This thesis examines water mixing and exchange in a drinking water reservoir operated by themunicipal association Norrvatten. Recent water samples from the reservoir’s outgoing waterhave shown an increase in culturable bacteria during late summer and fall. This thesis utilizesComputational Fluid Dynamics (CFD) modeling and analysis in OpenFOAM to simulatereservoir inflow and outflow, analyzing mixing processes and their relationship to operationalstrategies. The objective is to understand the correlation between the residence time of waterand microbial growth and propose operational improvements to increase the exchange of waterin order to achieve improved water quality. A trace element was implemented in the CFDmodel to simulate the residence time of water. Initial simulations were based on the reservoir’shistorical operational data, utilizing temperature and water level measurements providedby Norrvatten. After the initial simulations, four alternative simulations were performed,comparing different operational strategies by modifying inflow parameters. Inflow parametersthat were changed were the volumetric inflow rate, water level variation, and the temperatureof the inflowing water. The post­processing in ParaView focused on the thermal stratificationand residence time distribution near the outlet during each mixing process. The study revealeda complex relationship between flow conditions and microbial growth, making it challengingto identify a clear pattern. However, based on the simulations with the alternative operationalstrategies it was concluded that the set of operational strategies called ”Strategy 1” generated themost optimal flow conditions. This strategy involves a three times larger volumetric inflow rate(an increase from 0.05 to 0.15 m^3/s) and a water level that is kept at the same values comparedto the original simulation. Strategy 1 resulted in a 3.6 % higher water exchange compared to theoriginal simulation. In comparison to the other simulated strategies, Strategy 1 generates thehighest water exchange, with a 63.6 % increase compared to the worst­-case scenario involvingcolder inflow. The conclusion that could be drawn is that the most favorable operationalstrategies involve higher volumetric inflow rates, lower water levels, and an incoming watertemperature that is higher than the initial reservoir temperature.

Drinking water reservoirs are an important water source in many regions around the globe. As man-made structures, they are actively managed to provide raw water of adequate quality. Global change, and in particular global warming, is influencing the available water quantity and quality and is putting pressure on the management of drinking water reservoirs. To adapt management to the changing conditions, tools to evaluate different measurements, as well as predictions of potential future conditions, are required. Mechanistic models are the tool of choice to test adaptation measures and estimate potential future conditions. In this thesis, the impact of climate warming on the water quality of drinking water reservoirs, as well as potential mitigation strategies are investigated using one-dimensional mechanistic lake models. Therefore, three reservoirs located in Saxony, Germany are chosen as study sites. Specifically, the thesis evaluates the potential of adapting the withdrawal depth to mitigate the impact of global warming onto the water quality of reservoirs. To estimate the uncertainty associated with the used models, a framework for ensemble modeling of lakes and reservoirs is developed and applied to simulate climate impact predictions for a drinking water reservoir. The main findings of this thesis are: Adapting the withdrawal depth has the potential to mitigate some impact of global warming. Especially, the simulated impact of withdrawal depth on deep water temperature was about the same magnitude as the observed impact of global warming in the last 30 years. By using an ensemble approach, different sources of uncertainty were quantified and compared. For the climate impact simulation the largest uncertainty was found to be the epistemic uncertainty, which is related to the model structure. Nevertheless, the estimated trends for the climate scenario from all five applied models were coherent. The thesis furthers our knowledge