https://doi.org/10.1029/2021WR030070

Amankwah, S. K., Ireson, A. M., Maulé, C., Brannen, R., & Mathias, S. A. (2021). A Model for the Soil Freezing Characteristic Curve That Represents the Dominant Role of Salt Exclusion. Water Resources Research, 57(8), e2021WR030070. https://doi.org/10.1029/2021WR030070

The phenomenon of freezing point depression in frozen soils results in the co-existence of ice and liquid water in soil pores at temperatures below 273.15 K (0°C), and is thought to have two causes: (a) capillary and adsorption effects, where the phase transition relationship is modified due to soil-air-water-ice interactions, and (b) solute effects, where the presence of salts lowers the freezing temperature. The soil freezing characteristic curve (SFC) characterizes the relationship between liquid water content and temperature in frozen soils. Most hydrological models represent the SFC using only capillary and adsorption effects with a relationship known as the Generalized Clapeyron Equation (GCE). In this study, we develop and test a salt exclusion model for characterizing the SFC, comparing this with the GCE-based model and a combined salt-GCE effect model. We test these models against measured SFCs in laboratory and field experiments with diverse soil textures and salinities. We consistently found that the GCE-based models under-predicted freezing-point depression. We were able to match the observations with the salt exclusion model and the combined model, suggesting that salinity is a dominant control on the SFC in real soils that always contain solutes. In modeling applications where the salinity is unknown, the soil bulk solute concentration can be treated as a single fitting parameter. Improved characterization of the SFC may result in improvements in coupled mass-heat transport models for simulating hydrological processes in cold regions, particularly the hydraulic properties of frozen soils and the hydraulic head in frozen soils that drives cryosuction.

Key Points
- The soil freezing characteristic curve is an important property of frozen soils, and is required by cold regions hydrological models
- The Generalized Clapeyron Equation (GCE) is found to under predict freezing point depression
- A salt exclusion model and combined salt-GCE model perform well in simulating observed soil freezing characteristic curves

Plain Language Summary
When the ground freezes during the winter, not all the water stored in the soil turns into ice, which is because soil particles hold tightly onto some of the water making it impossible to freeze the water and because of the presence of dissolved salts within the soil pore water. The presence of unfrozen water in frozen soils determines the hydraulic properties of the soil which are vital for models of flood forecasting during spring melt, snowmelt infiltration for crop growth and the mechanical properties that determine the stability of the ground for infrastructure in cold regions. In this study, we use laboratory and field experiments, as well as different theoretical models to understand the effect of either or both dissolved salt and soil particles on the amount of unfrozen water stored in the frozen soil, and we suggest that dissolved salts may often be the dominant control. We propose a new relationship for this phenomenon that could improve cold regions hydrological models.

Process-Frozen-Soil, Refereed Publications

https://doi.org/10.1029/2021WR030070