Determining Soil Water Flux and Pore Water Velocity by a Heat Pulse Technique

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Abstract
A method is presented for measuring soil water flux density (J) with a thermo‐TDR (time domain reflectometry) probe. Constant heat input during a small time interval (15 s) is used to emit a heat pulse from a line heat source. Asymmetry in the thermal field near the heat source is quantified by computing the maximum dimensionless temperature difference (MDTD) between upstream and downstream locations. Heat transfer theory was used to relate MDTD to J A thermo‐TDR probe was used to obtain measurements of MDTD in water‐saturated soil materials of different textures (sand, sandy loam, and clay loam) with imposed water flux densities ranging from 1.16 × 10−5 to 6.31 × 10−5 m3 m−2 s−1 A nearly linear relationship between measured MDTDs and fluxes was observed for all soil materials. Measured and predicted MDTDs agreed well for flow experiments in sand. Greater discrepancies were observed for flow experiments in sandy loam and clay loam. Despite the lack of universal agreement between measured and predicted MDTDs, the experimental results indicate that the proposed method may provide a useful means of measuring J The method presented herein improves upon earlier methods by reducing distortion of the water flow field and minimizing heat‐induced soil water redistribution. Because the thermo‐TDR probe can be used to make TDR‐based measurements of volumetric water content (θ), the proposed method also may permit measurement of pore water velocity (J/θ).