinking water in southern Sweden and the nutrient load from the catchment makes it a eutrophic lake with recurring algal blooms. This project developed a hydrological model by Soil and Water Assessment Tool (SWAT) to model the impact of climate and socio-economic changes on nutrient dynamics on Lake Vomb. Two combinations of future scenarios were selected as combinations of Representative Concentration Pathways (RCP) and Shared Socioeconomic Pathways (SSP): 1) SSP1/RCP4.5 and 2) SSP5/RCP8.5. For the calibration and validation period, satisfactory results were obtained for monthly flow (R2 and NSE were 0.92 and 0.82, respectively) and yearly nutrient load simulation obtained overall convincing results (R2, NSE and PBIAS were 0.67, 0, 17% for total nitrogen (TN) and 0.78, -0.11, -1% for total phosphorus (TP) at the main inflow and R2, NSE and PBIAS were 0.46, 0.27, 15% for TN and 0.62, -0.01, -0.06% for TP at the second inflow). Simulation results of increased future flow scenarios showed higher peaks under the RCP8.5 scenario than under RCP4.5. Also, there was an increasing trend that flow will continuously rise during the simulation period. Results from future nutrient load simulation showed that the TN load was below the baseline in most scenarios and the TP load was all above the baseline. Besides, nutrient loading is more sensitive to the combination of SSPs and RCPs and got the highest loads under RCP4.5/SSP1. The challenge of reducing nutrient load increases over time, especially for P because the simulated load was higher for 2090-2100 than the load for 2040-2050. This project has limitations such as uncertain agriculture activities input and nutrient observations. This project provides projections of flows and nutrient loads into Lake Vomb for a combination of possible future scenarios and more different scenarios such as different RCPs and different designed SSPs deserve to be studied in the future.

The modelling of the processes of advection and dispersion-diffusion is the most crucial factor in solute transport simulations. It is generally appreciated that the first order upwind difference scheme gives rise to excessive numerical diffusion, whereas the conventional second order central difference scheme exhibits severe oscillations for advection dominated transport, especially in regions of high solute gradients or discontinuities. Higher order schemes have therefore become increasingly used for improved accuracy and for reducing grid scale oscillations. Two such schemes are the QUICK (Quadratic Upwind Interpolation for Convective Kinematics) and TOASOD (Third Order Advection Second Order Diffusion) schemes, which are similar in formulation but different in accuracy, with the two schemes being second and third order accurate in space respectively for finite difference models. These two schemes can be written in various finite difference forms for transient solute transport models, with the different representations having different numerical properties and computational efficiencies. Although these two schemes are advectively (or convectively) stable, it has been shown that the originally proposed explicit QUICK and TOASOD schemes become numerically unstable for the case of pure advection. The stability constraints have been established for each scheme representation based upon the von Neumann stability analysis. All the derived schemes have been tested for various initial solute distributions and for a number of continuous discharge cases, with both constant and time varying velocity fields. The 1-D QUICKEST (QUICK with Estimated Streaming Term) scheme is third order accurate both in time and space. It has been shown analytically and numerically that a previously derived quasi 2-D explicit QUICKEST scheme, with a reduced accuracy in time, is unstable for the case of pure advection. The modified 2-D explicit QUICKEST, ADI-TOASOD and ADI-QUICK schemes have been developed herein and proved to be numerically stable, with the bility sta- region of each derived 2-D scheme having also been established. All these derived 2-D schemesh ave been tested in a 2-D domain for various initial solute distributions with both uniform and rotational flow fields. They were further tested for a number of 2-D continuous discharge cases, with the corresponding exact solutions having also been derived herein. All the numerical tests in both the 1-D and 2-D cases were compared with the corresponding exact solutions and the results obtained using various other difference schemes, with the higher order schemes generally producing more accurate predictions, except for the characteristic based schemes which failed to conserve mass for the 2-D rotational flow tests. The ADI-TOASOD scheme has also been applied to two water quality studies in the U. K., simulating nitrate and faecal coliform distributions respectively, with the results showing a marked improvement in comparison with the results obtained by the second order central difference scheme. Details are also given of a refined numerical representation of flooding and drying of tidal flood plains for hydrodynamic modelling, with the results showing considerable improvements in comparison with a number of existing models and in good agreement with the field measured data in a natural harbour study.

