Quantifying the soil freezing characteristic: the dominant role of salt
exclusion
Abstract
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, and is thought to have two causes: i) capillary effects,
where the phase transition relationship is modified due to
soil-air-water-ice interactions, and ii) 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 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 capillary-solute 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 modelling 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.