Soil Moisture, Thermal Property and Salinity Measurements with a Button Heat Pulse Probe.

Water flow, energy flux and solute transport through the vadose zone are highly variable, and are interrelated with soil moisture. Crop health and production as well as biodiversity in natural ecosystems are largely governed by soil moisture. Not surprisingly, agricultural practices, among many other hydrological applications, rely on timely and accurate characterization of the temporal and spatial soil moisture distribution. However, few available sensors allow in-situ measurements of these properties. We use the multi-functional heat-pulse technique, and present laboratory results from soil measurements conducted with a newly designed button heat pulse probe (BHPP). Unlike conventional heat pulse probe (HPP), the BHPP does not use needles, but instead consists of a ring-shaped 6-mm-radius heating element and a central thermistor, both embedded in the face of a plastic disk. Measurement accuracy and robustness of the BHPP were significantly improved compared with the conventional HPP. Errors associated with needle deflection were eliminated by the design, whereas the water content measurement sensitivity was increased by the central placement of the thermistor.