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W. J. Riley - One of the best experts on this subject based on the ideXlab platform.
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A new Top Boundary condition for modeling surface diffusive exchange of a generic volatile tracer: theoretical analysis and application to soil evaporation
Hydrology and Earth System Sciences, 2013Co-Authors: Jinyun Tang, W. J. RileyAbstract:Abstract. We describe a new Top Boundary condition (TBC) for representing the air–soil diffusive exchange of a generic volatile tracer. This new TBC (1) accounts for the multi-phase flow of a generic tracer; (2) accounts for effects of soil temperature, pH, solubility, sorption, and desorption processes; (3) enables a smooth transition between wet and dry soil conditions; (4) is compatible with the conductance formulation for modeling air–water volatile tracer exchange; and (5) is applicable to site, regional, and global land models. Based on the new TBC, we developed new formulations for bare-soil resistance and corresponding soil evaporation efficiency. The new soil resistance is predicted as the reciprocal of the harmonic sum of two resistances: (1) gaseous and aqueous molecular diffusion and (2) liquid mass flow resulting from the hydraulic pressure gradient between the soil surface and center of the Topsoil control volume. We compared the predicted soil evaporation efficiency with those from several field and laboratory soil evaporation measurements and found good agreement with the typically observed two-stage soil evaporation curves. Comparison with the soil evaporation efficiency equation of Lee and Pielke (1992; hereafter LP92) indicates that their equation can overestimate soil evaporation when the atmospheric resistance is low and underestimate soil evaporation when the soil is dry. Using a synthetic inversion experiment, we demonstrated that using inverted soil resistance data from field measurements to derive empirical soil resistance formulations resulted in large uncertainty because (1) the inverted soil resistance data are always severely impacted by measurement error and (2) the derived empirical equation is very sensitive to the number of data points and the assumed functional form of the resistance. We expect the application of our new TBC in land models will provide a consistent representation for the diffusive tracer exchange at the soil–air interface.
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A new Top Boundary condition for modeling surface diffusive exchange of a generic volatile tracer: theoretical analysis and application to soil evaporation
2012Co-Authors: J. Y. Tang, W. J. RileyAbstract:Abstract. We describe a new Top Boundary condition (TBC) for representing the air-soil diffusive exchange of a generic volatile tracer. This new TBC (1) accounts for the multi-phase flow of a generic tracer; (2) accounts for effects of soil temperature, pH, solubility, sorption, and desorption processes; (3) enables a smooth transition between wet and dry soil conditions; (4) is compatible with the conductance formulation for modeling air-water volatile tracer exchange; and (5) is applicable to site, regional, and global land models. Based on the new TBC, we developed new formulations for bare-soil resistance and corresponding soil evaporation efficiency. The new soil resistance is predicted as the reciprocal of the harmonic sum of two resistances: (1) gaseous and aqueous molecular diffusion and (2) liquid mass flow resulting from the hydraulic pressure gradient between the soil surface and center of the Topsoil control volume. The resulting soil evaporation efficiency reasonably explains the two-stage soil evaporation curves typically observed in field and laboratory soil evaporation measurements. Comparison with the soil evaporation efficiency equation of Lee and Pielke (1992; hereafter LP92) indicates that their equation can overestimate soil evaporation when the atmospheric resistance is low and underestimate soil evaporation when the soil is dry. Using a synthetic inversion experiment, we demonstrated that using inverted soil resistance data from field measurements to derive empirical soil resistance formulations resulted in large uncertainty because (1) the inverted soil resistance data is always severely impacted by measurement error and (2) the derived empirical equation is very sensitive to the number of data points and the assumed functional form of the resistance. We expect the application of our new TBC in land models will provide a consistent representation for the diffusive tracer exchange at the soil–air interface.
Gregory A. Houseman - One of the best experts on this subject based on the ideXlab platform.
