@phdthesis{Mamede2008,
  author    = {Mamede, George Leite},
  title     = {Reservoir sedimentation in dryland catchments : modelling and management},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-17047},
  school      = {Universit{\"a}t Potsdam},
  year      = {2008},
  abstract  = {Semi-arid environments are mainly characterized by scarce water resources and are usually subject to risks of water stress. In these regions, water supply for drinking and irrigation purposes depends strongly on storage in surface reservoirs and sediment deposition in these reservoirs affects adversely the water storage. In order to reproduce the complex behaviour of sediment deposition in reservoirs located in semi-arid environments and the effects of using sediment management techniques, a reservoir sedimentation model is developed and coupled within the WASA-SED model, which simulates rainfall-runoff processes and sediment transport at the hillslope and river network. The reservoir sedimentation model consists of two modelling approaches, which may be applied according to reservoir size and data availability. For reservoirs with information about their geometric features (reservoir topography, stage-area and stage-volume curves) and physical properties of sediment deposits, such as deposition thickness, grain size distribution of sediment deposits and sediment densities, a detailed modelling approach of reservoir sedimentation may be applied. For reservoirs without those characteristics, a simplified modelling approach is used. The detailed modelling approach of reservoir sedimentation enables the assessment of sediment deposition pattern in reservoirs and the evaluation of sediment release efficiency of sediment management techniques. It simulates sediment transport along the longitudinal profile of a reservoir. The reservoir is divided into cross sections to elaborate the sediment budget. The sediment transport component is calculated using a non-uniform sediment transport approach based on the concept of sediment carrying capacity. Four different sediment-transport equations can be selected for the simulations. The simplified modelling approach of reservoir sedimentation is suitable to simulate water and sediment transfer in dense reservoirs network. Nevertheless, it allows simulating neither sediment management techniques, nor spatial distribution of sedimentation. In this approach, the reservoirs are classified into small and strategic reservoirs according to their location and size. Strategic reservoirs are medium and large-sized reservoirs located on main rivers at the sub-basin's outlet or reservoirs of particular interest. The small reservoirs are located at tributary streams and represented in the model in an aggregate manner by grouping them into size classes according to their storage capacity. A cascade routing scheme is used to describe the upstream-downstream position of the reservoir classes. The water and sediment balances of small reservoirs are computed for one hypothetical representative reservoir of mean characteristics. Sediment trapping efficiency and effluent grain size distribution are estimated using the overflow rate concept. Three model applications are carried out within this research, as follows: • The detailed modelling approach of reservoir sedimentation is applied to the 92.2 Mm³ Barasona Reservoir, located in the foothills of the Central Pyrenees (Aragon, Spain). A two-stage calibration was performed to account for changes on the sediment deposition pattern caused by sediment management. The reservoir sedimentation model is then validated for another simulation period which confirms that the processes related to reservoir sedimentation are well represented by the model. • An application is carried out to the 933-km² Bengu{\^e} catchment, located in the semi-arid region of Northeast Brazil. The catchment is characterized by a dense reservoir network, covering almost 45\% of the catchment area, with a significant lack of data. Water and sediment balances of those reservoirs are computed using the simplified modelling approach. Three spatial configurations describing the cascade routing scheme are tested. • The reservoir sedimentation model is applied again to the Barasona reservoir to evaluate the sediment release efficiency of sediment management strategies. Cost analysis is presented to help in the choice of the most promising sediment management technique for that situation. Thus, the model enables the assessment of technical features of the sediment management strategies. Overall, simulation results are characterized by large uncertainties, partly due to low data availability and also due to uncertainties of the model structure to adequately represent the processes related to reservoir sedimentation.},
  language  = {de}
}
@phdthesis{Wriedt2004,
