Determining in-situ soil freeze–thaw cycle dynamics using an access tube-based dielectric sensor

Dielectric sensors have been widely used for determining soil water content, but until now few have been effectively applied for observing the freeze–thaw cycle in soil at the field scale. With this intention we tested a frequency domain (FD) probe designed for use in access tubes installed into the ground at five sites. For accurate depth-referenced measurements, a series of magnetism-sensing Hall-switches was used. The experiment was carried out over the winter of 2009–2010. The temperature-dependent dielectric response was interpreted using a mixing model to fit four soil components, solid, air, liquid water and ice. To illustrate the soil frost dynamics over time, the measured liquid water content of frozen soil (LWCFS) was transformed into different expressions including frozen soil profile, frost penetration map, freezing front trace and thawing front trace. In contrast to Time Domain Reflectometry (TDR) instruments, the proposed technique is easy to operate, reasonably accurate, non-destructive, and has a rapid soil profiling output. In addition, there is a significant cost advantage using a single profiling probe across multiple sites.