TY  - THES
A1  - Reich, Marvin
T1  - Advances in hydrogravimetry
T1  - Weiterentwicklung der Hydrogravimetrie
BT  - terrestrial gravimeters as field tools for hydrological applications
BT  - terrestrische Gravimeter als Messgeräte für hydrologische Anwendungen
N2  - The interest of the hydrological community in the gravimetric method has steadily increased within the last decade. This is reflected by numerous studies from many different groups with a broad range of approaches and foci. Many of those are traditionally rather hydrology-oriented groups who recognized gravimetry as a potential added value for their hydrological investigations. While this resulted in a variety of interesting and useful findings, contributing to extend the respective knowledge and confirming the methodological potential, on the other hand, many interesting and unresolved questions emerged.
This thesis manifests efforts, analyses and solutions carried out in this regard. Addressing and evaluating many of those unresolved questions, the research contributes to advancing hydrogravimetry, the combination of gravimetric and hydrological methods, in showing how gravimeters are a highly useful tool for applied hydrological field research.

In the first part of the thesis, traditional setups of stationary terrestrial superconducting gravimeters are addressed. They are commonly installed within a dedicated building, the impermeable structure of which shields the underlying soil from natural exchange of water masses (infiltration, evapotranspiration, groundwater recharge). As gravimeters are most sensitive to mass changes directly beneath the meter, this could impede their suitability for local hydrological process investigations, especially for near-surface water storage changes (WSC). By studying temporal local hydrological dynamics at a dedicated site equipped with traditional hydrological measurement devices, both below and next to the building, the impact of these absent natural dynamics on the gravity observations were quantified. A comprehensive analysis with both a data-based and model-based approach led to the development of an alternative method for dealing with this limitation. Based on determinable parameters, this approach can be transferred to a broad range of measurement sites where gravimeters are deployed in similar structures. Furthermore, the extensive considerations on this topic enabled a more profound understanding of this so called umbrella effect.

The second part of the thesis is a pilot study about the field deployment of a superconducting gravimeter. A newly developed field enclosure for this gravimeter was tested in an outdoor installation adjacent to the building used to investigate the umbrella effect. Analyzing and comparing the gravity observations from both indoor and outdoor gravimeters showed performance with respect to noise and stable environmental conditions was equivalent while the sensitivity to near-surface WSC was highly increased for the field deployed instrument. Furthermore it was demonstrated that the latter setup showed gravity changes independent of the depth where mass changes occurred, given their sufficiently wide horizontal extent. As a consequence, the field setup suits monitoring of WSC for both short and longer time periods much better. Based on a coupled data-modeling approach, its gravity time series was successfully used to infer and quantify local water budget components (evapotranspiration, lateral subsurface discharge) on the daily to annual time scale.

The third part of the thesis applies data from a gravimeter field deployment for applied hydrological process investigations. To this end, again at the same site, a sprinkling experiment was conducted in a 15 x 15 m area around the gravimeter. A simple hydro-gravimetric model was developed for calculating the gravity response resulting from water redistribution in the subsurface. It was found that, from a theoretical point of view, different subsurface water distribution processes (macro pore flow, preferential flow, wetting front advancement, bypass flow and perched water table rise) lead to a characteristic shape of their resulting gravity response curve. Although by using this approach it was possible to identify a dominating subsurface water distribution process for this site, some clear limitations stood out. Despite the advantage for field installations that gravimetry is a non-invasive and integral method, the problem of non-uniqueness could only be overcome by additional measurements (soil moisture, electric resistivity tomography) within a joint evaluation. Furthermore, the simple hydrological model was efficient for theoretical considerations but lacked the capability to resolve some heterogeneous spatial structures of water distribution up to a needed scale. Nevertheless, this unique setup for plot to small scale hydrological process research underlines the high potential of gravimetery and the benefit of a field deployment. 

