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The climate is a complex dynamical system involving interactions and feedbacks among different processes at multiple temporal and spatial scales. Although numerous studies have attempted to understand the climate system, nonetheless, the studies investigating the multiscale characteristics of the climate are scarce. Further, the present set of techniques are limited in their ability to unravel the multi-scale variability of the climate system. It is completely plausible that extreme events and abrupt transitions, which are of great interest to climate community, are resultant of interactions among processes operating at multi-scale. For instance, storms, weather patterns, seasonal irregularities such as El Niño, floods and droughts, and decades-long climate variations can be better understood and even predicted by quantifying their multi-scale dynamics. This makes a strong argument to unravel the interaction and patterns of climatic processes at different scales. With this background, the thesis aims at developing measures to understand and quantify multi-scale interactions within the climate system.
In the first part of the thesis, I proposed two new methods, viz, multi-scale event synchronization (MSES) and wavelet multi-scale correlation (WMC) to capture the scale-specific features present in the climatic processes. The proposed methods were tested on various synthetic and real-world time series in order to check their applicability and replicability. The results indicate that both methods (WMC and MSES) are able to capture scale-specific associations that exist between processes at different time scales in a more detailed manner as compared to the traditional single scale counterparts.
In the second part of the thesis, the proposed multi-scale similarity measures were used in constructing climate networks to investigate the evolution of spatial connections within climatic processes at multiple timescales. The proposed methods WMC and MSES, together with complex network were applied to two different datasets.
In the first application, climate networks based on WMC were constructed for the univariate global sea surface temperature (SST) data to identify and visualize the SSTs patterns that develop very similarly over time and distinguish them from those that have long-range teleconnections to other ocean regions. Further investigations of climate networks on different timescales revealed (i) various high variability and co-variability regions, and (ii) short and long-range teleconnection regions with varying spatial distance. The outcomes of the study not only re-confirmed the existing knowledge on the link between SST patterns like El Niño Southern Oscillation and the Pacific Decadal Oscillation, but also suggested new insights into the characteristics and origins of long-range teleconnections.
In the second application, I used the developed non-linear MSES similarity measure to quantify the multivariate teleconnections between extreme Indian precipitation and climatic patterns with the highest relevance for Indian sub-continent. The results confirmed significant non-linear influences that were not well captured by the traditional methods. Further, there was a substantial variation in the strength and nature of teleconnection across India, and across time scales.
Overall, the results from investigations conducted in the thesis strongly highlight the need for considering the multi-scale aspects in climatic processes, and the proposed methods provide robust framework for quantifying the multi-scale characteristics.
Mechanical and/or chemical removal of material from the subsurface may generate large subsurface cavities, the destabilisation of which can lead to ground collapse and the formation of sinkholes. Numerical simulation of the interaction of cavity growth, host material deformation and overburden collapse is desirable to better understand the sinkhole hazard but is a challenging task due to the involved high strains and material discontinuities. Here, we present 2-D distinct element method numerical simulations of cavity growth and sinkhole development. Firstly, we simulate cavity formation by quasi-static, stepwise removal of material in a single growing zone of an arbitrary geometry and depth. We benchmark this approach against analytical and boundary element method models of a deep void space in a linear elastic material. Secondly, we explore the effects of properties of different uniform materials on cavity stability and sinkhole development. We perform simulated biaxial tests to calibrate macroscopic geotechnical parameters of three model materials representative of those in which sinkholes develop at the Dead Sea shoreline: mud, alluvium and salt. We show that weak materials do not support large cavities, leading to gradual sagging or suffusion-style subsidence. Strong materials support quasi-stable to stable cavities, the overburdens of which may fail suddenly in a caprock or bedrock collapse style. Thirdly, we examine the consequences of layered arrangements of weak and strong materials. We find that these are more susceptible to sinkhole collapse than uniform materials not only due to a lower integrated strength of the overburden but also due to an inhibition of stabilising stress arching. Finally, we compare our model sinkhole geometries to observations at the Ghor Al-Haditha sinkhole site in Jordan. Sinkhole depth ∕ diameter ratios of 0.15 in mud, 0.37 in alluvium and 0.33 in salt are reproduced successfully in the calibrated model materials. The model results suggest that the observed distribution of sinkhole depth ∕ diameter values in each material type may partly reflect sinkhole growth trends.
