Calculating groundwater response times for flow in heterogeneous porous media

Predicting the amount of time required for a transient groundwater response to take place is a practical question that is of interest in many situations. This time scale is often called the response time. In the groundwater hydrology literature there are two main methods used to calculate the response time: (i) both the transient and steady state groundwater flow equations are solved, and the response time is taken to be amount of time required for the transient solution to approach the steady solution within some tolerance; and (ii) simple scaling arguments are adopted. Certain limitations restrict both of these approaches. Here we outline a third method, based on the theory of mean action time. We derive the governing boundary value problem for both the mean and variance of action time for confined flow in two-dimensional heterogeneous porous media. Importantly, we show that these boundary value problems can be solved using widely available software. Applying these methods to a test case reveals the advantages of the theory of mean action time relative to standard methods.Comment: 15 pages, 2 Figure

A Bayesian approach to the aperture problem of 3D motion perception

We suggest a geometric-statistical approach that can be ap- plied to the 3D aperture problem of motion perception. In simulations and psychophysical experiments we study per- ceived 3D motion direction in a binocular viewing geometry by systematically varying 3D orientation of a line stimulus moving behind a circular aperture. Although motion direc- tion is inherently ambiguous perceived directions show sys- tematic trends and a Bayesian model with a prior for small depth followed by slow motion in 3D gives reasonable fits to individual data. We conclude that the visual system tries to minimize velocity in 3D but that earlier disparity processing strongly influences perceived 3D motion direction. We discuss implications for the integration of disparity and motion cues in the human visual system

Mixed-State Quasiparticle Spectrum for d-wave Superconductors

Controversy concerning the pairing symmetry of high-$T_c$ materials has motivated an interest in those measurable properties of superconductors for which qualitative differences exist between the s-wave and d-wave cases. We report on a comparison between the microscopic electronic properties of d-wave and s-wave superconductors in the mixed state. Our study is based on self-consistent numerical solutions of the mean-field Bogoliubov-de Gennes equations for phenomenological BCS models which have s-wave and d-wave condensates in the absence of a magnetic field. We discuss differences between the s-wave and the d-wave local density-of-states, both near and away from vortex cores. Experimental implications for both scanning-tunneling-microscopy measurements and specific heat measurements are discussed.Comment: 10 pages, REVTEX3.0, 3 figures available upon reques

Magnetic field and pressure effects on charge density wave, superconducting, and magnetic states in Lu5_5Ir4_4Si10_{10} and Er5_5Ir4_4Si10_{10}

We have studied the charge-density-wave (CDW) state for the superconducting Lu$_5$Ir$_4$Si$_{10}$ and the antiferromagnetic Er$_5$Ir$_4$Si$_{10}$ as variables of temperature, magnetic field, and hydrostatic pressure. For Lu$_5$Ir$_4$Si$_{10}$, the application of pressure strongly suppresses the CDW phase but weakly enhances the superconducting phase. For Er$_5$Ir$_4$Si$_{10}$, the incommensurate CDW state is pressure independent and the commensurate CDW state strongly depends on the pressure, whereas the antiferromagnetic ordering is slightly depressed by applying pressure. In addition, Er$_5$Ir$_4$Si$_{10}$ shows negative magnetoresistance at low temperatures, compared with the positive magnetoresistance of Lu$_5$Ir$_4$Si$_{10}$.Comment: 12 pages, including 6 figure