The fourth and last part is dedicated to the evaluation of potential uncertainties arising from the processing of gravity observations. The gravimeter senses all mass variations in an integral way, with the gravitational attraction being directly proportional to the magnitude of the change and inversely proportional to the square of the distance of the change. Consequently, all gravity effects (for example, tides, atmosphere, non-tidal ocean loading, polar motion, global hydrology and local hydrology) are included in an aggregated manner. To isolate the signal components of interest for a particular investigation, all non-desired effects have to be removed from the observations. This process is called reduction. The large-scale effects (tides, atmosphere, non-tidal ocean loading and global hydrology) cannot be measured directly and global model data is used to describe and quantify each effect. Within the reduction process, model errors and uncertainties propagate into the residual, the result of the reduction. The focus of this part of the thesis is quantifying the resulting, propagated uncertainty for each individual correction. Different superconducting gravimeter installations were evaluated with respect to their topography, distance to the ocean and the climate regime. Furthermore, different time periods of aggregated gravity observation data were assessed, ranging from 1 hour up to 12 months. It was found that uncertainties were highest for a frequency of 6 months and smallest for hourly frequencies. Distance to the ocean influences the uncertainty of the non-tidal ocean loading component, while geographical latitude affects uncertainties of the global hydrological component. It is important to highlight that the resulting correction-induced uncertainties in the residual have the potential to mask the signal of interest, depending on the signal magnitude and its frequency. These findings can be used to assess the value of gravity data across a range of applications and geographic settings.

In an overarching synthesis all results and findings are discussed with a general focus on their added value for bringing hydrogravimetric field research to a new level. The conceptual and applied methodological benefits for hydrological studies are highlighted. Within an outlook for future setups and study designs, it was once again shown what enormous potential is offered by gravimeters as hydrological field tools.
N2  - Gravimetrie ist eine geophysikalische Methode, bei der Massen und deren Veränderungen beobachtet und gemessen werden. Die Messgeräte der Gravimetrie heißen Gravimeter. Wenn man diese Methode in der Erforschung von Wasser-relevanten Fragestellungen, Prozessen und Zuständen einsetzt (Hydrologie), spricht man auch von Hydrogravimetrie.
Die vorliegende Dissertation beschäftigt sich damit wie diese hydrogravimetrische Methode für angewandte Forschung im Feld benutzt wird und weiterentwickelt werden kann.

Zuerst wird thematisiert, wie konventionelle Aufbauten mit Gravimetern aussehen und was daran aus der hydrologischen Perspektive problematisch ist. Das Gebäude in dem sich das Gravimeter befindet, stellt eine große versiegelte Fläche dar, die es verhindert, dass in der direktem Umgebung natürliche Prozesse ablaufen. Das ist so problematisch, weil das Gravimeter besonders empfindlich auf Massänderungen in nächster räumlicher Nähe reagiert. Als Lösung wird mit Hilfe einer neuen Methode aufgezeigt, wie man unter Benutzung von traditionellen hydrologischen Messinstrumenten um das Gebäude herum diese verhinderten natürlichen Prozesse beschreiben kann.

Darauf folgend wird anhand eines erfolgreich getesteten Aufbaus eines Gravimeters außerhalb von einem Gebäude, also direkt im Gelände, demonstriert, was solch eine Außeninstallation für einen großen Vorteil für die hydrologische Feldforschung mit sich bringt. Darüberhinaus wird gezeigt, dass dieser alternative Aufbau keinerlei Nachteile hinsichtlich Genauigkeit, Qualität, Rauschen oder Beherrschbarkeit von Umwelteinflüssen mit sich bringt, sondern vor allem die Empfindlichkeit für Messungen von Wassermassenänderungen in Oberflächennähe stark verbessert.

Anhand eines Beregnungsexperiments auf der Fläche um dieses im Gelände installierten Gravimeters werden die Vorzüge der gravimetrischen Methode für die hydrologische Prozessforschung aufgezeigt. Verschiedene mögliche Ausbreitungen des verregneten Wassers im Untergrund können mittels dieser Methode charakterisiert und identifiziert werden.

