Comparing unfrozen water content measurements of frozen soil using recently developed commercial sensors

Many soil moisture probes that use the bulk electromagnetic properties of the soil have been introduced to the market over the last decade. These electric devices have been developed by the semiconductor industry and have improved dramatically both in terms of size and price. In this study, we test and calibrate five different soil moisture sensors for measuring the unfrozen water content of frozen materials under laboratory and field conditions. The soil volume to which each probe is sensitive is one of the important parameters influencing moisture measurement, particularly in a heterogeneous medium such as ice rich frozen soils. Each sensor has a unique response to changing temperature and type of soil, which we characterize using porosity. All of the probes examined here require unique exponential calibration equations to obtain unfrozen water content from probe output. We provide equations for clay (bentonite) and Fairbanks silt. Field application of empirical equations for two different types of sensors demonstrates the change in values obtained and the convergence of measured water content values using calibrations specific for frozen soils.