Multi‐Functional Heat Pulse Probe for the Simultaneous Measurement of Soil Water Content, Solute Concentration, and Heat Transport Parameters

Abstract

Water, solute, and heat transport processes in soils are mutually interdependent as each includes convective water flow and each transport mechanism is partly controlled by fluid saturation, pore geometry, temperature, and other soil environmental conditions. Therefore, their measurement in approximately identical measurement locations and volume is essential for understanding transport phenomena in soils. We introduce a 2.7‐cm‐diameter multi‐functional heat pulse probe (MFHPP), which consists of a single central heater, four thermistors, and four electrodes (Wenner array) that together are incorporated in six 1.27‐mm‐o.d. stainless‐steel tubes. The bulk soil thermal properties and volumetric water content of Tottori Dune sand were determined from the measurement of the temperature response of all four thermistor sensors after application of an 8‐s heat pulse by the heater sensor. Simultaneously with the temperature measurements, the bulk soil electrical conductivity (ECb) was measured using the Wenner array, from which soil solution concentration (ECw) can be obtained after calibration. All measurements were taken during multistep outflow experiments, which also allowed estimation of the soil's hydraulic properties. We demonstrated that the MFHPP can effectively measure volumetric water content, thermal properties, and ECb, and can be used to indirectly estimate soil water fluxes at rates larger than 0.7 m d−1 in the sand.