Im letzten Teil wird das Problem von Unsicherheiten besprochen, die aus der notwendigen Datenbearbeitung resultieren. Um die gravimetrischen Beobachtungen auf die Anteile zu reduzieren, die innerhalb einer Studie betrachtet werden sollen, müssen alle Komponenten die das Gravimeter misst, die aber die hydrologische Interpretation stören, beseitigt werden. Dabei handelt es sich vor allem um globale Komponenten wie Gezeiten, Luftdruckschwankungen, Gezeiten-unabhängige Meeresströmungen und globale Hydrologie. Es wird untersucht, welche Unsicherheiten bei deren Korrektur auftreten, wenn verschiedene Zeitintervalle von zu beobachtenden hydrologischen Signalen vorherrschen.

Alle gewonnenen Resultate und Erfahrungen werden in einer gesamtheitlichen Betrachtung dahingehend diskutiert, wie die hydrogravimetrische Methode aufgrund dieser neuen Erkenntnisse verbessert und vorangebracht werden konnte.
KW  - hydrology
KW  - gravimetry
KW  - hydrogravimetry
KW  - fieldwork
KW  - hydrological modelling
KW  - geophysical methods
KW  - Feldarbeit
KW  - geophysikalische Methoden
KW  - Gravimetrie
KW  - Hydrogravimetrie
KW  - hydrologische Modellierung
KW  - Hydrologie
Y1  - 2023
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-604794
ER  - 
TY  - JOUR
A1  - Vormoor, Klaus Josef
A1  - Heistermann, Maik
A1  - Bronstert, Axel
A1  - Lawrence, Deborah
T1  - Hydrological model parameter (in)stability
BT  - "crash testing" the HBV model under contrasting flood seasonality conditions
JF  - Hydrological sciences journal = Journal des sciences hydrologiques
N2  - This paper investigates the transferability of calibrated HBV model parameters under stable and contrasting conditions in terms of flood seasonality and flood generating processes (FGP) in five Norwegian catchments with mixed snowmelt/rainfall regimes. We apply a series of generalized (differential) split-sample tests using a 6-year moving window over (i) the entire runoff observation periods, and (ii) two subsets of runoff observations distinguished by the seasonal occurrence of annual maximum floods during either spring or autumn. The results indicate a general model performance loss due to the transfer of calibrated parameters to independent validation periods of -5 to -17%, on average. However, there is no indication that contrasting flood seasonality exacerbates performance losses, which contradicts the assumption that optimized parameter sets for snowmelt-dominated floods (during spring) perform particularly poorly on validation periods with rainfall-dominated floods (during autumn) and vice versa.
KW  - hydrological modelling
KW  - flood seasonality
KW  - differential split-sample test
KW  - flood generating processes
KW  - Nordic catchments
Y1  - 2018
U6  - https://doi.org/10.1080/02626667.2018.1466056
SN  - 0262-6667
SN  - 2150-3435
VL  - 63
IS  - 7
SP  - 991
EP  - 1007
PB  - Routledge, Taylor & Francis Group
CY  - Abingdon
ER  - 
TY  - GEN
A1  - Vormoor, Klaus Josef
A1  - Heistermann, Maik
A1  - Bronstert, Axel
A1  - Lawrence, Deborah
T1  - Hydrological model parameter (in)stability
BT  - “crash testing” the HBV model under contrasting flood seasonality conditions
T2  - Hydrological Sciences Journal
N2  - This paper investigates the transferability of calibrated HBV model parameters under stable and contrasting conditions in terms of flood seasonality and flood generating processes (FGP) in five Norwegian catchments with mixed snowmelt/rainfall regimes. We apply a series of generalized (differential) split-sample tests using a 6-year moving window over (i) the entire runoff observation periods, and (ii) two subsets of runoff observations distinguished by the seasonal occurrence of annual maximum floods during either spring or autumn. The results indicate a general model performance loss due to the transfer of calibrated parameters to independent validation periods of −5 to −17%, on average. However, there is no indication that contrasting flood seasonality exacerbates performance losses, which contradicts the assumption that optimized parameter sets for snowmelt-dominated floods (during spring) perform particularly poorly on validation periods with rainfall-dominated floods (during autumn) and vice versa.
T3  - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 459 
KW  - hydrological modelling
KW  - flood seasonality
KW  - differential split-sample test
KW  - flood generating processes
KW  - Nordic catchments
Y1  - 2018
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-413008
ER  - 
TY  - GEN
A1  - Kormann, Christoph
A1  - Bronstert, Axel
A1  - Francke, Till
A1  - Recknagel, Thomas
A1  - Gräff, Thomas
T1  - Model-Based attribution of high-resolution streamflow trends in two alpine basins of Western Austria
N2  - Several trend studies have shown that hydrological conditions are changing considerably in the Alpine region. However, the reasons for these changes are only partially understood and trend analyses alone are not able to shed much light. Hydrological modelling is one possible way to identify the trend drivers, i.e., to attribute the detected streamflow trends, given that the model captures all important processes causing the trends. We modelled the hydrological conditions for two alpine catchments in western Austria (a large, mostly lower-altitude catchment with wide valley plains and a nested high-altitude, glaciated headwater catchment) with the distributed, physically-oriented WaSiM-ETH model, which includes a dynamical glacier module. The model was calibrated in a transient mode, i.e., not only on several standard goodness measures and glacier extents, but also in such a way that the simulated streamflow trends fit with the observed ones during the investigation period 1980 to 2007. With this approach, it was possible to separate streamflow components, identify the trends of flow components, and study their relation to trends in atmospheric variables. In addition to trends in annual averages, highly resolved trends for each Julian day were derived, since they proved powerful in an earlier, data-based attribution study. We were able to show that annual and highly resolved trends can be modelled sufficiently well. The results provide a holistic, year-round picture of the drivers of alpine streamflow changes: Higher-altitude catchments are strongly affected by earlier firn melt and snowmelt in spring and increased ice melt throughout the ablation season. Changes in lower-altitude areas are mostly caused by earlier and lower snowmelt volumes. All highly resolved trends in streamflow and its components show an explicit similarity to the local temperature trends. Finally, results indicate that evapotranspiration has been increasing in the lower altitudes during the study period.
T3  - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 364 
KW  - trend attribution
KW  - trend detection
KW  - climate change
KW  - trend drivers
KW  - hydrological modelling
KW  - alpine catchments
KW  - streamflow
KW  - hydroclimatology
Y1  - 2017
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-400641
ER  - 
TY  - THES
A1  - Pilz, Tobias
T1  - Pursuing the understanding of uncertainties in hydrological modelling
N2  - Hydrological models are important tools for the simulation and quantification of the water cycle.
They therefore aid in the understanding of hydrological processes, prediction of river discharge, assessment of the impacts of land use and climate changes, or the management of water resources.
However, uncertainties associated with hydrological modelling are still large.
While significant research has been done on the quantification and reduction of uncertainties, there are still fields which have gained little attention so far, such as model structural uncertainties that are related to the process implementations in the models.
This holds especially true for complex process-based models in contrast to simpler conceptual models.
Consequently, the aim of this thesis is to improve the understanding of structural uncertainties with focus on process-based hydrological modelling, including methods for their quantification.

