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  • 1
    Publication Date: 2015-05-16
    Description: In porous media, the nonwetting phase is trapped on water saturation due to capillary forces acting in a heterogeneous porous structure. Within the capillary fringe, the gas phase is trapped and released along with the fluctuation of the water table, creating a highly active zone for biological transformations and mass transport. We conducted column experiments to observe and quantify the magnitude and structure of the trapped gas phase at the pore scale using computed microtomography. Different grain size distributions of glass beads were used to study the effect of the pore structure on trapping at various capillary numbers. Viscous forces were found to have negligible impact on phase trapping compared with capillary and buoyancy forces. Residual gas saturations ranged from 0.5 to 10%, while residual saturation increased with decreasing grain size. The gas phase was trapped by snap-off in single pores but also in pore clusters, while this single-pore trapping was dominant for grains larger than 1 mm in diameter. Gas surface area was found to increase linearly with increasing gas volume and with decreasing grain size.
    Electronic ISSN: 1539-1663
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 4496-4504 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Highly resolved (Δν〈100 MHz) UV-REMPI (ultraviolet-resonantly enhanced multiphoton ionization) spectra of different vibronic bands in the phenol–water complex are presented. The torsional splitting caused by the hindered rotation of the water moiety in the hydrogen-bonded system is investigated. An autocorrelation procedure reveals torsional subbands, a correlation automated rotational fitting (CARF) of the spectra yields the rotational constants: The analyzed vibrations are classified by the rotational constants of the corresponding vibronic band and the symmetry of the torsional state. The transition to the stretching vibration at 156 cm−1 excess energy is shown to consist of two different torsional transitions similar to the electronic origin. The torsional splitting in the origin band is 0.8491(2) cm−1 and that of the stretching vibration is 0.8915(3) cm−1, demonstrating a very small coupling between the stretching and the torsional motion. We assign the 121 cm−1 band as the negative parity component of the transition to the wagging vibration β2 while the band at 125 cm−1 is tentatively assigned as the positive parity component of the same band. The resulting large torsional splitting of −4.596(3) cm−1 points to a strong coupling to the torsional motion. The band at 95 cm−1 has only one torsional parity component within its rotational envelope. The observed large change of its rotational constants does not fit to the pattern of the other vibrations and the band is tentatively assigned as an overtone of the torsional vibration τ with positive parity. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 3
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    Soil Science Society of America (SSSA)
    Publication Date: 2015-05-16
    Description: Processes in capillary fringes (CFs) have a complex nature due to the interactions between the solid, liquid, and gaseous environments. Despite a considerable body of literature on CFs coming from different disciplines, the ongoing processes and their complex interactions are yet only partially understood.
    Electronic ISSN: 1539-1663
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
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  • 4
    Publication Date: 2015-02-18
    Description: Water flow and solute transport in unsaturated porous media are affected by the highly nonlinear material properties and nonequilibrium effects. This makes experimental procedures and modeling of water flow and solute transport challenging. In this study, we present an extension to the well-known multistep-outflow (MS-O) and the newly introduced multistep-flux (MS-F) approaches to measure solute dispersivity as a function of water content under well-defined conditions (i.e., constant pressure head and uniform water content). The new approach is termed multistep-transport (MS-T) and complements the MS-O and MS-F approaches. Our setup allows for applying all three approaches in a single experimental setting. Hence, it provides a comprehensive data set to parameterize unsaturated flow and transport processes in a consistent way. We demonstrate this combined approach (MS-OFT) for sand (grain diameter: 0.1–0.3 mm) and complemented the experimental results with an analysis of the underlying pore structure using X-ray computed tomography (CT). The results show that dispersivity is a nonlinear function of water content, and a critical water content (0.2) exists at which dispersivity increased significantly. The results could be explained by marked change in the geometry of the flow field as derived from X-ray CT measurements. It is characterized by a reduced connectivity of the water phase. The results demonstrate the potential of a combined approach linking pore structure, hydraulic functions, and transport characteristic.
    Electronic ISSN: 1539-1663
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
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  • 5
    Publication Date: 2012-08-01
    Description: Knowledge of local water fluxes across the soil–root interface is essential to understand and model root water uptake. Despite its importance, there is a lack of direct methods to measure the location of water uptake along roots. The aim of this study was to develop a technique to quantify local flux of water from soil to the roots of living plants. To this end, we used neutron radiography to trace the transport of deuterated water (D2O) into individual roots. We grew lupin (Lupinus albus L.) in 30- by 25- by 1-cm containers filled with a sandy soil, which was partitioned into different compartments using 1-cm-thick layers of coarse sand. We locally injected D2O into a selected soil compartment near the roots of 18-d-old lupin during the day (transpiring) and night (not transpiring). The transport of D2O into roots was then monitored using time-series neutron radiography. The results show that: (i) the transport of D2O into roots was faster during the day than at night; and (ii) during the day, D2O was quickly transported along the roots toward the shoots, while at night this transport was insignificant. The differences between day and night measurements were explained by convective transport of D2O into the roots driven by transpiration. To quantify the local transport of D2O into roots, we developed a simple convection–diffusion model that assumed the endodermis as the main resistance to water transport. The D2O uptake predicted by the model was in agreement with the axial flow within the roots as derived from D2O transport behind the capillary barrier. This new method allows quantification of local water uptake in different parts of the root system.