  author    = {Wriedt, Gunter},
  title     = {Modelling of nitrogen transport and turnover during soil and groundwater passage in a small lowland catchment of Northern Germany},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-0001307},
  school      = {Universit{\"a}t Potsdam},
  year      = {2004},
  abstract  = {Stoffumsatzreaktionen und hydraulische Prozesse im Boden und Grundwasser k{\"o}nnen in Tieflandeinzugsgebieten zu einer Nitratretention f{\"u}hren. Die Untersuchung dieser Prozesse in Raum und Zeit kann mit Hilfe geeigneter Modelle erfolgen. Ziele dieser Arbeit sind: i) die Entwicklung eines geeigneten Modellansatzes durch Kombination von Teilmodellen zur Simulation des N-Transportes im Boden und Grundwasser von Tieflandeinzugsgebieten und ii) die Untersuchung von Wechselwirkungen zwischen Gebietseigenschaften und N-Transport unter besonderer Ber{\"u}cksichtigung der potentiellen N-Zufuhr in die Oberfl{\"a}chengew{\"a}sser. Der Modellansatz basiert auf der Kombination verschiedener Teilmodelle: das Bodenwasser- und -stickstoffmodell mRISK-N, das Grundwassermodell MODFLOW und das Stofftransportmodell RT3D. Zur Untersuchung der Wechselwirkungen mit den Gebietseigenschaften muss die Verteilung und Verf{\"u}gbarkeit von Reaktionspartnern ber{\"u}cksichtigt werden. Dazu wurde ein Reaktionsmodul entwickelt, welches chemische Prozesse im Grundwasser simuliert. Hierzu geh{\"o}ren die Mineralisation organischer Substanz durch Sauerstoff, Nitrat und Sulfat sowie die Pyritoxidation durch Sauerstoff und Nitrat. Der Modellansatz wurde in verschiedenen Einzelstudien angewandt, wobei jeweils bestimmte Teilmodelle im Vordergrund stehen. Alle Modellstudien basieren auf Daten aus dem Schaugrabeneinzugsgebiet (ca. 25 km\&\#178;), in der N{\"a}he von Osterburg(Altmark) im Norden Sachsen-Anhalts. Die folgenden Einzelstudien wurden durchgef{\"u}hrt: i) Evaluation des Bodenmodells anhand von Lysimeterdaten, ii) Modellierung eines Tracerexperimentes im Feldmaßstab als eine erste Anwendung des Reaktionsmoduls, iii) Untersuchung hydraulisch-chemischer Wechselwirkungen an einem 2D-Grundwassertransekt, iv) Fl{\"a}chenverteilte Modellierung von Grundwasserneubildung und Bodenstickstoffaustrag im Untersuchungsgebiet als Eingangsdaten f{\"u}r nachfolgende Grundwassersimulationen, und v) Untersuchung der Ausbreitung von Nitrat im Grundwasser und des Durchbruchs in die Oberfl{\"a}chengew{\"a}sser im Untersuchungsgebiet auf Basis einer 3D-Modellierung von Grundwasserstr{\"o}mung und reaktivem Stofftransport. Die Modellstudien zeigen, dass der Modellansatz geeignet ist, die Wechselwirkungen zwischen Stofftransport und \–umsatz und den hydraulisch-chemischen Gebietseigenschaften zu modellieren. Die Ausbreitung von Nitrat im Sediment wird wesentlich von der Verf{\"u}gbarkeit reaktiver Substanzen sowie der Verweilzeit im Grundwasserleiter bestimmt. Bei der Simulation des Untersuchungsgebietes wurde erst nach 70 Jahren eine der gegebenen Eintragssitutation entsprechende Nitratkonzentration im Grundwasserzustrom zum Grabensystem erreicht (konservativer Transport). Die Ber{\"u}cksichtigung von reaktivem Stofftransport f{\"u}hrt zu einer deutlichen Reduktion der Nitratkonzentrationen. Die Modellergebnisse zeigen, dass der Grundwasserzustrom die beobachtete Nitratbelastung im Grabensystem nicht erkl{\"a}ren kann, da der Großteil des Nitrates durch Denitrifikation verloren geht. Andere Quellen, wie direkte Eintr{\"a}ge oder Dr{\"a}nagenzufl{\"u}sse m{\"u}ssen ebenfalls in Betracht gezogen werden. Die Prognosef{\"a}higkeit des Modells f{\"u}r das Untersuchungsgebiet wird durch die Datenunsicherheiten und die Sch{\"a}tzung der Modellparameter eingeschr{\"a}nkt. Dennoch ist der Modellansatz eine wertvolle Hilfe bei der Identifizierung von belastungsrelevanten Teilfl{\"a}chen (Stoffquellen und -senken) sowie bei der Modellierung der Auswirkungen von Managementmaßnahmen oder Landnutzungsver{\"a}nderungen auf Grundlage von Szenario-Simulationen. Der Modellansatz unterst{\"u}tzt auch die Interpretation von Beobachtungsdaten, da so die lokalen Informationen in einen r{\"a}umlichen und zeitlichen Zusammenhang gestellt werden k{\"o}nnen.},
  language  = {en}
}
@phdthesis{Reusser2011,
  author    = {Reusser, Dominik Edwin},
  title     = {Combining smart model diagnostics and effective data collection for snow catchments},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-52574},
  school      = {Universit{\"a}t Potsdam},
  year      = {2011},
