Implications of measurement metrics on soil freezing curves: A
simulation of freeze-thaw hysteresis
Abstract
Soil freeze-thaw events have important implications for water resources,
flood risk, land productivity, and climate change. A property of these
phenomena is the relationship between unfrozen water content and
sub-freezing temperature, known as the soil freezing characteristic
curve (SFC). It is documented that this relationship exhibits hysteretic
behaviour when frozen soil thaws, leading to the definition of the soil
thawing characteristic curve (STC). Although explanations have been
given for SFC/STC hysteresis, the effect that “scale”—particularly
“measurement scale”—may have on these curves has received little
attention. 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 show (1) measurement support can influence the range
and shape of the SFC and (2) hysteresis can be, at least partially,
attributed to the support and location of the measurements comprising
the SFC/STC. We simulated lab measured temperature, volumetric water
content (VWC), and permittivity from soil samples undergoing freeze-thaw
transitions using Hydrus-1D and a modified Dobson permittivity model. To
assess the effect of measurement support and location on SFC/STC, we
masked the simulated temperature and VWC/permittivity 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. We show it is possible to simulate
hysteresis in homogenous media with purely geometric considerations,
suggesting that SFC/STC hysteresis may be more of an apparent phenomenon
than mechanistically real. Lastly, we develop an understanding of how
the location and support of soil temperature and VWC/permittivity
measurements influence the temperature range over which water freezing
events are captured.