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'In-situ Soil Water Content Estimation with Heat Pulse Technology'
Kashifa Rumana (kashifarumana@aggiemail.usu.edu), Utah State University; Markus Tuller (mtuller@cals.arizona.edu), University of Arizona; Scott Jones (scott.jones@usu.edu), Utah State University

Scientists from a wide range of disciplines – agriculturists, environmentalists, meteorologists, ecologists – are interested in monitoring and managing the soil hydrologic regime in the vadose zone. Soil moisture influences the exchange and partitioning of water and energy fluxes at the land surface from regional to global scale. Time- and frequency-domain electromagnetic methods are commonly used for soil water content measurements in laboratory and field applications. In order to improve the characterization and quantification of soil hydraulic parameters and hydrologic fluxes, we attempted to measure soil moisture variation with heat pulse probes on a millimeter scale. In-situ soil water dynamics were measured using a heat pulse probe designed to determine soil temperature and thermal properties. The probe consists of a heater needle and five or six thermistor needles. A small heat input is applied to a resistance wire in the heater needle, while the remaining thermistor needles measure the temperature response at fixed distances of 6.5mm from the heater. The heat pulse probe is rotated 27.3° from a vertical orientation yielding measurements every 3mm within the soil profile. The system-on-chip embedded design of the heat pulse probe facilitates real-time optimization of soil thermal properties – thermal conductivity and thermal diffusivity – using an inverse-fitting algorithm. Soil water content at the receding drying front is a function of soil heat capacity and soil bulk density. The soil water content has a commensurate effect on thermal conductivity; as the water content increases, thermal conductivity increases due to improved thermal contact between the soil particles. Volumetric heat capacity is determined from the ratio of thermal conductivity and thermal diffusivity in soil; hence, leading to water content estimates in a drying medium. Further investigation is ongoing to validate and understand the accuracy and benefit of using heat pulse probe method for soil hydraulic properties assessment at an unprecedented resolution.