The concept of hydrologic connectivity summarizes all flow processes that link separate regions of a landscape. As such, it is a central theme in the field of catchment hydrology, with influence on neighboring disciplines such as ecology and geomorphology. It is widely acknowledged to be an important key in understanding the response behavior of a catchment and has at the same time inspired research on internal processes over a broad range of scales. From this process-hydrological point of view, hydrological connectivity is the conceptual framework to link local observations across space and scales.
This is the context in which the four studies this thesis comprises of were conducted. The focus was on structures and their spatial organization as important control on preferential subsurface flow. Each experiment covered a part of the conceptualized flow path from hillslopes to the stream: soil profile, hillslope, riparian zone, and stream.
For each study site, the most characteristic structures of the investigated domain and scale, such as slope deposits and peat layers were identified based on preliminary or previous investigations or literature reviews. Additionally, further structural data was collected and topographical analyses were carried out. Flow processes were observed either based on response observations (soil moisture changes or discharge patterns) or direct measurement (advective heat transport). Based on these data, the flow-relevance of the characteristic structures was evaluated, especially with regard to hillslope to stream connectivity.
Results of the four studies revealed a clear relationship between characteristic spatial structures and the hydrological behavior of the catchment. Especially the spatial distribution of structures throughout the study domain and their interconnectedness were crucial for the establishment of preferential flow paths and their relevance for large-scale processes. Plot and hillslope-scale irrigation experiments showed that the macropores of a heterogeneous, skeletal soil enabled preferential flow paths at the scale of centimeters through the otherwise unsaturated soil. These flow paths connected throughout the soil column and across the hillslope and facilitated substantial amounts of vertical and lateral flow through periglacial slope deposits.
In the riparian zone of the same headwater catchment, the connectivity between hillslopes and stream was controlled by topography and the dualism between characteristic subsurface structures and the geomorphological heterogeneity of the stream channel. At the small scale (1 m to 10 m) highest gains always occurred at steps along the longitudinal streambed profile, which also controlled discharge patterns at the large scale (100 m) during base flow conditions (number of steps per section). During medium and high flow conditions, however, the impact of topography and parafluvial flow through riparian zone structures prevailed and dominated the large-scale response patterns.
In the streambed of a lowland river, low permeability peat layers affected the connectivity between surface water and groundwater, but also between surface water and the hyporheic zone. The crucial factor was not the permeability of the streambed itself, but rather the spatial arrangement of flow-impeding peat layers, causing increased vertical flow through narrow “windows” in contrast to predominantly lateral flow in extended areas of high hydraulic conductivity sediments.
These results show that the spatial organization of structures was an important control for hydrological processes at all scales and study areas. In a final step, the observations from different scales and catchment elements were put in relation and compared. The main focus was on the theoretical analysis of the scale hierarchies of structures and processes and the direction of causal dependencies in this context. Based on the resulting hierarchical structure, a conceptual framework was developed which is capable of representing the system’s complexity while allowing for adequate simplifications.
The resulting concept of the parabolic scale series is based on the insight that flow processes in the terrestrial part of the catchment (soil and hillslopes) converge. This means that small-scale processes assemble and form large-scale processes and responses. Processes in the riparian zone and the streambed, however, are not well represented by the idea of convergence. Here, the large-scale catchment signal arrives and is modified by structures in the riparian zone, stream morphology, and the small-scale interactions between surface water and groundwater. Flow paths diverge and processes can better be represented by proceeding from large scales to smaller ones. The catchment-scale representation of processes and structures is thus the conceptual link between terrestrial hillslope processes and processes in the riparian corridor.
Schwarz-Rot-Geil
(2018)
The Aral Sea desiccation and related changes in hydroclimatic conditions on a regional level is a hot topic for past decades. The key problem of scientific research projects devoted to an investigation of modern Aral Sea basin hydrological regime is its discontinuous nature – the only limited amount of papers takes into account the complex runoff formation system entirely. Addressing this challenge we have developed a continuous prediction system for assessing freshwater inflow into the Small Aral Sea based on coupling stack of hydrological and data-driven models. Results show a good prediction skill and approve the possibility to develop a valuable water assessment tool which utilizes the power of classical physically based and modern machine learning models both for territories with complex water management system and strong water-related data scarcity. The source code and data of the proposed system is available on a Github page (https://github.com/SMASHIproject/IWRM2018).