  abstract  = {Complete protection against flood risks by structural measures is impossible. Therefore flood prediction is important for flood risk management. Good explanatory power of flood models requires a meaningful representation of bio-physical processes. Therefore great interest exists to improve the process representation. Progress in hydrological process understanding is achieved through a learning cycle including critical assessment of an existing model for a given catchment as a first step. The assessment will highlight deficiencies of the model, from which useful additional data requirements are derived, giving a guideline for new measurements. These new measurements may in turn lead to improved process concepts. The improved process concepts are finally summarized in an updated hydrological model. In this thesis I demonstrate such a learning cycle, focusing on the advancement of model evaluation methods and more cost effective measurements. For a successful model evaluation, I propose that three questions should be answered: 1) when is a model reproducing observations in a satisfactory way? 2) If model results deviate, of what nature is the difference? And 3) what are most likely the relevant model components affecting these differences? To answer the first two questions, I developed a new method to assess the temporal dynamics of model performance (or TIGER - TIme series of Grouped Errors). This method is powerful in highlighting recurrent patterns of insufficient model behaviour for long simulation periods. I answered the third question with the analysis of the temporal dynamics of parameter sensitivity (TEDPAS). For calculating TEDPAS, an efficient method for sensitivity analysis is necessary. I used such an efficient method called Fourier Amplitude Sensitivity Test, which has a smart sampling scheme. Combining the two methods TIGER and TEDPAS provided a powerful tool for model assessment. With WaSiM-ETH applied to the Weisseritz catchment as a case study, I found insufficient process descriptions for the snow dynamics and for the recession during dry periods in late summer and fall. Focusing on snow dynamics, reasons for poor model performance can either be a poor representation of snow processes in the model, or poor data on snow cover, or both. To obtain an improved data set on snow cover, time series of snow height and temperatures were collected with a cost efficient method based on temperature measurements on multiple levels at each location. An algorithm was developed to simultaneously estimate snow height and cold content from these measurements. Both, snow height and cold content are relevant quantities for spring flood forecasting. Spatial variability was observed at the local and the catchment scale with an adjusted sampling design. At the local scale, samples were collected on two perpendicular transects of 60 m length and analysed with geostatistical methods. The range determined from fitted theoretical variograms was within the range of the sampling design for 80\% of the plots. No patterns were found, that would explain the random variability and spatial correlation at the local scale. At the watershed scale, locations of the extensive field campaign were selected according to a stratified sample design to capture the combined effects of elevation, aspect and land use. The snow height is mainly affected by the plot elevation. The expected influence of aspect and land use was not observed. To better understand the deficiencies of the snow module in WaSiM-ETH, the same approach, a simple degree day model was checked for its capability to reproduce the data. The degree day model was capable to explain the temporal variability for plots with a continuous snow pack over the entire snow season, if parameters were estimated for single plots. However, processes described in the simple model are not sufficient to represent multiple accumulation-melt-cycles, as observed for the lower catchment. Thus, the combined spatio-temporal variability at the watershed scale is not captured by the model. Further tests on improved concepts for the representation of snow dynamics at the Weißeritz are required. From the data I suggest to include at least rain on snow and redistribution by wind as additional processes to better describe spatio-temporal variability. Alternatively an energy balance snow model could be tested. Overall, the proposed learning cycle is a useful framework for targeted model improvement. The advanced model diagnostics is valuable to identify model deficiencies and to guide field measurements. The additional data collected throughout this work helps to get a deepened understanding of the processes in the Weisseritz catchment.},
  language  = {en}
}
@phdthesis{Francke2009,
  author    = {Francke, Till},
  title     = {Measurement and modelling of water and sediment fluxes in meso-scale dryland catchments},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-31525},
  school      = {Universit{\"a}t Potsdam},
  year      = {2009},