To identify common deficits of frequently used hydrological models and develop further strategies on how to reduce them, a survey among modellers was conducted.
It was found that there is a certain degree of subjectivity in the perception of modellers, for instance with respect to the distinction of hydrological models into conceptual groups.
It was further found that there are ambiguities on how to apply a certain hydrological model, for instance how many parameters should be calibrated, together with a large diversity of opinion regarding the deficits of models.
Nevertheless, evapotranspiration processes are often represented in a more physically based manner, while processes of groundwater and soil water movement are often simplified, which many survey participants saw as a drawback.
A large flexibility, for instance with respect to different alternative process implementations or a small number of parameters that needs to be calibrated, was generally seen as strength of a model.

Flexible and efficient software, which is straightforward to apply, has been increasingly acknowledged by the hydrological community.
This work further elaborated on this topic in a twofold way.
First, a software package for semi-automated landscape discretisation has been developed, which serves as a tool for model initialisation.
This was complemented by a sensitivity analysis of important and commonly used discretisation parameters, of which the size of hydrological sub-catchments as well as the size and number of hydrologically uniform computational units appeared to be more influential than information considered for the characterisation of hillslope profiles.
Second, a process-based hydrological model has been implemented into a flexible simulation environment with several alternative process representations and a number of numerical solvers.
It turned out that, even though computation times were still long, enhanced computational capabilities nowadays in combination with innovative methods for statistical analysis allow for the exploration of structural uncertainties of even complex process-based models, which up to now was often neglected by the modelling community.

