Dielectric permittivity method to monitor (liquid) water content in unsaturated soils, frozen soils, and snow

Monitoring (liquid) water content is key to inform early-warning systems for unsaturated, permafrost, and snow slopes. Dielectric permittivity-based sensors are widely used to monitor (liquid) water content in unsaturated soils and include the well-known TDR and capacitive probes. In contrast to unsaturated soils, there are no established methods to measure liquid water content in frozen soils and snow. This paper introduces the ‘refractive index’ mixing model for four-phase materials (solids, air, liquid water, and solid water) as a unifying equation to interpret dielectric permittivity measurements in unsaturated soils, frozen soils, and snow. The mixing model is first used to revisit empirical equations used to estimate the water content in unsaturated soils. It is then validated against TDR measurements of bulk density in dry snow. Its use to monitor the liquid water content in frozen soils and snow is finally discussed.