  abstract  = {Water shortage is a serious threat for many societies worldwide. In drylands, water management measures like the construction of reservoirs are affected by eroded sediments transported in the rivers. Thus, the capability of assessing water and sediment fluxes at the river basin scale is of vital importance to support management decisions and policy making. This subject was addressed by the DFG-funded SESAM-project (Sediment Export from large Semi-Arid catchments: Measurements and Modelling). As a part of this project, this thesis focuses on (1) the development and implementation of an erosion module for a meso-scale catchment model, (2) the development of upscaling and generalization methods for the parameterization of such model, (3) the execution of measurements to obtain data required for the modelling and (4) the application of the model to different study areas and its evaluation. The research was carried out in two meso-scale dryland catchments in NE-Spain: Ribera Salada (200 km²) and Is{\´a}bena (450 km²). Adressing objective 1, WASA-SED, a spatially semi-distributed model for water and sediment transport at the meso-scale was developed. The model simulates runoff and erosion processes at the hillslope scale, transport processes of suspended and bedload fluxes in the river reaches, and retention and remobilisation processes of sediments in reservoirs. This thesis introduces the model concept, presents current model applications and discusses its capabilities and limitations. Modelling at larger scales faces the dilemma of describing relevant processes while maintaining a manageable demand for input data and computation time. WASA-SED addresses this challenge by employing an innovative catena-based upscaling approach: the landscape is represented by characteristic toposequences. For deriving these toposequences with regard to multiple attributes (eg. topography, soils, vegetation) the LUMP-algorithm (Landscape Unit Mapping Program) was developed and related to objective 2. It incorporates an algorithm to retrieve representative catenas and their attributes, based on a Digital Elevation Model and supplemental spatial data. These catenas are classified to provide the discretization for the WASA-SED model. For objective 3, water and sediment fluxes were monitored at the catchment outlet of the Is{\´a}bena and some of its sub-catchments. For sediment yield estimation, the intermittent measurements of suspended sediment concentration (SSC) had to be interpolated. This thesis presents a comparison of traditional sediment rating curves (SRCs), generalized linear models (GLMs) and non-parametric regression using Random Forests (RF) and Quantile Regression Forests (QRF). The observed SSCs are highly variable and range over six orders of magnitude. For these data, traditional SRCs performed poorly, as did GLMs, despite including other relevant process variables (e.g. rainfall intensities, discharge characteristics). RF and QRF proved to be very robust and performed favourably for reproducing sediment dynamics. QRF additionally excels in providing estimates on the accuracy of the predictions. Subsequent analysis showed that most of the sediment was exported during intense storms of late summer. Later floods yielded successively less sediment. Comparing sediment generation to yield at the outlet suggested considerable storage effects within the river channel. Addressing objective 4, the WASA-SED model was parameterized for the two study areas in NE Spain and applied with different foci. For Ribera Salada, the uncalibrated model yielded reasonable results for runoff and sediment. It provided quantitative measures of the change in runoff and sediment yield for different land-uses. Additional land management scenarios were presented and compared to impacts caused by climate change projections. In contrast, the application for the Is{\´a}bena focussed on exploring the full potential of the model's predictive capabilities. The calibrated model achieved an acceptable performance for the validation period in terms of water and sediment fluxes. The inadequate representation of the lower sub-catchments inflicted considerable reductions on model performance, while results for the headwater catchments showed good agreement despite stark contrasts in sediment yield. In summary, the application of WASA-SED to three catchments proved the model framework to be a practicable multi-scale approach. It successfully links the hillslope to the catchment scale and integrates the three components hillslope, river and reservoir in one model. Thus, it provides a feasible approach for tackling issues of water and sediment yield at the meso-scale. The crucial role of processes like transmission losses and sediment storage in the river has been identified. Further advances can be expected when the representation of connectivity of water and sediment fluxes (intra-hillslope, hillslope-river, intra-river) is refined and input data improves.},
  language  = {en}
}