Flood polders are part of the flood risk management strategy for many lowland rivers. They are used for the controlled storage of flood water so as to lower peak discharges of large floods. Consequently, the flood hazard in adjacent and downstream river reaches is decreased in the case of flood polder utilisation. Flood polders are usually dry storage reservoirs that are typically characterised by agricultural activities or other land use of low economic and ecological vulnerability. The objective of this thesis is to analyse hydraulic, environmental and economic impacts of the utilisation of flood polders in order to draw conclusions for their management. For this purpose, hydrodynamic and water quality modelling as well as an economic vulnerability assessment are employed in two study areas on the Middle Elbe River in Germany. One study area is an existing flood polder system on the tributary Havel, which was put into operation during the Elbe flood in summer 2002. The second study area is a planned flood polder, which is currently in the early planning stages. Furthermore, numerical models of different spatial dimensionality, ranging from zero- to two-dimensional, are applied in order to evaluate their suitability for hydrodynamic and water quality simulations of flood polders in regard to performance and modelling effort. The thesis concludes with overall recommendations on the management of flood polders, including operational schemes and land use. In view of future changes in flood frequency and further increasing values of private and public assets in flood-prone areas, flood polders may be effective and flexible technical flood protection measures that contribute to a successful flood risk management for large lowland rivers. / Flutpolder werden zum gezielten Rückhalt von Wasser eingesetzt, um Spitzenabflüsse von großen Hochwassern zu senken. Dadurch wird im Falle des Flutpoldereinsatzes die Hochwassergefährdung für flussab gelegene Flussabschnitte verringert. Flutpolder sind meist trockene Staubecken, die typischerweise durch landwirtschaftliche Nutzung gekennzeichnet sind. Ziel der Dissertation ist die Analyse von hydraulischen, ökologischen und ökonomischen Auswirkungen des Einsatzes von Flutpoldern, um daraus Schlussfolgerungen für ihre Bewirtschaftung zu ziehen. Dazu werden numerische Modelle zur Simulation der Hydrodynamik und Wassergüte sowie ein landwirtschaftliches Schadenmodell gemeinsam in einem integrativen Ansatz eingesetzt. Ein Untersuchungsgebiet ist ein existierendes Flutpoldersystem am Nebenfluss Havel, welches während der Elbeflut im Sommer 2002 zum Einsatz kam. Das zweite Untersuchungsgebiet ist ein geplanter Flutpolder, welcher sich bisher noch in einem frühen Planungsstadium befindet. Darüber hinaus werden numerische Modelle verschiedener räumlicher Dimensionalität von null- bis zwei-dimensional angewandt, um ihre Eignung für hydrodynamische und Wassergütesimulationen von Flutpoldern hinsichtlich der Leistungsfähigkeit und des Modellierungsaufwands zu bewerten. Die Dissertation schließt mit übergreifenden Empfehlungen zur Bewirtschaftung von Flutpoldern einschließlich Kontrollstrategien und Landnutzung ab. Im Hinblick auf zukünftige Änderungen in der Auftretenshäufigkeit von Hochwassern und weiterhin ansteigenden Werten von privatem und öffentlichem Vermögen in überflutungsgefährdeten Gebieten stellen Flutpolder ein effektive und flexible Maßnahmen des technischen Hochwasserschutzes dar, welche zu einem erfolgreichen Hochwasserrisikomanagement großer Tieflandflüsse beitragen.