    Electronic ISSN: 1539-1663
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
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  • 6
    Publication Date: 2012-08-01
    Description: Prediction of fluid phase dynamics in the vadose zone is hampered by the presence of different types of sharp interfaces, questioning the validity of standard theory that is based on assumption of continuity of air and water phases, uniform distribution of fluids in a control volume, local equilibrium of phase contents and pressures, and slow process velocities. Complexity of fluid front morphology and burst-like redistribution processes may be accentuated across material contrasts or at the interface between soil and atmosphere. This special section presents eleven contributions highlighting the role of interfaces on water and air distributions and outlining methods to improve prediction of interfacial displacement.
    Electronic ISSN: 1539-1663
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
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  • 7
    Publication Date: 2014-08-12
    Description: Root system architecture and associated root–soil interactions exhibit large changes over time. Nondestructive methods for the quantification of root systems and their temporal development are needed to improve our understanding of root activity in natural soils. X-ray computed tomography (X-ray CT) was used to visualize and quantify growth of a single Vicia faba L. root system during a drying period. The plant was grown under controlled conditions in a sandy soil mixture and imaged every second day. Minkowski functionals and Euclidean distance transform were used to quantify root architectural traits. We were able to image the root system with water content decreasing from 29.6 to 6.75%. Root length was slightly underestimated compared with destructive measurements. Based on repeated measurements over time it was possible to quantify the dynamics of root growth and the demography of roots along soil depth. Measurement of Euclidean distances from any point within the soil to the nearest root surface yielded a frequency distribution of travel distances for water and nutrients towards roots. Our results demonstrate that a meaningful quantitative characterization of root systems and their temporal dynamics is possible.
    Electronic ISSN: 1539-1663
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
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  • 8
    Publication Date: 2016-05-17
    Description: The remarkable complexity of soil and its importance to a wide range of ecosystem services presents major challenges to the modeling of soil processes. Although major progress in soil models has occurred in the last decades, models of soil processes remain disjointed between disciplines or ecosystem services, with considerable uncertainty remaining in the quality of predictions and several challenges that remain yet to be addressed. First, there is a need to improve exchange of knowledge and experience among the different disciplines in soil science and to reach out to other Earth science communities. Second, the community needs to develop a new generation of soil models based on a systemic approach comprising relevant physical, chemical, and biological processes to address critical knowledge gaps in our understanding of soil processes and their interactions. Overcoming these challenges will facilitate exchanges between soil modeling and climate, plant, and social science modeling communities. It will allow us to contribute to preserve and improve our assessment of ecosystem services and advance our understanding of climate-change feedback mechanisms, among others, thereby facilitating and strengthening communication among scientific disciplines and society. We review the role of modeling soil processes in quantifying key soil processes that shape ecosystem services, with a focus on provisioning and regulating services. We then identify key challenges in modeling soil processes, including the systematic incorporation of heterogeneity and uncertainty, the integration of data and models, and strategies for effective integration of knowledge on physical, chemical, and biological soil processes. We discuss how the soil modeling community could best interface with modern modeling activities in other disciplines, such as climate, ecology, and plant research, and how to weave novel observation and measurement techniques into soil models. We propose the establishment of an international soil modeling consortium to coherently advance soil modeling activities and foster communication with other Earth science disciplines. Such a consortium should promote soil modeling platforms and data repository for model development, calibration and intercomparison essential for addressing contemporary challenges.
    Electronic ISSN: 1539-1663
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
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  • 9
    Publication Date: 2013-11-16
    Electronic ISSN: 1539-1663
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
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  • 10
    Publication Date: 2013-11-16
    Description: The hydraulic behavior of soil is determined by its hydraulic properties and their variability in space. In agricultural soils, this heterogeneity may stem from tillage or may have natural origin. The root distribution of plants will adapt to some extent to this soil heterogeneity. However, the combined impact of soil heterogeneity and root water uptake (RWU) on long-term soil water budgets has not received much attention. Numerical experiments helped identify how soil heterogeneity affects plant transpiration, soil evaporation, and groundwater recharge. Two-dimensional virtual soils with hierarchical heterogeneity, both natural and tillage induced, served as a basis for modeling soil water dynamics for a 10-yr climate record from two weather stations in Germany that vastly differ in annual precipitation. The complex interactions between soil and vegetation were explored by (i) comparing different RWU strategies (depth-, structure-, and time-dependent root profiles), (ii) land use types (perennial grass and annual winter crops), (iii) a combination of textures (silt above sand and sand above loam), and (iv) RWU with or without a compensation mechanism. The simulations were repeated with one-dimensional, effective representations of these virtual soils. In the framework of hydropedology, this study shed some light on the interaction between plants and pedological features and its impact on the macroscopic soil water budget. We demonstrated that land use has a major impact on the annual water balance through the partitioning of evapotranspiration into bare soil evaporation and plant transpiration. Compensational RWU becomes important for the annual water balance when the root zone comprises contrasting materials with respect to water holding capacity. Soil heterogeneity has in fact a minor impact on long-term soil water budgets. As a consequence, the relative contribution of plant transpiration, soil evaporation, and groundwater recharge to the total soil water loss was well reproduced by simulations in one-dimensional effective soil profiles. This advocates the application of one-dimensional soil–atmosphere–vegetation transfer (SVAT) models at larger scales. These findings only hold for assumptions made in our numerical simulations including flat area without lateral flow and no macropore flow.
    Electronic ISSN: 1539-1663
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
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