Search for: All records

Abstract A petrophysical model that accurately relates bulk electrical conductivity (σ) to pore fluid conductivity (σ_w) is critical to the interpretation of geophysical measurements. Classical models are either only applicable over a limited salinity regime or incorrectly explain the nonlinear‐to‐linear behavior of the σ(σ_w) relationship. In this study, asymptotic limits at zero and infinite salinity are first established in which, σ is expressed as a linear function of σ_wwith four parameters: cementation exponent (m), the equivalent value of volumetric surface electrical conductivity (σ_s), the volume fraction of overlapped diffuse layer (ϕ_od) and parameter χ representing the ratio of the volume fraction of the water phase to that of the solid phases in the surface conduction pathway. Subsequently, we bridge the gap between the two extremes by employing the Padé approximant (PA). Given that parameter χ exhibits a marginal influence on the σ(σ_w) curve, based on measurements for 15 samples, we identify its optimal value to be 0.4. After setting the optimal value ofχ, we proceed to evaluate the performance of the PA model by comparing its estimates and estimates made by two existing models to measured values from 27 rock samples and eight sediment samples. The comparison confirms that the PA model estimates are more accurate than estimates made by existing models, particularly at low salinity and for samples with higher cation exchange capacity. The PA model is advantageous in scenarios involving the interpretation of electrical data in freshwater environments. 

Abstract Soil compaction leads to an increase in bulk density () and results in a shift in pore‐size distribution toward smaller pores. These changes alter the soil hydraulic properties (SHPs), that is, the water retention curve and the hydraulic conductivity curve. Most existing models that address the impact of changes in on SHP have been confined to SHP models that consider only capillary water, neglecting water stored and transmitted within adsorbed films (noncapillary water). Recently, a new prediction model was developed that combines the Peters–Durner–Iden (PDI) SHP model system, which accounts for capillary and noncapillary water, with a prediction scheme for compaction effects. However, this new approach has yet to be calibrated and tested against data from soils with varying textures. The objective of this study was to calibrate and evaluate the new water retention model using a comprehensive dataset from the literature. Two different variants, which vary in the number of degrees of freedom have been tested. Remarkably, the variant with only one adjustable parameter, the one that shifts the pore‐size distribution by scaling the pressure head, was sufficient to accurately describe the data. All other parameters can either be fixed at the reference value or scaled based on straightforward physical reasoning. The model achieved low calibration errors (median root mean square error [RMSE]: 0.013; median mean error [ME]: 0.0014) and performed satisfactorily in validation (median RMSE: 0.025; median ME: −0.014). Based on our results, we hypothesize that the scaling approach is independent of the capillary saturation function and that this method might be applied to other models within the PDI system without new calibration. 

Abstract Biocrusts are a critical surface cover in global drylands, but knowledge about their influences on surface soil thermal properties are still lacking because it is quite challenging to make accurate thermal property measurements for biocrust layers, which are only millimetres thick. In this study, we repacked biocrust layers (moss‐ and cyanobacteria‐dominated, respectively) that had the same material as the original intact biocrusts but was more homogeneous and thicker. The thermal conductivity (λ), heat capacity (C) and thermal diffusivity (k) of the repacked and intact biocrusts were measured by the heat pulse (HP) technique at different mass water contents (θ_m) and mass ratios (W_t), and the differences between repacked and intact biocrusts were analysed. Our results show that biocrusts substantially alter the thermal properties of the soil surface. The averageλof moss (0.37 W m⁻¹ K⁻¹) and cyanobacteria biocrusts (0.90 W m⁻¹ K⁻¹) were reduced by 63.0% and 10.3% compared with bare soil (1.00 W m⁻¹ K⁻¹), respectively. Edge effects including heat loss and water evaporation caused theλandkof the biocrusts to be underestimated, but theCto be overestimated. The differences in thermal properties were significant (p <0.001), except for the differences in thermal conductivity between repacked and intact cyanobacteria biocrusts, which were not significant (p = 0.379). Specifically, in the volumetric water content (θ_v) range of 0 to 20%, theλandkof the repacked moss biocrusts were underestimated by 59.1% and 61.8%, respectively, and theCwas overestimated by 23.9% compared with the intact moss biocrusts. Theλandkof the repacked cyanobacteria biocrusts were underestimated by 15.8% and 79.2%, respectively, and theCwas overestimated by 34.8% compared with the intact cyanobacteria biocrusts at theθ_vrange of 0 to 30%. Typically, this difference increased as theθ_vrises between repacked and intact biocrusts. Our new measurements provide evidence that the thermal properties of biocrusts were previously misjudged due to the measurement limitations imposed by their limited thickness when measured in situ. Biocrusts are likely more significant in regulating soil heat and temperature in drylands than was previously assumed. 

Abstract A thermo‐time domain reflectometry (thermo‐TDR) sensor combines a heat‐pulse sensor with a TDR waveguide to simultaneously measure coupled processes of water, heat, and solute transfer. The sensor can provide repeated in situ measurements of several soil state properties (temperature, soil water content, and ice content), thermal properties (thermal diffusivity, thermal conductivity, heat capacity), and electromagnetic properties (dielectric constant and bulk electrical conductivity) with minimal soil disturbance. Combined with physical or empirical models, structural indicators, such as bulk density and air‐filled porosity, can be derived from measured soil thermal and electrical properties. Successful applications are available to determine fine‐scale heat, water, and vapor fluxes with thermo‐TDR sensors. Applications of thermo‐TDR sensors in complicated scenarios, such as heterogeneous root zones and saline environments, are also possible. Therefore, the multi‐functional uses of thermo‐TDR sensors are invaluable for in situ observations of several soil physical properties and processes in critical zone soils.