Markus Berli

Effects of root-induced compaction on rhizosphere hydraulic properties--X-ray microtomography imaging and numerical simulations.

Soil compaction represents one of the most ubiquitous environmental impacts of human development, decreasing bulk-scale soil porosity and hydraulic conductivity, thereby reducing soil productivity and fertility. At the aggregate-scale however, this study shows that natural root-induced compaction increases contact areas between aggregates, leading to an increase in unsaturated hydraulic conductivity of the soils adjacent to the roots. Contrary to intuition, water flow may therefore be locally enhanced due to root-induced compaction. This study investigates these processes by using recent advances in X-ray microtomography (XMT) imaging and numerical water flow modeling to show evolution in interaggregate contact and its implications for water flow between aggregates under partially saturated conditions. Numerical modeling showed that the effective hydraulic conductivity of a pair of aggregates undergoing uniaxial deformation increased following a nonlinear relationship as the interaggregate contact area increased due to increasing aggregate deformation. Numerical modeling using actual XMT images of aggregated soil around a root surrogate demonstrated how root-induced deformation increases unsaturated water flow toward the root, providing insight into the growth, function, and water uptake patterns of roots in natural soils.

Deformation and permeability of aggregated soft earth materials

[ 1] This study develops a framework for modeling deformation of individual pores in elastoviscoplastic earth material accounting for the effects of evolving pore size and shape on material hydraulic permeability. We describe the velocity field of a fluid within deforming pores of hypotrochoidal cross-sectional areas as a function of remote stress or deformation and elastoviscoplastic material properties using finite element analysis. We find that pore permeability decreases with increasing stress and deformation. Pore cross-sectional areas are mainly reduced in size while the shape remains constant. Under stress-controlled conditions, change in permeability depends on matrix constitutive laws and loading time while there is no such dependency for controlled strain. Permeability estimates based on the hydraulic radius, Saint-Venant, and Aissen approximations were in good agreement with numerical calculations for a deforming hypotrochoidal pore. We also show that permeability of a deforming hypotrochoidal pore can be modeled using a pore with equivalent elliptical cross-sectional area ( equal initial permeability and size), providing the ellipse has the correct orientation. The study shows that fluid flow in deforming elastoviscoplastic earth material can be modeled on the pore scale knowing the evolution of pore size and shape employing rather simple relations between pore cross-sectional geometry and permeability.

Evolution of unsaturated hydraulic conductivity of aggregated soils due to compressive forces

Prediction of water flow and transport processes in soils susceptible to structural alteration such as compaction of tilled agricultural lands or newly constructed landfills rely on accurate description of changes in soil unsaturated hydraulic conductivity. Recent studies have documented the critical impact of aggregate contact characteristics on water flow rates and pathways in unsaturated aggregated soils. We developed an analytical model for aggregate contact size evolution as a basis for quantifying effects of compression on saturated and unsaturated hydraulic conductivity of aggregated soil. Relating confined one-dimensional sample strain with aggregate deformation facilitates prediction of the increase in interaggregate contact area and concurrent decrease in macropore size with degree of sample compression. The hydrologic component of the model predicts unsaturated hydraulic conductivity of a pack of idealized aggregates (spheres) on the basis of contact size and saturation conditions under prescribed sample deformation. Calculated contact areas and hydraulic conductivity for pairs of aggregates agreed surprisingly well with measured values, determined from compaction experiments employing neutron and X-ray-radiography and image analysis. Model calculations for a unit cell of uniform spherical aggregates in cubic packing were able to mimic some of the differences in saturated and unsaturated hydraulic conductivity observed for aggregates and bulk soil.

Permeability of deformable soft aggregated earth materials: From single pore to sample cross section

[1] Fluid flow in deformable porous media is of interest in many areas of hydrology, geophysics, and environmental engineering at scales ranging from individual pore to field scale. With increasing interest in pore-scale hydraulics, there is also the need to connect microscale with bulk material properties by suitable upscaling. In this study, fluid permeability through a stack of deforming spherical aggregates is described using analytical and finite element (FE) analyses. The permeability of an entire sample cross section was constructed from analytical estimates of individual pore permeabilities and from numerical solution to steady flow through the entire cross section of the sample. Experimental results of pore deformation within a stack of modeling clay aggregates were compared to FE calculations for two-dimensional cross sections. Results showed that pores within a cubic pack of aggregates deform isotropically even under anisotropic stress conditions. Therefore aggregate arrangement seems to be as important as stress anisotropy to predicting pore shape evolution. Observations of cross sections through a porous sample under compaction showed reduction in both the mean and variance of pore permeability with increasing compaction. Despite predominantly vertical compaction, sample permeability remained nearly isotropic (with only a slight additional decrease in the vertical direction). The Aissen analytical approximation for pore permeability was very similar to numerical results for the entire range of sample deformation, thereby providing a useful tool for estimating sample permeability from images of complex pore cross-sectional areas.

Challenges in the Application of Fractional Derivative Models in Capturing Solute Transport in Porous Media: Darcy-Scale Fractional Dispersion and the Influence of Medium Properties

Heterogeneous media consisting of segregated flow regions are fractional-order systems, where the regional-scale anomalous diffusion can be described by the fractional derivative model (FDM). The standard FDM, however, first, cannot characterize the Darcy-scale dispersion through repacked sand columns, and second, the link between medium properties and model parameters remains unknown. To fill these two knowledge gaps, this study applies a tempered fractional derivative model (TFDM) to capture bromide transport through laboratory repacked sand. Column transport experiments are conducted first, where glass beads and silica sand with different diameters are repacked individually. Late-time tails are observed in the breakthrough curves (BTC) of bromide even in relatively homogeneous glass beads. The TFDM can capture the observed subdiffusion, especially the late-time BTC with a transient declining rate. Results also show that both the size distribution of repacked sand and the magnitude of fluid velocity can affect subdiffusion. In particular, a wider sand size distribution or a smaller flow rate can enhance the subdiffusion, leading to a smaller time index and a higher truncation parameter in the TFDM. Therefore, the Darcy-scale dispersion follows the tempered stable law, and the model parameters might be related to the soil size and flow conditions.

Unsupervised segmentation of soil x-ray microtomography images

Advances in X-ray microtomography (XMT) are opening new opportunities for examining soil structural properties and fluid distribution around living roots in-situ. The low contrast between moist soil, root and air-filled pores in XMT images presents a problem with respect to image segmentation. In this paper, we develop an unsupervised method for segmenting XMT images to pores (air and water), soil, and root regions. A feature-based segmentation method is provided to isolate regions that consist of similar texture patterns from an image based on the normalized inverse difference moment of gray-level co-occurrence matrix. The results obtained show that the combination of features, clustering, and post-processing techniques has advantageous over other advanced segmentation methods.