Numerical Assessment of Morphological and Hydraulic Properties of Moss,
Lichen and Peat from a Permafrost Peatland
Abstract
The hydraulic properties of ground vegetation cover are important for
high resolution hydrological modeling of permafrost regions, due to its
insulating and draining role. In this study, the morphological and
effective hydraulic properties of Western Siberian Lowland ground
vegetation samples (lichens, Sphagnum mosses, peat) are
numerically assessed based on tomography scans. After numerical
pre-processing, porosity is estimated through a void voxels counting
algorithm, showing the existence of representative elementary volumes
(REV) of porosity for most samples. Then, two methods are used to
estimate hydraulic conductivity depending on the sample’s homogeneity.
For the most homogeneous samples, Direct Numerical Simulations (DNS) of
a single-phase flow are performed, leading to a definition of hydraulic
conductivity related to REV, which is larger than those obtained for
porosity. For more heterogeneous samples, no adequate REV may be
defined. To bypass this issue, a pore network representation of the
whole sample is created from computerized scans. Morphological and
hydraulic properties are then estimated through this simplified
representation. Both methods converged on similar results for porosity.
Some discrepancies are observed in the morphological properties
(specific surface area). Hydraulic conductivity fluctuates by two orders
of magnitude, depending on the method used, and yet this uncertainty is
less than that found in experimental studies. Therefore, biological and
sampling artifacts are predominant over numerical biases. Porosity
values are in line with previous values found in the literature, showing
that arctic cryptogamic cover can be considered as an open and
well-connected porous medium (over 99% of overall porosity is open
porosity). Meanwhile, digitally estimated hydraulic conductivity is
higher compared to previously obtained results based on field and
laboratory experiments. This could be related to compressibility
effects, occurring during field or laboratory measurements. Thus, some
supplementary studies are compulsory for assessing syn-sampling and
syn-measurement perturbations in experimentally estimated, effective
hydraulic properties of such a biological porous medium.