of drinking water reservoir management in a warming climate. Using mechanistic models and ensemble techniques were shown to be an effective tool to compare different management strategies and evaluate uncertainties related to the modeling process. Further mitigation measures need to be developed to safeguard drinking water production under global warming, and methods presented in this thesis help to better evaluate strategies for managing drinking water reservoirs.:1 Introduction 1.1 Background 1.1.1 Down by the river: Drinking water reservoirs 1.1.2 Take care of business: Impact of management 1.1.3 All the world is green: Phytoplankton and lake physics 1.1.4 The times they are a-changing: Climate warming 1.1.5 OK computer: Process-based models 1.2 Study area 1.2.1 Investigated reservoirs 1.2.2 Local impact of climate warming 1.3 Aims and structure of the thesis 1.3.1 Research questions 1.3.2 Structure of the thesis 2 Managing climate change in drinking water reservoirs: potentials and limitations of dynamic withdrawal strategies 2.1 Background 2.2 Methods 2.2.1 Study area 2.2.2 Model input 2.2.3 Model setup 2.2.4 Management strategies 2.2.5 Statistical evaluation 2.3 Results 2.3.1 Observed trend 2.3.2 Validation of simulated temperature profiles 2.3.3 Modeled impact of management 2.3.4 External and internal forcing 2.4 Discussion 2.4.1 Observed trends in hydrophysical features 2.4.2 Modeled impact of management 2.4.3 Statistical model 2.4.4 Climate change and implications for management 2.5 Conclusion 3 LakeEnsemblR: An R package that facilitates ensemble modelling of lakes 3.1 Introduction 3.2 Methods 3.2.1 Model description 3.2.2 R package description 3.2.3 Getting started 3.2.4 Calibration algorithms 3.2.5 Combining multiple ensemble runs 3.3 Example application of LakeEnsemblR 3.3.1 Lough Feeagh: water temperature dynamics 3.3.2 Langtjern: lake ice dynamics 3.3.3 Uncertainty partitioning 3.3.4 Multi-parameter ensemble 3.3.5 Discussion 3.4 Summary 3.4.1 Framework 3.4.2 Recommendations for use 3.4.3 Outlook 4 Ensemble of models shows coherent response of a reservoir’s stratification and ice cover to climate warming 4.1 Introduction 4.2 Methods 4.2.1 Study site 4.2.2 Climate scenarios and data 4.2.3 Lake model ensemble 4.2.4 Calibration 4.2.5 Data evaluation 4.2.6 Uncertainty partitioning 4.3 Results 4.3.1 Observed trends 4.3.2 Model performance 4.3.3 Climate predictions 4.3.4 Uncertainty partitioning 4.4 Discussion 4.5 Conclusions 5 From T to P: Impact of withdrawal on water quality of a drinking water reservoir 5.1 Introduction 5.2 Methods 5.2.1 Study site and data 5.2.2 Applied models 5.2.3 Model calibration and sensitivity analysis 5.2.4 Withdrawal strategies 5.3 Results 5.3.1 Model calibration 5.3.2 Withdrawal strategies 5.4 Discussion 5.5 Conclusion 6 Synthesis 6.1 Lessons learned 6.1.1 Impact of global warming on reservoir water quality 6.1.2 Mitigation through adaptive withdrawal strategies 6.1.3 Using models for scenario analysis 6.2 Next steps 6.2.1 Further management and climate simulations 6.2.2 Model based real time decision support 6.2.3 Improve biogeochemical models 6.3 Concluding remarks A Appendix Chapter 2 A.1 Additional plots A.2 Principal component analysis A.3 Full linear model B Appendix Chapter 3 B.1 Format input files B.2 Additional figures B.3 Additional tables C Appendix Chapter 4 C.1 Additional Tables C.2 Additional Figures D Appendix Chapter 5 D.1 rodeoFABM D.2 Model description D.2.1 State variables D.2.2 Processes D.2.3 Parameter D.2.4 Stoichiometry D.3 Sensitivity analysis D.4 Additional Tables D.5 Additional Figures

In den letzten 15 Jahren wurde in einigen Mittelgebirgsregionen Zentraleuropas ein regional un-terschiedlich starker Anstieg der Konzentration gelösten organischen Kohlenstoffes (DOC) in Oberflächengewässern festgestellt. Besonders betroffen von dieser Entwicklung sind für die Trink-wasserproduktion genutzte Talsperren. Durch den erhöhten DOC-Gehalt wird die Trinkwasser-aufbereitung nicht nur kostenintensiver, sondern bekommt auch zunehmend Probleme bei der Wahrung der Trinkwasserqualität. Die Quellen des DOC sind in den Einzugsgebieten befindliche Moore und hydromorphe Nassstandorte. Die Ursachen für den in den vergangenen Jahren doku-mentierten Mehraustrag sind äußerst komplex und beinhalten sowohl klimatische