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Rayleigh-Taylor instability under a shear stress free Top Boundary condition and its relevance to removal of mantle lithosphere from beneath the Sierra Nevada
Tectonics, 2008Co-Authors: Christopher Harig, Peter Molnar, Gregory A. HousemanAbstract:[1] The separation of zones of apparent downwelling flow at the ends of the Sierra Nevada suggests a relatively large wavelength (∼500 km) of unstable growth, but Rayleigh-Taylor instabilities for plausible rheological structures with a fixed Top Boundary condition require much shorter wavelengths (
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rayleigh taylor instability under a shear stress free Top Boundary condition and its relevance to removal of mantle lithosphere from beneath the sierra nevada
Tectonics, 2008Co-Authors: Christopher Harig, Peter Molnar, Gregory A. HousemanAbstract:[1] The separation of zones of apparent downwelling flow at the ends of the Sierra Nevada suggests a relatively large wavelength (∼500 km) of unstable growth, but Rayleigh-Taylor instabilities for plausible rheological structures with a fixed Top Boundary condition require much shorter wavelengths (<100 km) for maximum growth rates. To understand this difference, we analyze analytical solutions and perform numerical 2-D plane strain experiments for Rayleigh-Taylor instability of a dense layer overlying a less dense substratum, representing the instability between the mantle lithosphere and the underlying asthenosphere, focusing on the effects of a shear stress free Boundary condition at the Top. The overall effect of this condition is an enhancement of growth rate factors at long wavelengths, which depends greatly on the exponential viscosity variation with depth of the layer. With large or little variation across the unstable layer, the solutions approximate those with a fixed Top Boundary condition or for constant viscosity, respectively. An intermediate zone showing the enhanced growth rates includes ratios of layer thickness to viscosity e-folding length, h/L, of ∼1–8 for Newtonian viscosity and ∼1–4 for nonlinear viscosity. The free Top condition is likely applicable to geologic situations where the lower crust is weak. Olivine flow laws and low-temperature estimates at 35 km depth (255–355°C) place the Sierra Nevada viscosity scaling ratio, h/L, between 5 and 9. Thus longer wavelengths than commonly assumed for Rayleigh-Taylor instabilities seem permissible when viscosity decreases with depth and the Top surface of the layer is only weakly constrained.
Peter Bayer - One of the best experts on this subject based on the ideXlab platform.
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a finite line source model with cauchy type Top Boundary conditions for simulating near surface effects on borehole heat exchangers
Energy, 2016Co-Authors: Jaime A Rivera, Philipp Blum, Peter BayerAbstract:BHEs (borehole heat exchangers) are the most common shallow geothermal applications. By approximating the BHE as a line source, semi-analytical models can describe the heat exchange within the ground. These models though always assume prescribed temperature at the ground surface. This work presents a formulation which expands existing finite line source models by implementing a more general Cauchy-type Top Boundary condition and in this way, a better estimation of the heat fluxes at the ground surface. The new formulation is numerically verified and examined in a dimensionless analysis. It is demonstrated that the discrepancy to prescribed temperature settings is significant near to the ground surface, and it propagates deeper when groundwater flow is absent and when strong decoupling between the thermal regimes interacting at the land surface is assumed. The new approach shows to be suited especially for short BHEs, both for more flexible and accurate prediction of the ground thermal regime as well as for long-term analysis of technological performance.
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Influence of spatially variable ground heat flux on closed-loop geothermal systems: Line source model with nonhomogeneous Cauchy-type Top Boundary conditions
Applied Energy, 2016Co-Authors: Jaime A Rivera, Philipp Blum, Peter BayerAbstract:Abstract Borehole heat exchangers (BHEs) utilize the shallow ground to extract geothermal energy. Mostly they are installed in urbanized areas, where the thermal regime is strongly influenced by pavements, buildings and other urban infrastructures. In order to account for the spatial and temporal variability in the above-ground urban temperatures, a new semi-analytical model with a Cauchy-type Top Boundary is introduced. With this model, it is possible to estimate the transient three-dimensional temperature field in the near-surface ground influenced by the interaction of BHEs, horizontal groundwater flow, land use type and associated surface air temperature (SAT). It is verified with a numerical model and sensitivity analyses are conducted to examine the relevance of the prevailing thermal regime. By adopting a dimensionless formulation, it is shown that the decoupling between temperature fields at the ground surface restraints heat fluxes and penetration depth of thermal signals above ground. A systematic comparison with traditional Dirichlet-type Boundary conditions shows that a fixed temperature formulation generally overestimates the thermal effect of land surface signals on thermal plumes of BHEs. This is also addressed by investigating the ground energy balance during operation of the geothermal system.
