Marvin Reich

• 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 terrestrialThe 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.…
• 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 vonGravimetrie 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.…