Studies of smectite membrane behavior: Importance of layer thickness and fabric in experiments at 20°C

The 0.5–2.0 μm diameter size fraction of the Cheto clay from Chambers, Arizona, was sedimented to form 0.5–2.0 cm-thick cakes, either with a relatively random or a relatively well-oriented fabric as defined by the criterion of Meade (1961). The cakes were compacted under a confining pressure of 34.7 MPa (5000 psi) and a solution 0.79 m in NaCI and 0.079 m in CaCl₂ was forced through at 20°C by a fluid pressure differential of either 6.94 MPa (1000 psi) or 13.0 MPa (2000 psi). The filtration efficiencies of the cakes with oriented fabric are roughly twice as great as those of the others, reaching 50–68%. Ca²⁺ passed through the cake preferentially, relative to Na⁺ in three of four experimental runs. Cake thickness, over the fourfold range studied, has no significant effect upon filtration efficiency. The stable oxygen isotopic fractionation for brine water passing through a cake with relatively well-oriented fabric is ?0.96%..     

Ionic diffusion in naturally-occurring aqueous solutions: transition-state models that use either empirical expressions or statistically-derived relationships to predict mutual diffusion coefficients in the concentrated-solution regions of 8 binary systems

Mutual diffusion coefficients for the systems NaCl-H₂O, KCl-H₂O, CaCl₂-H₂O, SrCl₂-H₂O, BaCl₂-H₂O, MgCl₂-H₂O, Na₂SO₄-H₂O, and MgSO₄-H₂O are computed using a model that postulates exchanges between ions and water molecules. Limiting ionic equivalent conductances, a solution-density function, and a mean ionic activity-coefficient function are required as input. A region of changing solution structure extends up to concentrations ranging from about 0.01 molar in MgCl₂-H₂O to about 0.2 molar in KCl-H₂O. In the remaining concentration range to saturation, a single expression in each system containing one variable parameter can be fitted empirically to reproduce selected sets of experimentally measured D^v₁₂ with maximum errors for individual compositions ranging from about 0.25% in KCl-H₂O and Na₂SO₄-H₂O to about 4% in MgCl₂-H₂O. The experimentally measured D^v₁₂ can be reproduced with errors comparable to those of the empirical fits by further postulating that individual ion-water molecule exchanges are coupled to yield hydrated neutral exchange complexes (the activated complexes), and defining probability expressions that describe the following exchanges: Ba²⁺ + 2Cl^? for 3H₂O, 2Ca²⁺ + 4Cl^? for 6H₂O, 2Sr²⁺ + 4Cl^? for 6H₂O, Na⁺ + Cl^? for 3H₂O, 2K⁺ + 2Cl^? for 5H₂O, NaSO^?₄ + Na⁺ for 5H₂O, Mg²⁺ + SO^2?₄ (+MgSO⁰₄) for 4H₂O, and MgCl⁺ + Cl^? for 2H₂O. This calculation also contains one variable parameter. Solute transport between exchange sites is by movement of ions, except for the ion-pair contribution indicated in parentheses.     

Detection of Entrainment Influences on Surface-Layer Measurements and Extension of Monin–Obukhov Similarity Theory

We present a method to detect influences of boundary-layer processes on surface-layer measurements, using statistics and spectra of surface-layer variables only. We validated our detection method with boundary-layer measurements. Furthermore, we confirm that Monin–Obukhov similarity functions fit well to temperature-variance data obtained at two different homogeneous surfaces. However, we found that humidity variance measurements deviate from the universal functions above one of the two studied surfaces for days on which entrained air reached the surface layer. These results confirm that Monin–Obukhov similarity theory should be used with care in the analysis of surface-layer data. Finally, we propose the use of an extra term in flux-variance relations that depends on the entrainment ratio for humidity and on the boundary-layer height. If boundary-layer measurements are not available, we show how the entrainment ratio for humidity can be approximated from the skewness of the humidity distribution.     

Intercomparison of Methods for the Simultaneous Estimation of Zero-Plane Displacement and Aerodynamic Roughness Length from Single-Level Eddy-Covariance Data

We applied three approaches to estimate the zero-plane displacement $d$ through the aerodynamic measurement height $z$ (with $z = z_{m}- d$ and $z_{m}$ being the measurement height above the surface), and the aerodynamic roughness length $z_{0}$ , from single-level eddy covariance data. Two approaches (one iterative and one regression-based) were based on the universal function in the logarithmic wind profile and yielded an inherently simultaneous estimation of both $d$ and $z_{0}$ . The third approach was based on flux–variance similarity, where estimation of $d$ and consecutive estimation of $z_{0}$ are independent steps. Each approach was further divided into two methods differing either with respect to the solution technique (profile approaches) or with respect to the variable (variance of vertical wind and temperature, respectively). All methods were applied to measurements above a large, growing wheat field where a uniform canopy height and its frequent monitoring provided plausibility limits for the resulting estimates of time-variant $d$ and $z_{0}$ . After applying, for each approach, a specific data filtering that accounted for the range of conditions (e.g. stability) for which it is valid, five of the six methods were able to describe the temporal changes of roughness parameters associated with crop growth and harvest, and four of them agreed on $d$ to within 0.3 m most of the time. Application of the same methods to measurements with a more heterogeneous footprint consisting of fully-grown sugarbeet and a varying contribution of adjacent harvested fields exhibited a plausible dependence of the roughness parameters on the sugarbeet fraction. It also revealed that the methods producing the largest outliers can differ between site conditions and stability. We therefore conclude that when determining $d$ for canopies with unknown properties from single-level measurements, as is increasingly done, it is important to compare the results of a number of methods rather than rely on a single one. An ensemble average or median of the results, possibly after elimination of methods that produce outliers, can help to yield more robust estimates. The estimates of $z_{0}$ were almost exclusively physically plausible, although $d$ was considered unknown and estimated simultaneously with the methods and results described above.