The instrumentation of unfrozen water content: Considering the importance of conversion curves

As permafrost regions warm and thaw, the parametrization of unfrozen water content (θ_uwc) in cryotic (< 0^oC) soil has become a priority in the study of permafrost behavior under a warming climate. Many of the physical properties of soils are governed by θ_uwc, but it is difficult to measure because all current measurement techniques are indirect.

One of the standard procedures to measure θ_uwc  involves the use of a capacitance base moisture sensor. In this case, the sensor measures an electrical property of the soil: its relative dielectric permittivity (ε_eff). It then transforms this measurement into θ_uwc through a mathematical function called a conversion curve. The current default conversion curve for many moisture sensors was never meant to be used in the range of temperatures relevant to cryotic soils. Researchers from Queen’s University suggest a new physics-based conversion curve based on a dielectric mixing model which encompasses elementary physical parameters representative of the soil studied. 

Using this new conversion curve greatly alters the final readings of θ_uwc, tripling its value. This paper’s findings provide evidence supporting the reevaluation of the default conversion curve in moisture sensors when applied to cryotic soils.