In a further study it could be shown that process-based models may even be employed as tools for seasonal operational forecasting.
In contrast to statistical models, which are faster to initialise and to apply, process-based models produce more information in addition to the target variable, even at finer spatial and temporal scales, and provide more insights into process behaviour and catchment functioning.
However, the process-based model was much more dependent on reliable rainfall forecasts.

It seems unlikely that there exists a single best formulation for hydrological processes, even for a specific catchment.
This supports the use of flexible model environments with alternative process representations instead of a single model structure.
However, correlation and compensation effects between process formulations, their parametrisation, and other aspects such as numerical solver and model resolution, may lead to surprising results and potentially misleading conclusions.
In future studies, such effects should be more explicitly addressed and quantified.
Moreover, model functioning appeared to be highly dependent on the meteorological conditions and rainfall input generally was the most important source of uncertainty.
It is still unclear, how this could be addressed, especially in the light of the aforementioned correlations.
The use of innovative data products, e.g.\ remote sensing data in combination with station measurements, and efficient processing methods for the improvement of rainfall input and explicit consideration of associated uncertainties is advisable to bring more insights and make hydrological simulations and predictions more reliable.
N2  - Hydrologische Modelle sind wichtige Werkzeuge zur Simulation und Quantifizierung des Wasserkreislaufs.
Sie helfen bei der explorativen Analyse hydrologischer Prozesse, Abflussvorhersage, Abschätzung der Folgen von Klima- und Landnutzungswandel oder dem Management von Wasserressourcen.
Allerdings sind die mit der hydrologischen Modellierung einhergehenden Unsicherheiten noch immer groß.
Trotz der zahlreichen Forschungsarbeiten auf dem Gebiet der Quantifizierung und Reduktion der Unsicherheiten gibt es einige Bereiche, die bisher wenig erforscht wurden, wie beispielsweise strukturelle Unsicherheiten, welche sich unter anderem auf die Prozessimplementation in den Modellen beziehen.
Dies betrifft vor allem komplexe prozessbasierte Modelle im Gegensatz zu einfacheren konzeptionellen Modellen.
Gegenstand dieser Arbeit ist es daher, das Verständnis struktureller Unsicherheiten sowie Methoden für deren Quantifizierung innerhalb prozessbasierter hydrologischer Modellanwendungen zu erweitern.

Zur Identifikation typischer Defizite hydrologischer Modelle und Erarbeitung von Lösungsstrategien, um diese zu reduzieren, wurde eine Umfrage unter Modellanwendern durchgeführt.
Dabei stellte sich heraus, dass ein hohes Maß an Subjektivität in der Wahrnehmung des Themas unter Modellieren herrscht, beispielsweise bei der Einordnung hydrologischer Modelle in konzeptionelle Klassen.
Des Weiteren gibt es Unklarheiten in der Art und Weise, wie ein bestimmtes hydrologisches Modell angewendet werden sollte, wie etwa hinsichtlich der Kalibrierung bestimmter Parameter, sowie vielschichtige Auffassungen bezüglich der Modelldefizite.
Letztlich stellte sich jedoch heraus, dass Verdunstungsprozesse vor allem physikalisch basiert abgebildet werden, während Prozesse im Bereich des Grundwassers und der Bodenwasserbewegung häufig vereinfacht abgebildet werden, was von vielen Umfrageteilnehmern als Nachteil empfunden wurde.
Generell als Stärke wurde die Flexibilität einiger Modelle empfunden, zum Beispiel wenn diese verschiedene Implementationen eines Prozesses enthalten oder wenn nur eine geringe Zahl an Parametern kalibriert werden muss.