Causes for slow weathering and erosion in the steep, warm, monsoon-subjected Highlands of Sri Lanka
(2018)
In the Highlands of Sri Lanka, erosion and chemical weathering rates are among the lowest for global mountain denudation. In this tropical humid setting, highly weathered deep saprolite profiles have developed from high-grade metamorphic charnockite during spheroidal weathering of the bedrock. The spheroidal weathering produces rounded corestones and spalled rindlets at the rock-saprolite interface. I used detailed textural, mineralogical, chemical, and electron-microscopic (SEM, FIB, TEM) analyses to identify the factors limiting the rate of weathering front advance in the profile, the sequence of weathering reactions, and the underlying mechanisms. The first mineral attacked by weathering was found to be pyroxene initiated by in situ Fe oxidation, followed by in situ biotite oxidation. Bulk dissolution of the primary minerals is best described with a dissolution – re-precipitation process, as no chemical gradients towards the mineral surface and sharp structural boundaries are observed at the nm scale. Only the local oxidation in pyroxene and biotite is better described with an ion by ion process. The first secondary phases are oxides and amorphous precipitates from which secondary minerals (mainly smectite and kaolinite) form. Only for biotite direct solid state transformation to kaolinite is likely. The initial oxidation of pyroxene and biotite takes place in locally restricted areas and is relatively fast: log J = -11 molmin/(m2 s). However, calculated corestone-scale mineral oxidation rates are comparable to corestone-scale mineral dissolution rates: log R = -13 molpx/(m2 s) and log R = -15 molbt/(m2 s). The oxidation reaction results in a volume increase. Volumetric calculations suggest that this observed oxidation leads to the generation of porosity due to the formation of micro-fractures in the minerals and the bedrock allowing for fluid transport and subsequent dissolution of plagioclase. At the scale of the corestone, this fracture reaction is responsible for the larger fractures that lead to spheroidal weathering and to the formation of rindlets. Since these fractures have their origin from the initial oxidational induced volume increase, oxidation is the rate limiting parameter for weathering to take place. The ensuing plagioclase weathering leads to formation of high secondary porosity in the corestone over a distance of only a few cm and eventually to the final disaggregation of bedrock to saprolite. As oxidation is the first weathering reaction, the supply of O2 is a rate-limiting factor for chemical weathering. Hence, the supply of O2 and its consumption at depth connects processes at the weathering front with erosion at the surface in a feedback mechanism. The strength of the feedback depends on the relative weight of advective versus diffusive transport of O2 through the weathering profile. The feedback will be stronger with dominating diffusive transport. The low weathering rate ultimately depends on the transport of O2 through the whole regolith, and on lithological factors such as low bedrock porosity and the amount of Fe-bearing primary minerals. In this regard the low-porosity charnockite with its low content of Fe(II) bearing minerals impedes fast weathering reactions. Fresh weatherable surfaces are a pre-requisite for chemical weathering. However, in the case of the charnockite found in the Sri Lankan Highlands, the only process that generates these surfaces is the fracturing induced by oxidation. Tectonic quiescence in this region and low pre-anthropogenic erosion rate (attributed to a dense vegetation cover) minimize the rejuvenation of the thick and cohesive regolith column, and lowers weathering through the feedback with erosion.
Continental rift systems open up unique possibilities to study the geodynamic system of our planet: geodynamic localization processes are imprinted in the morphology of the rift by governing the time-dependent activity of faults, the topographic evolution of the rift or by controlling whether a rift is symmetric or asymmetric. Since lithospheric necking localizes strain towards the rift centre, deformation structures of previous rift phases are often well preserved and passive margins, the end product of continental rifting, retain key information about the tectonic history from rift inception to continental rupture.
Current understanding of continental rift evolution is based on combining observations from active rifts with data collected at rifted margins. Connecting these isolated data sets is often accomplished in a conceptual way and leaves room for subjective interpretation. Geodynamic forward models, however, have the potential to link individual data sets in a quantitative manner, using additional constraints from rock mechanics and rheology, which allows to transcend previous conceptual models of rift evolution. By quantifying geodynamic processes within continental rifts, numerical modelling allows key insight to tectonic processes that operate also in other plate boundary settings, such as mid ocean ridges, collisional mountain chains or subduction zones.
