Abstract
AbstractThe 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.
Highlights
The way that ice and liquid water are held in the soil pore space depends on the soil physical and chemical properties, the total water content and the temperature, and is expressed through a relationship known as the soil freezing characteristic curve (SFC)
soil moisture characteristic curve (SMC) and SFCs were measured for soils with varying texture and salinity in laboratory and field conditions, using dielectric impedance probes to measure the liquid water content
In our seasonally frozen field sites, SFCs have a number of important characteristics, which we summarize conceptually in Figure 12, and describe here: (a) the antecedent water content prior to freezing are normally not saturated, and may be quite dry, meaning that assuming saturated soils for frozen conditions is likely to introduce significant errors; (b) the freezing and thawing curves are distinctly hysteretic; and (c) the soils are wetter, and perhaps saturated, at the end of thawing, which is due to a combination of possible soil moisture redistribution by cryosuction during the winter, and snowmelt infiltration during the melt period
Summary
The way that ice and liquid water are held in the soil pore space depends on the soil physical and chemical properties, the total water content and the temperature, and is expressed through a relationship known as the soil freezing characteristic curve (SFC). Information about these properties are employed in infrastructure development such as the construction of roads, pavements, airport runways, bridges and railway lines. In agronomy, these properties are useful in understanding microbial metabolism (He et al, 2016; Oquist et al, 2009; Watanabe & Osada, 2017) and crop water uptake in frozen soils as well as estimating water requirements for winter crops.
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