Measurement metrics as a cause of soil freezing curve hysteresis: Simulating the geophysical observation of soil freeze-thaw

Abstract:

Frozen ground is the single largest component of the cryosphere by areal extent, with up to a third of Earth’s land surface experiencing a transition between seasonally frozen or thawed conditions. These freeze/thaw events are critical factors affecting terrestrial water, carbon, and energy balances with consequential effects on hydrological, climatic, ecological, and biogeochemical processes. A fundamental feature of these phenomena is the relationship between unfrozen water content and sub-freezing temperature, known as the soil freezing curve (SFC). It is well documented that this relationship exhibits hysteresis when frozen soil thaws, leading to the definition of the soil thawing curve (STC). Although many explanations have been given for SFC/STC hysteresis, the effect that “scale” – particularly “measurement scale” – may have on these curves has not been explored. The most commonly used measurement scale metric is the “grain” or “support,” which is the spatial (or temporal) unit within which the measured variable is integrated – in this case, the soil volume sampled.

We demonstrate that measurement support can influence the range and shape of the SFC and hysteresis can be, at least partially, attributed to the support and location of the measurements comprising the SFC/STC. We simulated lab measured temperature and volumetric water content (VWC) from soil samples undergoing freeze-thaw transitions using Hydrus-1D. To assess the effect of measurement support and location on SFC/STC, we masked the simulated temperature and VWC extent to match the instrument’s grain and location. By creating a detailed simulation of the intra- and inter-grain variability associated with the penetration of a freezing front, we demonstrate how measurement support and location can influence the temperature range over which water freezing events are captured. This realization allows us to reconcile theoretical and empirical SFCs with results from undisturbed samples. We show that it is possible to simulate hysteresis in homogenous media with these purely geometric considerations, suggesting that SFC/STC hysteresis may be more of an apparent phenomenon than a mechanistically real one.