als auch nut-zungsbedingte Veränderungen. Ziel der vorliegenden Arbeit ist, den Charakter des DOC näher zu bestimmen, um die für die Wasseraufbereitung problematischen Fraktionen zu benennen und Ansatzpunkte für regulierende Maßnahmen aufzuzeigen. Die Charakterisierung erfolgte an einem LC-OCD-System (Liquid Chromatography – Organic Carbon Detection) am Beispiel der drei Tal-sperrensysteme Muldenberg, Carlsfeld (Westerzgebirge) und Fláje (Osterzgebirge, Tschechien). Die Talsperren weisen unterschiedliche DOC-Gehalte auf. Die Ergebnisse zeigen, dass der DOC aller Untersuchungsgebiete sowohl in den Zuflüssen, dem Talsperrenwasser bzw. Rohwasser als auch dem Reinwasser der Wasserwerke von Huminstoffen dominiert wird. DOC-Schwankungen sind folglich auf Schwankungen der Huminstoffkonzentrationen zurückzuführen. In den Zuflüs-sen treten die höchsten Huminstoffkonzentrationen während der Schneeschmelze im Frühjahr und bei Starkniederschlägen im Sommer und Herbst auf. Niedrige Konzentrationen sind an Bo-dengefrornis oder Trockenphasen im Sommer gebunden. In den Talsperren tritt im Sommer eine Schichtung der Huminstoffe auf, bei der an der Oberfläche durch photolytischen Abbau niedrige-re Konzentrationen und in Abhängigkeit von der Talsperrentiefe durch die Einschichtung der Zuflüsse am Grund höhere Konzentrationen auftreten. Die Verteilung im Winter ist allein von der Einschichtung der Zuflüsse abhängig. Die Verteilung der Building Blocks (Huminstoffhydrolysate) ist im Sommer der der Huminstoffe entgegengesetzt, da sich beim photolytischen Abbau von Hu-minstoffen Building Blocks bilden. Während der Zirkulationen sind die Konzentrationen der Hu-minstofffraktionen im gesamten Tiefenprofil gleich. In den Talsperren Muldenberg und Carlsfeld wurde im Untersuchungszeitraum unter Berücksichtigung der technischen Gegebenheiten und anderer Wassergütekriterien der optimale Rohwasserentnahmehorizont (niedrigstmögliche Hu-minstoffkonzentration) gewählt. In der Talsperre Fláje könnte die Rohwasserqualität durch Instal-lation eines schwenkbaren Entnahmerohres erheblich verbessert werden. In den Wasserwerken sind die Huminstofffraktionen mit abnehmender Molekülgröße schwieriger zu entfernen. Die relativ niedermolekularen Building Blocks sind die am schwersten entfernbaren DOC-Bestandteile. Auf Grundlage mathematischer Beziehungen zu SAK254 bzw. DOC wurde ein Programm (DOCQuaC – DOC-Quality Calculator) entwickelt, mit dessen Hilfe die Huminstofffraktionen über diese Grundparameter ermittelt werden können. Neben der Fraktionsquantifizie-rung besteht im Programm die Möglichkeit zur Berechnung der Eliminierungsleistung, so dass beispielsweise nach der Durchführung einer Maßnahme zur Verbesserung der DOC-Entfernung eine unmittelbare Aussage über den Effekt auf die Fraktionen vorliegt. / For the last 15 years a change of Dissolved Organic Carbon (DOC) has been taking place in surface waters of Middle European Highlands. The degree of this increase is regionally dependent. The reasons as well as the consequences of this development are of high complexity. The changes are expressed in an increase of middle DOC values and, additionally, an increase of annual changes. Reservoirs, used for drinking water production, are particularly affected, because the heightened DOC content increases the costs for drinking water treatment, and currently it is not possible to maintain a good drinking water quality. The main DOC source are peat bogs, situated in the catchments. The cause of the increasing DOC output from peat bogs are very complex and contain changes as well in climate as in land use. The aim of this presented dissertation was to gain more information about the character of DOC, so that those fractions can be determined which are problematically to remove inside the waterworks. On that base it should be possible to find some starting points for more regulating measures. The analytical characterisation was carried out by the help of a LC-OCD-System (Liquid Chromatography – Organic Carbon Detection), exemplary for the reservoir systems Muldenberg, Carlsfeld (Western Ore Mountains) and Fláje (Eastern Ore Mountains, Czech Republic). Although the DOC concentrations are very different in the reser-voirs, it was always dominated by humic substances, namely as well in the tributaries, as in the raw