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(2022) Zhang, QiThesisAs a dominant terrestrial ecosystem of the Earth, forest environments play profound roles in ecology, biodiversity, resource utilization, and management, which highlights the significance of forest characterization and monitoring. Some forest parameters can help track climate change and quantify the global carbon cycle and therefore attract growing attention from various research communities. Compared with traditional in-situ methods with expensive and time-consuming field works involved, airborne and spaceborne remote sensors collect cost-efficient and consistent observations at global or regional scales and have been proven to be an effective way for forest monitoring. With the looming paradigm shift toward data-intensive science and the development of remote sensors, remote sensing data with higher resolution and diversity have been the mainstream in data analysis and processing. However, significant heterogeneities in the multi-source remote sensing data largely restrain its forest applications urging the research community to come up with effective synergistic strategies. The work presented in this thesis contributes to the field by exploring the potential of the Synthetic Aperture Radar (SAR), SAR Polarimetry (PolSAR), SAR Interferometry (InSAR), Polarimetric SAR Interferometry (PolInSAR), Light Detection and Ranging (LiDAR), and multispectral remote sensing in forest characterization and monitoring from three main aspects including forest height estimation, active fire detection, and burned area mapping. First, the forest height inversion is demonstrated using airborne L-band dual-baseline repeat-pass PolInSAR data based on modified versions of the Random Motion over Ground (RMoG) model, where the scattering attenuation and wind-derived random motion are described in conditions of homogeneous and heterogeneous volume layer, respectively. A boreal and a tropical forest test site are involved in the experiment to explore the flexibility of different models over different forest types and based on that, a leveraging strategy is proposed to boost the accuracy of forest height estimation. The accuracy of the model-based forest height inversion is limited by the discrepancy between the theoretical models and actual scenarios and exhibits a strong dependency on the system and scenario parameters. Hence, high vertical accuracy LiDAR samples are employed to assist the PolInSAR-based forest height estimation. This multi-source forest height estimation is reformulated as a pan-sharpening task aiming to generate forest heights with high spatial resolution and vertical accuracy based on the synergy of the sparse LiDAR-derived heights and the information embedded in the PolInSAR data. This process is realized by a specifically designed generative adversarial network (GAN) allowing high accuracy forest height estimation less limited by theoretical models and system parameters. Related experiments are carried out over a boreal and a tropical forest to validate the flexibility of the method. An automated active fire detection framework is proposed for the medium resolution multispectral remote sensing data. The basic part of this framework is a deep-learning-based semantic segmentation model specifically designed for active fire detection. A dataset is constructed with open-access Sentinel-2 imagery for the training and testing of the deep-learning model. The developed framework allows an automated Sentinel-2 data download, processing, and generation of the active fire detection results through time and location information provided by the user. Related performance is evaluated in terms of detection accuracy and processing efficiency. The last part of this thesis explored whether the coarse burned area products can be further improved through the synergy of multispectral, SAR, and InSAR features with higher spatial resolutions. A Siamese Self-Attention (SSA) classification is proposed for the multi-sensor burned area mapping and a multi-source dataset is constructed at the object level for the training and testing. Results are analyzed by different test sites, feature sources, and classification methods to assess the improvements achieved by the proposed method. All developed methods are validated with extensive processing of multi-source data acquired by Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR), Land, Vegetation, and Ice Sensor (LVIS), PolSARproSim+, Sentinel-1, and Sentinel-2. I hope these studies constitute a substantial contribution to the forest applications of multi-source remote sensing.
