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Institute
Multidimensional visualization of preferential flow pathways using neutron and x-ray radiation
(2016)
The occurrence of preferential flow of water in soils is rather the rule than the exception particularly in heterogeneous structured soils. The quantitative prediction is difficult because of the spatial as well as temporal variability. One aim of the dissertation was to visualize the soil structure and flow patterns in mine soils using a combination of neutron radiography, neutron tomography and x-ray computed tomography experiments. Both radiation techniques were used to describe preferential pathways as a soil structure with CT and the process of preferential flow of water with NT. For the first time neutron radiation was applied on undisturbed soil cores to visualize water distributions in natural soils on three examples for heterogeneities of different origin.
In a first investigation we used neutron radiography to analyze the moisture distribution in undisturbed slab-type and cylindrical samples of mine soils containing lignitic fragments and inclined soil layers. 2D radiography sequences indicated that flow pathways are highly dependent on the shape, distribution, and internal structure of the lignitic fragments. For 3D experiments, the quantitative analysis of water contents failed due to a pronounced beam hardening effect. The tensiometer data indicated the existence of local non-equilibrium in pressure heads between fragments and matric pore regions.
The objective of the second study was to analyze and visualize the effects of initial soil structure formation at the surface of bare soils in an artificial hydrological catchment that has been left to undirected ecological succession for three years. The three-dimensional (3D) micro-morphology was described using the X-ray computed tomography (CT); two-dimensional (2D) infiltration patterns were obtained from drip infiltration experiments using time-resolved neutron radiography (NT). The analyses of CT data indicate the existence of (i) structural crusts on soil consisting of sandy to loamy sediments and (ii) depositional crusts on sandy sediments. The results of combined observations using CT and NT suggest that the initial soil surface structure alterations of young sandy sediment modify moisture patterns of infiltrating water only slightly.
The aim of the 3rd experiment was to study effect of two ‘ecological engineers’, moss vegetation and beetle larvae on pore structure and infiltration. The soil pore structure was characterized by micro-computed tomography analyses of bare soil, sparse and dense moss-vegetated samples containing a single burrow. Flow patterns of infiltration experiments were visualized using the neutron radiography technique. The results of this study suggest that moss vegetation and burrowing activity of ground beetles strongly control the initial soil development by modification of pore structure and surface water infiltration.
The central topic of this dissertation is the 3D spatial description of geomorphic and sediment mass balance development in initial phases of ecosystem development. The introductory chapters give an overview on the consideration of characteristics and relevance of the initial development phase in geomorphological landform development concepts and summarize the state of the art of research for experimental studies on landform development and for 3D soil-landscape modeling approaches. The central aim of the work is a 3D-spatially and temporally resolved description of the development of mass balances of the sediment solid phase during the initial years of ecosystem development and of its dependence on initial and boundary conditions in the 6 ha, artificially-created catchment ‘Hühnerwasser’. This aim is approached using remotely-sensed data, methods of quantitative soil landscape modeling and geomorphic change detection, and the application of a numerical landscape evolution model. The construction of a 3D volume model of the catchment’s sediment body based on digital elevation data is described. Possibilities for the quantification of sediment mass balances and for the 3D spatial description of sediment properties within this model are discussed. Digital elevation models based on airborne and terrestrial laser scanning and photogrammetry are evaluated for their suitability for sediment mass balance quantification and reconstruction of initial morphologic development; and methods for the modification and combination of elevation data for improved sediment mass balance quantification are described. The development of the catchment’s surface morphometry and of the geometry of the evolving erosion rill network is reconstructed and analysed based on aerial photographs and digital elevation models. Relations between structures of the initial surface and the developing hydro-geomorphic structures are discussed. Effects of initial surface morphology and precipitation characteristics during the initial development phase are further assessed by simulations with a numeric landscape evolution model. Results allow for a quantification of geometry and volume of the catchment’s initial sediment body and for a visualization of sediment layers deposited in time intervals. It is shown that a combination of different elevation data, based on their suitability for depicting the sediment surface in areas of different morphologic and vegetation characteristics, allows for an improved quantification of sediment mass balances. Results allow for a characterization of phases of rill network growth, contraction and stabilization and suggest influences of initial morphology, precipitation characteristics, and developing structure-process-interactions on rill network geometry in the catchment. The phases of hydro-geomorphic surface structure evolution can be related to the spatial organization of surface flow patterns during initial phases of landform development.