Jinyun Tang - One of the best experts on this subject based on the ideXlab platform.
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A new Top Boundary condition for modeling surface diffusive exchange of a generic volatile tracer: theoretical analysis and application to soil evaporation
Hydrology and Earth System Sciences, 2013Co-Authors: Jinyun Tang, W. J. RileyAbstract:Abstract. We describe a new Top Boundary condition (TBC) for representing the air–soil diffusive exchange of a generic volatile tracer. This new TBC (1) accounts for the multi-phase flow of a generic tracer; (2) accounts for effects of soil temperature, pH, solubility, sorption, and desorption processes; (3) enables a smooth transition between wet and dry soil conditions; (4) is compatible with the conductance formulation for modeling air–water volatile tracer exchange; and (5) is applicable to site, regional, and global land models. Based on the new TBC, we developed new formulations for bare-soil resistance and corresponding soil evaporation efficiency. The new soil resistance is predicted as the reciprocal of the harmonic sum of two resistances: (1) gaseous and aqueous molecular diffusion and (2) liquid mass flow resulting from the hydraulic pressure gradient between the soil surface and center of the Topsoil control volume. We compared the predicted soil evaporation efficiency with those from several field and laboratory soil evaporation measurements and found good agreement with the typically observed two-stage soil evaporation curves. Comparison with the soil evaporation efficiency equation of Lee and Pielke (1992; hereafter LP92) indicates that their equation can overestimate soil evaporation when the atmospheric resistance is low and underestimate soil evaporation when the soil is dry. Using a synthetic inversion experiment, we demonstrated that using inverted soil resistance data from field measurements to derive empirical soil resistance formulations resulted in large uncertainty because (1) the inverted soil resistance data are always severely impacted by measurement error and (2) the derived empirical equation is very sensitive to the number of data points and the assumed functional form of the resistance. We expect the application of our new TBC in land models will provide a consistent representation for the diffusive tracer exchange at the soil–air interface.
Anders Wörman - One of the best experts on this subject based on the ideXlab platform.
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Groundwater flow systems theory: research challenges beyond the specified-head Top Boundary condition
Hydrogeology Journal, 2016Co-Authors: Etienne Bresciani, Anders Wörman, Tom Gleeson, Pascal Goderniaux, J.-r. De Dreuzy, Adrian D. Werner, Wouter Zijl, Okke BatelaanAbstract:Groundwater flow systems theory : research challenges beyond the specified-head Top Boundary condition
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The use of spectral analysis-based exact solutions to characterize Topography-controlled groundwater flow
Hydrogeology Journal, 2011Co-Authors: Lars Marklund, Anders WörmanAbstract:Spectral analysis enhances the ability to analyze groundwater flow at a steady state by separating the Top Boundary condition into its periodic forms. Specifically, spectral analysis enables comparisons of the impact of individual spatial scales on the total flow field. New exact spectral solutions are presented for analyzing 3D groundwater flow with an arbitrarily shaped Top Boundary. These solutions account for depth-decaying, anisotropic and layered permeability while utilizing groundwater flux or the phreatic surface as a Top Boundary condition. Under certain conditions, groundwater flow is controlled by Topography. In areas where the groundwater flow is controlled by the Topography, the unknown water table is often approximated by the Topography. This approximation induces a systematic error. Here, the optimal resolution of digital elevation models (DEMs) is assessed for use as a Top Boundary in groundwater flow models. According to the analysis, the water-table undulation is smoother than the Topography; therefore, there is an upper limit to the resolution of DEMs that should be used to represent the groundwater surface. The ability to represent DEMs of various spectral solutions was compared and the results indicate that the fit is strongly dependent on the number of harmonics in the spectral solution.