In this thesis, I combine numerical, plate-tectonic, analytical, and analogue modelling approaches, whereas numerical thermomechanical modelling constitutes the primary tool. This method advanced rapidly during the last two decades owing to dedicated software development and the availability of massively parallel computer facilities. Nevertheless, only recently the geodynamical modelling community was able to capture 3D lithospheric-scale rift dynamics from onset of extension to final continental rupture.
The first chapter of this thesis provides a broad introduction to continental rifting, a summary of the applied rift modelling methods and a short overview of previews studies. The following chapters, which constitute the main part of this thesis feature studies on plate boundary dynamics in two and three dimension followed by global scale analyses (Fig. 1).
Chapter II focuses on 2D geodynamic modelling of rifted margin formation. It highlights the formation of wide areas of hyperextended crustal slivers via rift migration as a key process that affected many rifted margins worldwide. This chapter also contains a study of rift velocity evolution, showing that rift strength loss and extension velocity are linked through a dynamic feed-back. This process results in abrupt accelerations of the involved plates during rifting illustrating for the first time that rift dynamics plays a role in changing global-scale plate motions. Since rift velocity affects key processes like faulting, melting and lower crustal flow, this study also implies that the slow-fast velocity evolution should be imprinted in rifted margin structures.
Chapter III relies on 3D Cartesian rift models in order to investigate various aspects of rift obliquity. Oblique rifting occurs if the extension direction is not orthogonal to the rift trend. Using 3D lithospheric-scale models from rift initialisation to breakup I could isolate a characteristic evolution of dominant fault orientations. Further work in Chapter III addresses the impact of rift obliquity on the strength of the rift system. We illustrate that oblique rifting is mechanically preferred over orthogonal rifting, because the brittle yielding requires a lower tectonic force. This mechanism elucidates rift competition during South Atlantic rifting, where the more oblique Equatorial Atlantic Rift proceeded to breakup while the simultaneously active but less oblique West African rift system became a failed rift. Finally this Chapter also investigates the impact of a previous rift phase on current tectonic activity in the linkage area of the Kenyan with Ethiopian rift. We show that the along strike changes in rift style are not caused by changes in crustal rheology. Instead the rift linkage pattern in this area can be explained when accounting for the thinned crust and lithosphere of a Mesozoic rift event.
Chapter IV investigates rifting from the global perspective. A first study extends the oblique rift topic of the previous chapter to global scale by investigating the frequency of oblique rifting during the last 230 million years. We find that approximately 70% of all ocean-forming rift segments involved an oblique component of extension where obliquities exceed 20°. This highlights the relevance of 3D approaches in modelling, surveying, and interpretation of many rifted margins. In a final study, we propose a link between continental rift activity, diffuse CO2 degassing and Mesozoic/Cenozoic climate changes. We used recent CO2 flux measurements in continental rifts to estimate worldwide rift-related CO2 release, which we based on the global extent of rifts through time. The first-order correlation to paleo-atmospheric CO2 proxy data suggests that rifts constitute a major element of the global carbon cycle.
We explore the potential of spaceborne radar (SR) observations from the Ku-band precipitation radars onboard the Tropical Rainfall Measuring Mission (TRMM) and Global Precipitation Measurement (GPM) satellites as a reference to quantify the ground radar (GR) reflectivity bias. To this end, the 3-D volume-matching algorithm proposed by Schwaller and Morris (2011) is implemented and applied to 5 years (2012–2016) of observations. We further extend the procedure by a framework to take into account the data quality of each ground radar bin. Through these methods, we are able to assign a quality index to each matching SR–GR volume, and thus compute the GR calibration bias as a quality-weighted average of reflectivity differences in any sample of matching GR–SR volumes. We exemplify the idea of quality-weighted averaging by using the beam blockage fraction as the basis of a quality index. As a result, we can increase the consistency of SR and GR observations, and thus the precision of calibration bias estimates. The remaining scatter between GR and SR reflectivity as well as the variability of bias estimates between overpass events indicate, however, that other error sources are not yet fully addressed. Still, our study provides a framework to introduce any other quality variables that are considered relevant in a specific context. The code that implements our analysis is based on the wradlib open-source software library, and is, together with the data, publicly available to monitor radar calibration or to scrutinize long series of archived radar data back to December 1997, when TRMM became operational.