Cryosols as a Three-Part System - Cryopedology - page 9

2.2.2 Dynamics and Modeling the Active Layer

There have been many attempts to model the active-layer thickness from climate-
permafrost interactions (e.g., Mackay 1995 ). As an alternative to these approaches,
semi-empirical methods have been developed for the practical needs of cold-regions
engineering. These include the “frost index” (Nelson and Outcalt 1987 ), the
“n-factor” (Klene et al. 2001 ), and Kudryavtsev's formulation (Anisimov et al.
1997 ). The “frost index” is a dimensionless ratio defined by manipulation of either
freezing-and-thawing degree-day sums or frost and thaw penetration depths for
detecting the presence of permafrost. The n-factor is defined as the ratio of the sum
of thawing degree days at the soil-surface to that in the air. Kudryavtsev's solution
takes into account the effects of snow-cover, vegetation, soil moisture, and
soil thermal properties and has been widely used to estimate active-layer thickness.
The Basal Temperature of Snow (BTS) has been used to predict the occurrence of
isolated permafrost in mountainous areas (e.g., Lewkowicz and Ednie 2004 ).

2.2.3 Properties of the Active Layer

2.2.3.1 Thickness

Active layer thickness depends on terrain factors such as air temperature, vegeta-
tion, drainage, soil or rock type, texture, total water content, snow-cover, and degree
and orientation of slope. The active layer tends to be thin (0.1-0.15 m) in high
latitude areas of the zone of continuous permafrost, such as the High Arctic and at
high elevations in the Transantarctic Mountains (Table 2.1 ). In alpine regions of the
world, the active layer commonly ranges from 2 to 8 m or more. In the definition of
Gelisols, the active layer can not exceed 1 m for Histels (organic cryosols) and
Orthels (mineral cryosols without cryoturbation) or 2 m for Turbels (cryoturbated
mineral soils). Although these depths are hypothetical, they were chosen as the
maximum expression of permafrost effects in terms of soil development, response
to climate change, and engineering properties. However, in many mountainous
areas where the permafrost is sporadic or isolated, the active layer may extend to
depths of 8 m or more (Bockheim and Munroe 2014 ).

2.2.3.2 Physical Properties

Because the active layer involves the merging of freezing fronts during “freeze-
back” (seasonal refreezing of the thawed active layer), it is subject to compression
and rearrangement of coarse fragments and the soil matrix that results in changes in
physical properties such as structure, bulk density, dilatancy, and texture. Soil hori-
zons in the active layer have granular, platy, prismatic, and blocky structures because
of freeze-thaw processes (Tarnocai and Bockheim 2011 ). Massive structures associ-