and treated water. Thus changes in the DOC concentration are due to changes in the concen-tration of humic substances. In spring and summer occur the highest concentrations of humic substances in the tributaries. Low concentrations are linked with frost in winter or dry periods in summer. In the reservoirs in summer a stratification of humic substances shapes up, with lower concentration near the water surface (caused by photolytical degradation) and accordingly higher concentration near the ground (layering of influxes). Distribution in winter only depends on the layering of the influxes. In summer the concentration of Building Blocks (humic substance hydro-lysates) is opposite to that of humic substances, because of the creation of Building Blocks at the photolytical degradation. During circulation the concentration of all humic substance fractions are equal in each reservoir layer. In the reservoirs Muldenberg and Carlsfeld, a further optimisation of the raw water withdrawal horizon is not possible. In the reservoir Fláje the raw water could be much improved by installing a movable withdrawal device. With decreasing molecular size the elimination of the humic fractions gets more difficult in the waterworks. Most difficult to eliminate are Building Blocks. On the basis of the mathematical cor-relation between humic fractions and the SAC254 and DOC a program (DOCQuaC – DOC-Quality Calculator) has been developed, which enables die calculation of humic concentrations by enter-ing these general parameters. In addition to the calculation of the concentrations it is possible to calculate the elimination capacity for the humic fractions. Thus the effects of measures on the humic fractions, which could be done for optimisation of the elimination capacity, can be imme-diately observed.

In den letzten 15 Jahren wurde in einigen Mittelgebirgsregionen Zentraleuropas ein regional un-terschiedlich starker Anstieg der Konzentration gelösten organischen Kohlenstoffes (DOC) in Oberflächengewässern festgestellt. Besonders betroffen von dieser Entwicklung sind für die Trink-wasserproduktion genutzte Talsperren. Durch den erhöhten DOC-Gehalt wird die Trinkwasser-aufbereitung nicht nur kostenintensiver, sondern bekommt auch zunehmend Probleme bei der Wahrung der Trinkwasserqualität. Die Quellen des DOC sind in den Einzugsgebieten befindliche Moore und hydromorphe Nassstandorte. Die Ursachen für den in den vergangenen Jahren doku-mentierten Mehraustrag sind äußerst komplex und beinhalten sowohl klimatische als auch nut-zungsbedingte Veränderungen. Ziel der vorliegenden Arbeit ist, den Charakter des DOC näher zu bestimmen, um die für die Wasseraufbereitung problematischen Fraktionen zu benennen und Ansatzpunkte für regulierende Maßnahmen aufzuzeigen. Die Charakterisierung erfolgte an einem LC-OCD-System (Liquid Chromatography – Organic Carbon Detection) am Beispiel der drei Tal-sperrensysteme Muldenberg, Carlsfeld (Westerzgebirge) und Fláje (Osterzgebirge, Tschechien). Die Talsperren weisen unterschiedliche DOC-Gehalte auf. Die Ergebnisse zeigen, dass der DOC aller Untersuchungsgebiete sowohl in den Zuflüssen, dem Talsperrenwasser bzw. Rohwasser als auch dem Reinwasser der Wasserwerke von Huminstoffen dominiert wird. DOC-Schwankungen sind folglich auf Schwankungen der Huminstoffkonzentrationen zurückzuführen. In den Zuflüs-sen treten die höchsten Huminstoffkonzentrationen während der Schneeschmelze im Frühjahr und bei Starkniederschlägen im Sommer und Herbst auf. Niedrige Konzentrationen sind an Bo-dengefrornis oder Trockenphasen im Sommer gebunden. In den Talsperren tritt im Sommer eine Schichtung der Huminstoffe auf, bei der an der Oberfläche durch photolytischen Abbau niedrige-re Konzentrationen und in Abhängigkeit von der Talsperrentiefe durch die Einschichtung der Zuflüsse am Grund höhere Konzentrationen auftreten. Die Verteilung im Winter ist allein von der Einschichtung der Zuflüsse abhängig. Die Verteilung der Building Blocks (Huminstoffhydrolysate) ist im Sommer der der Huminstoffe entgegengesetzt, da sich beim photolytischen Abbau von Hu-minstoffen Building Blocks bilden. Während der Zirkulationen sind die Konzentrationen der Hu-minstofffraktionen im gesamten Tiefenprofil gleich. In den Talsperren Muldenberg und Carlsfeld wurde im Untersuchungszeitraum unter Berücksichtigung