Heat as a tracer to quantify water flow in near-surface sediments

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 heal pulse from a line heat source. Asymmetry in the thermal field near the heat source is quantified hy 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 x 10 -5 to 6.31 × 10 -5 m 3 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).

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

Accurate and continuous measurements of soil thermal and hydraulic propertiesare required for environmental, Earth and planetary science, and engineering applications, but they are not practicallyobtained by steady-state methods. The heat pulse (HP) method is a transient method for determinationof soil thermal properties and a wide range of other physical properties in laboratory and field conditions. The HP method is based on the line-heat source solution of the radial heat flow equation. This literature review begins with a discussion of the evolution of the HP method and related applications, followed by the principal theories, data interpretation methods and their differences. Important factors for HP probe construction are presented. The properties determined in unfrozen and frozen soilsare discussed, followed by a discussion of limitations and perspectives for the application of this method. The paper closes with a brief overview of future needs and opportunities for further development and application of the HP method.

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1572&context=agron_pubs

Development and Application of the Heat Pulse Method for Soil Physical Measurements

Unsaturated water flux densities are needed to quantify water and contaminant transfer within the vadose zone. However, there are few reliable and accurate methods available for direct measurement. A water flux meter was designed, constructed and tested to directly measure drainage fluxes in field soils. The flux meter consists of collector, designed to minimize divergence, which concentrates flow into a narrow sensing region filled with a fiberglass wick of known hydraulic properties. The wick applies suction, equal in magnitude to its length, to the collector to passively drain the meter. The meter can be easily installed below the root zone in augured borehole. Water flux through the sensing region can be determined from the time-of-flight measurements or with a self-calibrating tipping bucket. In this study, sensitivity of the tipping bucket was 0.167 mm per tip. Laboratory measurements confirmed predictions of a 2-D numerical model, which showed for optimal performance, precise control of the ratio of the collector height (H) to its width (W) and collector surface area to that of sensor region is required. In sandy soils, a H/W ratio of 1.5 to 2.0 reduced divergence to less than 10% of the applied flux for all fluxes above 10 mm more » yr{sup -1}. Because greater divergence can expected in fine-textured soils, larger H/W ratios will be required to achieve a similar levels of performance. The water flux meter is simple in concept, inexpensive, easy to install, and provides continuous and reliable monitoring of unsaturated water fluxes ranging from less than 10 mm yr{sup -1} to more than 1000 mm yr{sup -1}. « less

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2001WR000816

A vadose zone water fluxmeter with divergence control

The hyporheic zone (HZ) has the capability to eliminate and attenuate nutrients and contaminants in riverine systems. Biogeochemical reactions and the potential elimination of contaminants are strongly controlled by the flow paths and dynamics in the HZ. Nevertheless, an easily applicable method for the field determination of flow patterns in the HZ is still lacking. Therefore, a heat pulse technique, which traces the movement of a short heat pulse in the upper part of the HZ and other sand beds, was developed. Five rods are vertically driven into the sediment of the streambed; one rod with a heater as point source located in about 10-cm sediment depth and four rods with four temperature sensors in 3 cm distance, arranged concentrically with 7 cm diameter around the heating rod. Subsequently, a heat pulse is applied and the resulting breakthrough curves are indicative of flow velocities and flow directions in the streambed. A rough data analysis procedure is also suggested. In addition, laboratory experiments were performed to test the heat pulse technique. These experiments were validated based on coupled numerical modelling of flow and heat transport. First field tests of the method prove that the method is easily applicable under field conditions. These first field tests showed highly complex flow patterns with flow velocities from 1·8 to 4·9 cm min−1 and flow directions from parallel to surface flow to opposite to surface flow. This suggests the need for a robust method to quantify hyporheic flow patterns in situ. Copyright © 2011 John Wiley & Sons, Ltd.

A heat pulse technique for the determination of small-scale flow directions and flow velocities in the streambed of sand-bed streams

The theoretical analysis and laboratory calibration of a sensor to measure fluid flux in soil or other porous medium shows that a flow velocity of 10−4 cm sec−1 can be readily measured, and that sensor output is uniquely related to flux. The flow to be measured causes asymmetry in the temperature field,generated by an approximation to a point source of heat located centrally in the instrument. The temperature difference due to this asymmetry gives rise to the output of the instrument.

J. E. Drummond

Papers

A sensor for water flux in soil. “Point source” instrument