Der großräumige Abbau von Braunkohle in der Lausitz führte in der Vergangenheit zu einer extremen Beeinflussung des Wasserhaushaltes im Einzugsgebiet der Spree. Mit dem Beginn der Sanierung und Flutung der Tagebaue kommt es nun langfristig zu einer verstärkten Nutzung der existierenden Oberflächengewässer und der Einbindung der entstehenden Tagebaurestseen in das Fließgewässernetz. Die Kopplung von Mengenbewirtschaftungsmodellen mit Gütemodellen berücksichtigt die Verfügbarkeit und Verteilung der begrenzten Ressource Wasser im Einzugsgebiet und der aus der Bewirtschaftung resultierenden Gewässergüte. Dies entspricht auch dem Leitbild der EU-WRRL (2000) für ein integriertes Flussgebietsmanagement, was eine einzugsgebietsbezogene Betrachtung der vorhandenen Ressourcen unter Berücksichtigung aller beeinflussten und beeinflussenden Kriterien fordert. Werden Modelle, die unterschiedlich sensitive und komplexe Systeme abbilden, miteinander gekoppelt, erfordert dies eine Anpassung der Datenstruktur und der zeitlichen Skalen. Schwerpunkt dieser Arbeit war die Entwicklung einfacher, robuster Simulationswerkzeuge für die Prognose der Gewässergüte in den Talsperren Bautzen und Quitzdorf. Als Basis diente das komplexe Standgewässergütemodell SALMO. Das Modell wurde zunächst um einfache Algorithmen ergänzt, so dass es trotz einer angepassten, stark reduzierten Datengrundlage, plausible Ergebnisse simulierte. Stochastisch erzeugte Bewirtschaftungsszenarien und die komplex simulierten Modellergebnisse bezüglich der resultierenden Gewässergüte, wurden als Trainingsdaten für ein Künstliches Neuronales Netz (ANN) genutzt. Die für beide Talsperren trainierten ANN sind als effektive Black-Box-Module in der Lage, das komplexe Systemverhalten des deterministischen Modells SALMO widerzuspiegeln. Durch eine Kopplung der entwickelten ANN mit dem Bewirtschaftungsmodell WBalMo ist es möglich, Bewirtschaftungsalternativen hinsichtlich ihrer Konsequenzen für die Gewässergüte zu bewerten. ANN sind systemgebundene Modelle, die nicht auf andere Gewässersysteme übertragen werden können. Allerdings stellt die hier erarbeitete Methodik einen fundierten Ansatz dar, der für die Entwicklung weiterer aggregierter Gütemodule im Rahmen integrierter Bewirtschaftungsmodelle angewendet werden kann. / The large-scale extraction of lignite in Lusatia in the past had an extreme impact on the water balance of the Spree river catchment. The restoration and flooding of the opencast pits put heavy demand on existing surface waters for a long time period. The resulting artificial lakes have to be integrated in the riverine network. The coupling of management models and water quality models allows to consider both availability and distribution of limited water resources in the catchment and resulting water quality. This is corresponding to the principles of the EU-WFD for integrated river basin management, which is a basin-related consideration of available resources taking into account all influencing and influenced characteristics. Adjustment of data structure and time scale is necessary if models describing unequally sensitive and complex systems are to be coupled. Main focus of this task was to develop simple and robust simulation tools for the prediction of water quality in the reservoirs Bautzen and Quitzdorf. The complex water quality model SALMO served as a basis. In a first step, simple algorithms had to be amended in order to generate plausible simulation results despite of an adapted reduced data base. Stochastically generated management scenarios and complex simulated model results regarding the resulting water quality were employed as training data for an Artificial Neuronal Network (ANN). The trained ANN’s are efficient black box modules. As such they are able to mirror complex system behaviour of the deterministic model SALMO. By coupling the developed ANN with the management model WBalMo it is possible to evaluate management strategies in terms of their impact on the quality of the water bodies. ANN’s are system-linked models. A transfer to other aquatic systems is not possible. However, the methodology developed here represents an in-depth approach which is applicable to the development of further aggregated water quality models in the framework of integrated management models.