Showing papers by "Marcel G. Schaap published in 2006"

A Modified Mualem–van Genuchten Formulation for Improved Description of the Hydraulic Conductivity Near Saturation

Abstract: The unsaturated soil hydraulic properties are often described using Mualem–van Genuchten (MVG) type analytical functions. Recent studies suggest several shortcomings of these functions near saturation, notably the lack of second-order continuity of the soil water retention function at saturation and the inability of the hydraulic conductivity function to account for macroporosity. We present a modified MVG formulation that improves the description of the hydraulic conductivity near saturation. The modified model introduces a small but constant air-entry pressure ( h s ) into the water retention curve. Analysis of the UNSODA soil hydraulic database revealed an optimal value of −4 cm for h s , more or less independent of soil texture. The modified model uses a pressure dependent piece-wise linear correction to ensure that deviations between measured and fitted conductivities between pressure heads of 0 and −40 cm were eliminated. A small correction was found necessary between −4 and −40 cm, and a much larger correction was needed between 0 and −4 cm. An average RMSE in log K of only 0.26 remained for a data set of 235 samples. The resulting modified MVG model was found to have small systematic errors across the entire pressure range. The modified model appears well suited for large-scale vadose zone flow and transport simulations, including inverse modeling studies.

Percolation Theory for Flow in Porous Media

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Models for Indirect Estimation of Soil Hydraulic Properties

Abstract: retention and hydraulic conductivity characteristics. Indirect methods can be classified into semiphysical methods that are based on mechanical assumptions regarding particle and pore arrangements, and statistical methods that are known as pedotransfer functions. Both classes are described and evaluated on their merits, and some characteristic examples are given. A list of applicable software is described at the end of the article and an extensive reference list provides sufficient material for further background information.

Can Basin-Scale Recharge Be Estimated Reasonably with Water-Balance Models?

Abstract: We examine in-place recharge as an example of the complex, basin-scale hydrologic processes that are being represented with simplified numerical models. The rate and distribution of recharge depend on local meteorological conditions and hydrogeologic properties. The pattern of recharge is defined predominantly by the distribution of net precipitation (precipitation less evapotranspiration), but different pedotransfer functions (PTFs) predict different fractions of precipitation that become in-place recharge at a given location. At any single location, these differences can often be explained on the basis of the PTF characteristics, but because of the complex averaging that occurs across a basin, the combined effects of meteorological variation and soil textural variation on the basin-wide recharge rates cannot be predicted on the basis of the characteristics of different PTFs. In fact, we show that the same basin-scale numerical model, using identical inputs and modeling options, can produce almost an order of magnitude variation in predicted basin total recharge depending on the choice of PTF. This suggests that sensitivity analyses should be performed on the choice of constitutive relationship (e.g., PTF) when assessing the predictive capability of basin-scale hydrologic models.

Electrical properties cup (EPC) for characterizing water content of martian and lunar soils

Abstract: In this effort we used electrical impedance spectroscopy and a four-probe apparatus, the electrical properties cup (EPC), to measure the properties of various lunar and martian soil simulants. The impedance values are characterized by a resistance-capacitor network that is used to determine the soil conductivity and dielectric constant. In this effort we measured the impedance of different types of martian soil simulants (Silica sand, Atacama desert sand, and Moses lake basalt) and lunar simulants. The results show that the soil impedance measurements are strongly dependent on water content and soil type and to a lesser extent particle size and electrolyte concentration. This presentation describes the experimental four-probe apparatus, procedures used to prepare the samples including soil washing and loading, and soil impedance measurements.

Dynamic Effects in Oil/Water and Air/Water Capillary Pressure-Saturation Curves: Experiments and Lattice-Boltzmann Simulations

Abstract: The capillary pressure-saturation curve is widely used to characterize hydraulic properties of porous media It is often assumed that curves measured under equilibrium or steady-state flow conditions can be applied to transient flow conditions, and vice versa Yet, substantial experimental evidence suggests that capillary pressure-saturation curves obtained during transient conditions differ from those obtained under equilibrium or steady-state conditions It has been shown that the capillary pressure-saturation curve shows signs of dynamic behavior depending on the inflow and outflow rate applied to the porous system The exact cause of the observed shift is not yet fully understood It is hypothesized that the mechanisms responsible for dynamic behavior include: (1) the geometry of the pore space, (2) interfacial phenomena at the pore scale, and (3) the interplay of inertial and viscous forces In this investigation, air/water and oil/water imbibition and drainage experiments were conducted on a column of packed glass beads Various inflow and outflow rates were applied to each multi-phase system, which resulted in capillary pressure-saturation curves that exhibit varying degrees of dynamic behavior The dynamic behavior observed in preliminary oil/water experiments was less pronounced than the behavior observed in past air/water experiments This suggests that the viscous and inertial forces may only be a major factor when the density and viscosity ratios are large, as is the case for the air/water system The dynamic behavior was examined using conceptual 2D and 3D lattice-Boltzmann (LB) simulations We used the multi-phase, multi-component model developed by Shan and Chen for these simulations The conceptual LB simulations can provide insights into pore-scale interfacial phenomena and help explain the dynamic behavior observed in the experiments Scaling of time and space from LB parameters to physical parameters was performed to make comparisons between simulation and experimental results possible

Linking Experimental Capillary Pressure-Saturation Data with Lattice Boltzmann Simulations.

Abstract: Recent advances in observational and computational techniques have facilitated the study of fluid dynamics and interfacial geometry in porous media. Within some experimental limitations, computed tomography X-ray (CMT) and magnetic resonance imaging (MRI) are now able to accurately map the 3D structure of porous geometries. Computational advances largely concern Lattice Boltzmann (LB) method that has been shown to be useful in simulating microscale flow in porous media. With some phenomenological or thermodynamic extensions, the LB method is also able to deal with microscale interfacial phenomena in single or multiphase systems. The goal of this presentation is to provide insight into what is needed to make a link between 3D experimental observations of interfacial geometry and LB simulations. The experimental data consist of CMT observations several Sotrol-water displacements inside a glass bead system with a resolution of 17 microns. Also available are capillary pressure-saturation curves between 0 and 1kPa. The LB model is that of Shan-Chen as modified by Martys and Chen (1996). We present the most parsimonious way to calibrate the surface tension and contact angle in the model, define space, pressure and time scaling. We will also identify potential problems relating to pore-size and digitization effects that are present in the simulations, but not in the original observations. The analyses are partly performed on idealized systems and finally applied to large scale (107 voxel) simulations of the real physical systems. Observations are simulations are compared in terms of pressure-saturation curves, and where possible, in terms of fluid distribution and interfacial curvatures.