der technischen Gegebenheiten und anderer Wassergütekriterien der optimale Rohwasserentnahmehorizont (niedrigstmögliche Hu-minstoffkonzentration) gewählt. In der Talsperre Fláje könnte die Rohwasserqualität durch Instal-lation eines schwenkbaren Entnahmerohres erheblich verbessert werden. In den Wasserwerken sind die Huminstofffraktionen mit abnehmender Molekülgröße schwieriger zu entfernen. Die relativ niedermolekularen Building Blocks sind die am schwersten entfernbaren DOC-Bestandteile. Auf Grundlage mathematischer Beziehungen zu SAK254 bzw. DOC wurde ein Programm (DOCQuaC – DOC-Quality Calculator) entwickelt, mit dessen Hilfe die Huminstofffraktionen über diese Grundparameter ermittelt werden können. Neben der Fraktionsquantifizie-rung besteht im Programm die Möglichkeit zur Berechnung der Eliminierungsleistung, so dass beispielsweise nach der Durchführung einer Maßnahme zur Verbesserung der DOC-Entfernung eine unmittelbare Aussage über den Effekt auf die Fraktionen vorliegt. / For the last 15 years a change of Dissolved Organic Carbon (DOC) has been taking place in surface waters of Middle European Highlands. The degree of this increase is regionally dependent. The reasons as well as the consequences of this development are of high complexity. The changes are expressed in an increase of middle DOC values and, additionally, an increase of annual changes. Reservoirs, used for drinking water production, are particularly affected, because the heightened DOC content increases the costs for drinking water treatment, and currently it is not possible to maintain a good drinking water quality. The main DOC source are peat bogs, situated in the catchments. The cause of the increasing DOC output from peat bogs are very complex and contain changes as well in climate as in land use. The aim of this presented dissertation was to gain more information about the character of DOC, so that those fractions can be determined which are problematically to remove inside the waterworks. On that base it should be possible to find some starting points for more regulating measures. The analytical characterisation was carried out by the help of a LC-OCD-System (Liquid Chromatography – Organic Carbon Detection), exemplary for the reservoir systems Muldenberg, Carlsfeld (Western Ore Mountains) and Fláje (Eastern Ore Mountains, Czech Republic). Although the DOC concentrations are very different in the reser-voirs, it was always dominated by humic substances, namely as well in the tributaries, as in the raw and treated water. Thus changes in the DOC concentration are due to changes in the concen-tration of humic substances. In spring and summer occur the highest concentrations of humic substances in the tributaries. Low concentrations are linked with frost in winter or dry periods in summer. In the reservoirs in summer a stratification of humic substances shapes up, with lower concentration near the water surface (caused by photolytical degradation) and accordingly higher concentration near the ground (layering of influxes). Distribution in winter only depends on the layering of the influxes. In summer the concentration of Building Blocks (humic substance hydro-lysates) is opposite to that of humic substances, because of the creation of Building Blocks at the photolytical degradation. During circulation the concentration of all humic substance fractions are equal in each reservoir layer. In the reservoirs Muldenberg and Carlsfeld, a further optimisation of the raw water withdrawal horizon is not possible. In the reservoir Fláje the raw water could be much improved by installing a movable withdrawal device. With decreasing molecular size the elimination of the humic fractions gets more difficult in the waterworks. Most difficult to eliminate are Building Blocks. On the basis of the mathematical cor-relation between humic fractions and the SAC254 and DOC a program (DOCQuaC – DOC-Quality Calculator) has been developed, which enables die calculation of humic concentrations by enter-ing these general parameters. In addition to the calculation of the concentrations it is possible to calculate the elimination capacity for the humic fractions. Thus the effects of measures on the humic fractions, which could be done for optimisation of the elimination capacity, can be imme-diately observed.