Article

Estimation of soil waterfront velocity and saturated hydraulic conductivity in a Hele-Shaw apparatus with an optical flow method

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Abstract

The estimation of saturated hydraulic conductivity is a key parameter for studying the water flow in the unsaturated zone of the soil. In this work, a combined modelling approach of the waterfront movement is elaborated, integrating a physical analogue type device with image analysis. A customized Hele-Shaw apparatus is designed for the wetting process visual inspection, while an optical flow algorithm is used for the numerical calculation of flow velocity and saturated hydraulic conductivity values at a pixel level of the image processing. Saturated hydraulic conductivity values estimated by the proposed method are in close agreement with values found in the literature under similar soil conditions. Also, the proposed method seems to be a useful tool for a comprehensive analysis of the waterfront movement in the vadose zone.

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Evaporation rates from porous media may vary considerably due to changes in internal transport mechanisms and potential interruption of hydraulic continuity; both are influenced by media pore space properties. Evaporation behavior in layered porous media is affected by thickness and sequence of layering and capillary characteristics of each layer. We propose a composite characteristic length for predicting drying front depth at the end of a period with a high and constant drying rate (stage 1 evaporation) from layered porous media. The model was tested in laboratory experiments using Hele-Shaw cells filled with alternating layers of coarse and fine sands considering different combinations of thicknesses and positions. The presence of textural interfaces affects drying rate, modifies liquid phase configuration, and affects the dynamics of the receding drying front. Neutron radiography measurements were used to delineate dynamics of liquid phase distribution with high temporal and spatial resolution. Results show that air invading an interface between fine and coarse sand layers results in a capillary pressure jump and subsequent relaxation that significantly modify liquid phase distribution compared with evaporation from homogeneous porous media. Insights are potentially useful for designing mulching strategies and capillary barriers aimed at reducing evaporative losses.
Chapter
The accurate knowledge of hydraulic properties for unsaturated soils is critical in addressing problems in a variety of disciplines such as hydrology, ecology, environmental sciences, soil science, and agriculture. The purpose of this paper is to review the characterization of unsaturated soil hydraulic properties for their applicability in models simulating unsaturated water transport. In a theoretical section, we present the fundamentals for the definitions of the hydraulic properties in the framework of the continuum theory as well as provide the common parameterizations of the hydraulic functions. The characterization and parameterization of hysteresis and the phenomenon of dynamic effects in hydraulic properties are addressed in subsequent sections. Finally we discuss issues related to the handling of spatial and temporal variability, including geostatistical characterization, upscaling, and scale dependency of effective properties. The review closes with a summary on limits and opportunities for modeling water transport with Richards' equation.
Article
Many environmental studies on the protection of European soil and water resources make use of soil water simulation models. A major obstacle to the wider application of these models is the lack of easily accessible and representative soil hydraulic properties. In order to overcome this apparent lack of data, a project was initiated to bring together the available hydraulic data which resided within different institutions in Europe into one central database. This information was then used to derive a set of pedotransfer functions applicable to studies at a European scale. These pedotransfer functions predict the hydraulic properties from parameters collected during soil surveys and can be a good alternative for costly and time-consuming direct measurement of these properties. A total of 20 institutions from 12 European countries collaborated in establishing the database of draulic operties of uropean oils (HYPRES). This database has a flexible relational structure capable of holding a wide diversity of both soil pedological and hydraulic data. As these data were contributed by 20 different institutions it was necessary to standardise both the particle-size and the hydraulic data. A novel similarity interpolation procedure was successfully used to achieve standardization of particle-sizes according to the FAO clay, silt and sand particle-size ranges. Standardization of hydraulic data was achieved by fitting the Mualem-van Genuchten model parameters to the individual θ(h) and K(h) hydraulic properties stored in HYPRES. The HYPRES database contains information on a total of 5521 soil horizons (including replicates). Of these, 4030 horizons had sufficient data to be used in the derivation of pedotransfer functions. Information on both water retention and hydraulic conductivity was available for 1136 horizons whereas 2894 horizons had only information on water retention. Each soil horizon was allocated to one of 11 possible soil textural/pedological classes derived from the six FAO texture classes (five mineral and one organic) and the two pedological classes (topsoil and subsoil) recognised within the 1:1 000 000 scale Soil Geographical Data Base of Europe. Next, both class and continuous pedotransfer functions were developed. By using the class pedotransfer functions in combination with the 1:1 000 000 scale Soil Map of Europe, the spatial distribution of soil water availability within Europe was derived.
Article
Wetting front instability in the vadose zone causes the formation of fingers which can rapidly transport both water and solute to the phreatic surface. The development of the unstable flow field in laboratory experiments is described for an initially dry, two-layer sand system, in which the top layer has a finer texture than the bottom layer. The effect of repeated infiltration cycles and of initial moisture content at field capacity are presented. Fingers once formed in the dry porous media are found to not change location even after several infiltration events. Only saturation and subsequent drainage alters the finger structure within the chamber. In Eastern Long Island, New York, USA, field infiltration experiments using the combination of two dyes showed that water moved through finger-like structures.
Article
Functional evaluations of pedotransfer functions (PTFs) were performed. In a first sample problem, the influence of uncertainty in the PTF was examined on the basis of two functional criteria: the moisture supply capacity (MSC) and the downward flux below the root zone (DFR). In a second sample problem the effect of the uncertainty in the PTFs and the variability in soil properties within a map unit were analyzed. It was found that an improved estimate to the hydraulic properties, obtained by calibrating PTFs using detailed textural information, did not substantially reduce the dispersion of the MSC and DFR distributions. Results from the second sample problem indicated that >90% of the variation in the simulated MSC was caused by estimation errors in the hydraulic properties, overwhelming the map unit variability. -from Authors
Handbook of Machine and Computer Vision -. In: The Guide for Developers and Users
  • A Hornberg
Hornberg, A., 2017. Handbook of Machine and Computer Vision -. In: The Guide for Developers and Users. Wiley-VCH, Göppingen, Germany. https://doi.org/10.1142/ 9789814273398.
Preferential solute transport through layered soils
  • G Oosting
  • R Glass
  • T Steenhuis
Oosting, G., Glass, R., Steenhuis, T., 1987. Preferential solute transport through layered soils. In: Winter Meeting of. American Society of Agricultural Engineers. American Society of Agricultural Engineers, Chicago, US, pp. 87-2624.
SoilKsatDB: global soil saturated hydraulic conductivity measurements for geoscience applications
  • Gupta
Preferential solute transport through layered soils
  • Oosting
Modeling leakage pathways in subsurface formations: Fluid drainage through multiple fractures in porous media: insights from Hele Shaw cell experiments
  • Ray