(2021) Alohali, Ruaa Tawfiq AThesisThe Arabian basin was subject to several tectonic events, including Lower Cambrian Najd rifting, the Carboniferous Hercynian Orogeny, Triassic Zagros rifting, and the Early/Cretaceous and Late/Tertiary Alpine orogenic events. These events reactivated Precambrian basement structures and affected the structural configuration of the overlying Paleozoic cover succession. In addition to a 2D seismic array and several drill well logs, a newly acquired, processed 3D seismic image of the subsurface in part of the basin covering an area of approximately 1051 km2 has been provided to improve the understanding of the regional tectonic evolution associated with these deformation events. In this study, a manual interpretation is presented of six main horizons from the Late Ordovician to the Middle Triassic. Faults and folds were also mapped to further constrain the stratigraphic and structural framework. Collectively, this data is used to build a geological model of the region and develop a timeline of geological events. Results show that a lower Paleozoic sedimentary succession between the Late Silurian to the Early Permian was subject to localised tilting, uplift, and erosion during the Carboniferous Hercynian Orogeny, forming a regional unconformity. Subsequent deposition occurred from the Paleozoic to the Mesozoic, producing a relatively thick, conformable, upper succession. The juxtaposition of the Silurian rocks and Permian formations allows a direct fluid flow between the two intervals. Seismic analysis also indicated two major fault generations. A younger NNW-striking fault set with a component of reverse, east-side-up displacement affected the Lower Triassic succession and is most likely related to the Cretaceous and Tertiary Alpine Events that reactivated the Najd fault system. These fault structures allow vertical migration that could act as conduits to form structural traps. Manual mapping of fault structures in the study area required significant time and effort. To simplify and accelerate the manual faults interpretation in the study area, a fault segmentation method was developed using a Convolutional Neural Network. This method was implemented using the 3D seismic data acquired from the Arabian Basin. The network was trained, validated, and tested with samples that included a seismic cube and fault images that were labelled manually corresponding to the seismic cube. The model successfully identified faults with an accuracy of 96% and an error rate of 0.12 on the training dataset. To achieve a more robust model, the prediction results were further enhanced using postprocessing by linking discontinued segments of the same fault and thus, reducing the number of detected faults. This method improved the accuracy of the prediction results of the proposed model using the test dataset by 77.5%. Additionally, an efficient framework was introduced to correlate the predictions and the ground truth by measuring their average distance value. This technique was also applied to the F3 Netherlands survey, which showed promising results in another region with complex fault geometries. As a result of the automated technique developed here, fault detection and diagnosis were achieved efficiently with structures similar to the trained dataset and has a huge potential in improving exploration targets that are structurally controlled by faults.
Advancing understanding of development policy impacts on transboundary river basins: Integrated watershed modelling of the Lower Mekong Basin.(2021) Ly, KongmengThesisThe management of transboundary river basins across developing countries, such as the Lower Mekong River Basin (LMB), is frequently challenging given the development and conservation divergences of the basin countries. Driven by needs to sustain economic performance and reduce poverty, the LMB countries are embarking on significant land use changes in the form hydropower dams, to fulfill their energy requirements. This pathway could lead to irreversible changes to the ecosystem of the Mekong River, if not properly managed. This thesis aims to explore the potential effects of changes in land use —with a focus on current and projected hydropower operations— on the Lower Mekong River network streamflow and instream water quality. To achieve this aim, this thesis first examined the relationships between the basin land use/land cover attributes, and streamflow and instream water quality dynamics of the Mekong River, using total suspended solids and nitrate as proxies for water quality. Findings from this allowed framing challenges of integrated water management of transboundary river basins. These were used as criteria for selecting eWater’s Source modelling framework as a management tool that can support decision-making in the socio-ecological context of the LMB. Against a combination of predictive performance metrics and hydrologic signatures, the model’s application in the LMB was found to robustly simulate streamflow, TSS and nitrate time series. The model was then used for analysing four plausible future hydropower development scenarios, under extreme climate conditions and operational alternatives. This revealed that hydropower operations on either tributary or mainstream could result in annual and wet season flow reduction while increasing dry season flows compared to a baseline scenario. Conversely, hydropower operation on both tributary and mainstream could result in dry season flow reduction. Both instream TSS and nitrate loads were predicted to reduce under all three scenarios compared to the baseline. These effects were found to magnify under extreme climate conditions, but were less severe under improved operational alternatives. In the LMB where hydropower development is inevitable, findings from this thesis provide an enhanced understanding on the importance of operational alternatives as an effective transboundary cooperation and management pathway for balancing electricity generation and protection of riverine ecology, water and food security, and people livelihoods.
