The thermal conductivity and water content relationship is required for quantitative study of heat and water transfer processes in saturated and unsaturated soils. In this study, we developed an improved model that describes the relationship between thermal conductivity and volumetric water content of soils. With our new model, soil thermal conductivity can be estimated using soil bulk density, sand (or quartz) fraction, and water content. The new model was first calibrated using measured thermal conductivity from eight soils. As a first step in validation, predicted thermal conductivity with the calibrated model was compared with measured thermal conductivity on four additional soils. Except for the sand, the root mean square error (RMSE) of the new model ranged from 0.040 to 0.079 W m−1 K−1, considerably less than that of the Johansen model (0.073–0.203 W m−1 K−1) or the Côté and Konrad model (0.100–0.174 W m−1 K−1). A second validation test was performed by comparing the three models with literature data that were mostly used by Johansen and Côté and Konrad to establish their models. The RMSEs of the new model, the Johansen model, and the Côté and Konrad model were 0.176, 0.176, and 0.177 W m−1 K−1, respectively. The results show that the new model provided accurate approximations of soil thermal conductivity for a wide range of soils. All of the models tested demonstrated sensitivity to the quartz fraction of coarse‐textured soils.

The soil thermal properties—heat capacity (C), thermal diffusivity (α), and thermal conductivity (λ)—are important in many agricultural, engineering, and meteorological applications. Soil thermal properties are largely dependent on the volume fraction of water (θ), volume fraction of solids (vs), and volume fraction of air (na) in the soil. In many natural settings θ, vs, and na vary greatly over time and space, but data showing the effects of these variations on thermal properties are not readily available. We used a heat‐pulse method to measure the thermal properties of 59 packed columns of four medium‐textured soils covering large ranges of θ, vs, and na The measured data reveal the commonly overlooked but dominant influence of na on soil thermal properties. Notably, the measurements show that the λ of these soils at 20°C can be accurately described as a decreasing linear function of na Good agreement exists between the measured data and common models for λ and C

Several approaches are available for estimating soil water flux indirectly (Nielsen et al., 1973; Bresler, 1973), A method is presented for measuring soil water flux density (J) but these approaches can be time consuming, mathematwith a thermo-TDR (time domain reflectometry) probe. Constant heat input during a small time interval (15 s) is used to emit a heat ically complicated, and measurement-intensive. pulse from a line heat source. Asymmetry in the thermal field near the Byrne et al. (1967, 1968) first applied heat as a tracer heat source is quantified by computing the maximum dimensionless to measure soil water flux. Their instruments consisted temperature difference (MDTD) between upstream and downstream of temperature sensors positioned symmetrically with locations. Heat transfer theory was used to relate MDTD to J. A respect to point or line heat sources. Water flux was thermo-TDR probe was used to obtain measurements of MDTD in measured by characterizing distortion in the thermal water-saturated soil materials of different textures (sand, sandy loam, field around the instruments. Several limitations have and clay loam) with imposed water flux densities ranging from 1.16 ϫ prevented these instruments from being used as practi-10 Ϫ5 to 6.31 ϫ 10 Ϫ5 m 3 m Ϫ2 s Ϫ1. A nearly linear relationship between cal tools for characterizing soil water flux. One limitameasured MDTDs and fluxes was observed for all soil materials. tion is that they require constant heat input for relatively Measured and predicted MDTDs agreed well for flow experiments in sand. Greater discrepancies were observed for flow experiments long periods of time (30 min for average flow rates) in sandy loam and clay loam. Despite the lack of universal agreement before reaching thermal equilibrium. Thus, these instrubetween measured and predicted MDTDs, the experimental results ments will have limited applicability in unsaturated soil indicate that the proposed method may provide a useful means of where thermal gradients will result in soil water redistrimeasuring J. The method presented herein improves upon earlier bution. Another limitation is that calibration is required methods by reducing distortion of the water flow field and minimizing to relate flux to instrument response. In addition, the heat-induced soil water redistribution. Because the thermo-TDR size of these instruments results in distortion of the probe can be used to make TDR-based measurements of volumetric soil water flow field in the vicinity of the instrument. water content (), the proposed method also may permit measurement Experimental results showed poor agreement between of pore water velocity (J/).

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Soil thermal conductivity (λ) models are needed frequently in studying coupled heat and water transfer in soils. Several models are available, but some are complicated and some produce relatively large errors. In this study, we developed a simple model for estimating λ from soil texture, bulk density (ρb), and water content (θ). Three parameters, α, β, and λdry, are included in the model, where λdry is determined by ρb and α and β are shape factors estimated from soil texture and ρb. Empirical relations were developed for α and β by fitting the model to heat‐pulse (HP) measurements of λ(θ) on seven soils of various textures. The model performance was evaluated with independent λ(θ) data from independent HP measurements and literature values. The results show that the model is able to express the λ(θ) curves from oven dry to saturation at fixed ρb values. When ρb is varied, the estimated λ data agree well with measured values. The root mean square errors are <0.15 W m−1 K−1, and the bias is within 0.10 W m−1 K−1. The new model has the potential for use in studying heat movement in soils of varying texture, bulk density, and water content and can be incorporated into numerical algorithms for describing coupled heat and mass transfer processes.