Flexible und effiziente Software, die darüber hinaus einfach zu bedienen ist, wird von der hydrologischen Gemeinschaft immer stärker in den Vordergrund gebracht.
Daher greift diese Arbeit das Thema in zweifacher Hinsicht auf.
Zum einen wurde ein Softwarepaket zur halbautomatischen Landschaftsdiskretisierung entwickelt, welches zudem als Werkzeug zur Modellinitialisierung gedacht ist.
Damit einhergehend wurde eine Sensitivitätsanalyse wichtiger und häufig genutzter Diskretisierungsparameter durchgeführt, bei der die Größe hydrologischer Teileinzugsgebiete sowie die Anzahl und Größe hydrologischer Elementarflächen sich als maßgeblicher herausstellte als etwa raumbezogene Informationen zur Charakterisierung der Hangprofile.
Zum anderen wurde ein prozessbasiertes hydrologisches Modell in eine flexible Softwareumgebung integriert, der verschiedene alternative Prozessformulierungen sowie numerische Differentialgleichungslöser hinzugefügt wurden.
Die Analyse struktureller Unsicherheiten komplexer prozessbasierter Modelle wurde in der Vergangenheit von der hydrologischen Gemeinschaft mit Verweis auf zu lange Rechenzeit oft vernachlässigt.
Es zeigte sich jedoch, dass die mittlerweile zur Verfügung stehenden Computerressourcen, vor allem in Kombination mit innovativen statistischen Analyseverfahren, derartige Untersuchungen bereits ermöglichen.

In einer weiteren Studie konnte zudem gezeigt werden, dass auch prozessbasierte Modelle für den operationellen Einsatz in der saisonalen Vorhersage geeignet sind.
Im Gegensatz zu statistischen Modellen, welche schneller initialisierbar und anwendbar sind, produzieren prozessbasierte Modelle neben der eigentlichen Zielgröße weitere potentiell relevante Informationen in höherer räumlicher und zeitlicher Auflösung und geben zudem tiefere Einblicke in die generelle Wirkungsweise der hydrologischen Prozesse in einem Einzugsgebiet.
In der Studie stellte sich jedoch ebenso heraus, dass zuverlässige Niederschlagsvorhersagen für ein prozessbasiertes Modell umso wichtiger sind.

Allgemein erscheint es unwahrscheinlich, dass eine einzelne optimale Implementation für einen hydrologischen Prozess, selbst innerhalb eines bestimmten Einzugsgebietes, überhaupt existiert.
Die Nutzung flexibler Modellumgebungen mit alternativen Prozessbeschreibungen anstelle eines einzelnen Modells scheint deshalb große Vorteile zu bringen.
Mögliche Korrelationen zwischen Prozessbeschreibungen, deren Parametrisierung, sowie anderen Aspekte wie numerischen Lösern und Modellauflösung, können jedoch zu überraschenden Ergebnissen und letztlich falschen Schlussfolgerungen führen.
In zukünftigen Studien sollten solche Effekte daher explizit berücksichtigt und quantifiziert werden.
Darüber hinaus wird die Leistungsfähigkeit eines Modells maßgeblich von den meteorologischen Randbedingungen beeinflusst.
Vor allem der Niederschlag erwies sich innerhalb dieser Arbeit als wichtigste Ursache für Unsicherheiten in der Modellierung.
Allerdings ist nicht vollständig klar, wie dieser Umstand berücksichtigt werden kann und inwiefern die zuvor genannten Korrelationen hier einen Einfluss haben.
Die Nutzung innovativer Datenprodukte, zum Beispiel Fernerkundungsdaten verbunden mit Stationsmessungen, in Kombination mit effizienten Prozessierungsalgorithmen zur Verbesserung des Niederschlagsinputs und expliziten Beachtung einhergehender Unsicherheiten wird angeraten.
Dies verspricht bessere Einblicke in die Zusammenhänge verschiedener Unsicherheitsquellen zu gewinnen und letztlich hydrologische Simulationen und Vorhersagen zuverlässiger zu machen.
T2  - Beiträge zum Verständnis der Unsicherheiten in der
hydrologischen Modellierung
KW  - hydrology
KW  - hydrological modelling
KW  - uncertainties
KW  - Hydrologie
KW  - hydrologische Modellierung
KW  - Unsicherheiten
Y1  - 2020
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-476643
ER  -