Remote tree-ring proxies: methods, opportunities, and limitations for reconstructing South Pacific climate(2022) Higgins, PhilippaThesisIncreasing population and resource demands, a changing hydroclimate, and increasing risks of extreme events means that sustainable water management is more important now than ever before. Water planners are increasingly recognising that short instrumental records are insufficient to understand fully natural trends and variability in climate. High resolution paleoclimate proxies, like tree rings, can provide long time series of observations prior to the instrumental period, to better understand instrumental and pre-instrumental variability, the occurrence, trends, and drivers of extreme events, and provide insights into possible future hydroclimatic scenarios. However, tree-ring proxies are not evenly distributed in the landscape, and the South Pacific has very few high-resolution paleoclimate proxies to develop detailed reconstructions of climate variability. This thesis explores whether the relationships between tree-ring proxies in regions with strong teleconnections to the Pacific (i.e., ‘remote’ tree rings) can be exploited to reconstruct hydroclimatic indices across eastern Australia and the South Pacific Islands. Methods for hydroclimatic reconstruction are investigated, considering the unique challenges of the region: strong inter-annual and inter-decadal variability, very short data records, data gaps, and potential non-stationarities in climate teleconnections. Existing methods for tree-ring reconstructions have been successfully applied in the South Pacific (Chapter 2); however, overcoming the challenges posed by very short and non-continuous records required adaptations to existing methods (Chapter 3) and the development of new methods (Chapter 5). In the final two chapters, the thesis focuses on how catchment-scale tree-ring reconstructions can be most useful to water managers. In these chapters, methods of identifying, explaining, and representing extreme event frequency, return periods, and trends are explored, as are methods for using paleoclimate data along with climate model projections to help contextualise future risks of climate change. Overall, this thesis highlights the enormous potential of remote tree-rings for improving our understanding of past climate in the South Pacific. The reconstructions consistently demonstrate that the instrumental period underestimates the full range of natural climate variability and shows how century-long records provided by tree rings can help us better understand past climate drivers, contextualise the instrumental period, and refine estimates of future climate risks. This thesis builds upon a growing body of work that demonstrates the considerable value of tree-ring based reconstructions for current and future water resource decision making, most notably in remote regions that are highly vulnerable to climate change but where there are limited instrumental records. Maximising the potential of tree-ring data for water management will require ongoing collaboration between dendrochronologists and water managers.
The processes determining dissolved organic carbon (DOC) character and concentration in groundwater in different geological environments(2022) Oudone, PhetdalaThesisDissolved organic carbon is stored and processed in groundwater in three ways. It is stored on minerals by adsorption, it is biologically processed through biodegradation, and it also undergoes a process to return to groundwater called desorption. This biophysiochemical research shows that the groundwater system is therefore a vital part of the global carbon cycle and carbon sink. This research fills a gap in the existing understanding of how to calculate the global carbon budget, as does not yet include the dissolved organic carbon that is stored in groundwater. This thesis exclusively explores processes determining dissolved organic carbon character and concentration in groundwater in different geological environments. This is new, useful knowledge to describe the biophysiochemical process. This research did not examine human interference in adding carbon to groundwater. This research found how dissolved organic carbon is stored and processed in groundwater due to biodegradation and desorption, and how it is adsorbed onto sediment surface. This research explored the characteristics and concentration of Dissolved organic carbon in groundwater by using Liquid Chromatography-Organic Carbon Detection, and other techniques, to examine dissolved organic carbon in terms of its fractions: humic substances, hydrophobic organic carbon, biopolymers, building blocks (BB), low molecular weight neutrals and low molecular weight acids. There were several key findings. First, the results showed that both semi-arid inland low sedimentary organic carbon environments – i.e., Maules Creek and Wellington – were a carbon source; while the high rainfall temperate coastal peatland environment of Anna Bay was a carbon sink. Secondly, another key finding was that dissolved organic carbon was not processed as a whole chemical compound, but it was processed by its fractions where each fraction was processed distinctly. For example, humic substances were only adsorbed/desorbed in groundwater; while low molecular weight neutrals were only consumed by microbes in the biodegradation process in groundwater.