Evaluation of the CENTURY model with laboratory measured soil respiration

Non-Peer ReviewedThe CENTURY model is widely used in assessing the effect of management on soil C dynamics. However, recent testing of the model revealed that it performed unsatisfactorily in simulating soil C changes in southwestern Saskatchewan, suggesting that the model may need further testings and modifications. Evaluations of the model were made with measurements conducted in an laboratory experiment which included different straw placements (incorporated in the soil and applied on the soil surface) and soil water regimes (continuously moist and moist-dry). Results from model testing revealed some weaknesses of the model and modifications were made to improve model performance. The temperature function was modified to slightly increase the relative decomposition rate when temperatures were below the reference temperature. The moisture function was modified to reduce the relative decomposition rate when the soil moisture was very low. The modified model also assumed that soil mineral N is readily available for the use of decomposition of soil C pools, but only about 6.6 mg m-2 d-1 of soil N is available for C pools on the soil surface. When N availability is less than that required for maximum decomposition rates of soil or surface pools, decomposition rates of these pools were reduced until supply met demand. The modified model improved simulations of daily C fluxes, cumulative CO2 emissions and soil mineral N. To use this modified model for estimating soil respiration in the field, further studies on the N availability for soil surface C pools and the dryness of surface-placed residue are needed. The concentration of CO2 in the atmosphere has increased by about 25% since the beginning of the industrial revolution. There are concerns that continuing increases in levels of CO2 and other greenhouse gases will contribute to global warming. Soils contains about three times as much C as the atmosphere, and they have the potential to store additional C (Campbell and Zentner, 1993). Agricultural soils on the Canadian prairies contain about 3 Pg soil organic carbon (SOC) in the top 30 cm layer, which is about 20 times the amount of CO2-C emitted annually by fossil fuel combustion in Canada. Research shows that if properly managed agricultural lands could be an important sink for C. Management options to enhance C storage in Canadian prairie soils include: decreasing summer fallow frequency, reducing tillage, including legumes in crop rotations, proper fertilization, and growing forage and trees on marginal lands (Campbell and Zentner, 1993; Campbell et al., 1995). For example, reduction in tillage intensity, especially no-tillage (NT) cropping, has been shown to increase SOC at various locations (Janzen, et al. 1998). Although changes in SOC occur when soil management practices are altered (Mann, 1986), it is common for these changes to remain undetectable for 10 or 20 years. The reason is that because of the inherent spatial variability of SOC in the field, too many samples are required to be taken to ensure that small differences can be statistically separated (Campbell et al., 1976; Campbell et al., 2000). Thus, we often use a process-based simulation model that describes soil organic matter turnover and nitrogen cycling dynamics in soils to estimate management induced SOC changes. The CENTURY model (Parton et al., 1987) is one of such models that is the most widely used and has been extensively evaluated in various ecosystems (Scholes et al., 1997). However, the recent testing of the CENTURY model revealed that it performed unsatisfactorily in simulating soil C changes in a 30-yr crop rotation experiment in southwestern Saskatchewan (Campbell et al., 1999), suggesting that the model may need further testings and modifications for use on the Canadian prairies. Because of the problems associated with SOC measurement and the variability of environmental conditions in the field, it is difficult to rigorously test the mechanism of a process-based soil organic matter model. Alternatively, the model can be readily tested against measurements of CO2 emissions from a controlled laboratory experiment. The objectives of this study were thus: (1) to test the validity of the CENTURY model with the soil respiration measured from laboratory experiments and (2) to address the weaknesses revealed during the model testings by modifying the model

Analytic expressions for alpha particle preformation in heavy nuclei

The experimental alpha decay energies and half-lives are investigated systematically to extract the alpha particle preformation in heavy nuclei. Formulas for the preformation factors are proposed. They can be used to guide the microscopic studies on preformation factors and perform accurate calculations of the alpha decay half-lives. There is little evidence for the existence of an island of long stability of superheavy nuclei (SHN)

Influence of casting temperature on the thermal stability of Cu- and Zr-based metallic glasses: theoretical analysis and experiments

Influence of casting temperature on the thermal stability of Cu- and Zr-based metallic glasses (MGs) was analyzed based on the monomer-cluster structural model using the Johnson-Mehl-Avrami (JMA) equation. The result indicates that increasing the casting temperature can enhance the thermal stability of MGs. It is suggested that it be attributed to the decrease in the amount of the local ordering clusters induced by the elevating casting temperature. The prediction is confirmed by continuous heating transformation diagrams constructed for the Cu- and Zr-amorphous samples obtained under different casting temperatures