Geology Reference
In-Depth Information
Table 3.3.
Thermal parameters (n-factors) and soil moisture values assigned to certain
vegetation types in central Alaska.
Veget at ion t y p e
n-factor
Soil Moisture (% wt.)
thawed (
N
t
)
frozen (
N
f
)
Closed aspen forest
1.00
0.30
15
Closed birch forest
0.90
0.35
20
Mixed birch-spruce forest
0.85
0.35
20
Closed white-spruce forest
0.80
0.35
50
Open black-spruce forest
0.60
0.30
50
Closed black-spruce forest
0.50
0.30
45
Low shrub-scrub
0.85
0.30
40
Tussock bog
0.90
0.30
55
Source: Jorgenson and Kreig (1988). Reproduced by permission of Tapir Academic Press.
Table 3.4 .
Summary of snow characteristic at fi ve sites in the Takhini valley, central Yukon
Territory, for the winters 1997/98, 1998/99, and 1999/2000. The relation between air and near-
surface (ground) temperatures is expressed by values of the coeffi cient of determination (r
2
).
Freezing n-factors were computed using air and near-surface freezing degree-days for the fi ve sites.
Snow characteristic
Forest
Burned
Meadow
Snow fence
Cleared
Max snow depth (cm)
19
28
24
25
0
Mean snow depth (Dec-Feb) (cm)
10
23
15
19
0
Mean duration of snow cover (weeks)
21
20
21
21
0
Coeffi cient of determination (r
2
)
0.31
0.14
0.21
0.09
0.27
(Air: near-surface temperatures)
Freezing n-factors
0.54
0.32
0.47
0.32
0.55
Source: Karunaratne and Burn (2003).
3.3.2. The Thermal Offset
If the ground surface is conceived as a thermal rather than a physical boundary, the
mean annual temperature immediately above, at, and below the ground surface can
be represented schematically (Figure 3.3). Three levels can be recognized (Smith and
Riseborough, 2002): (1) air temperature, measured at standard height above seasonal
snow cover (MAAT), (2) ground surface temperature (MAGST), and (3) temperature at
the top of permafrost (TTOP).
The mean annual air temperature (MAAT) can then be expressed as the thawing index
for air temperature (thawing degree-days, TDD) minus the freezing index for air tem-
perature (freezing degree-days, FDD, expressed as a positive number) divided by the
number of days in a year. In contrast, the mean annual ground surface temperature
(MAGST) refl ects the effect of plant vegetation in summer and the effect of snow cover
in winter. Because of the insulating effect of the winter snow cover (known as the “nival”
offset), the MAGST exceeds the MAAT in winter. However, due to vegetation effects
(known as the “vegetation” offset), the situation is reversed in summer, when the MAGST
is less than the MAAT. The exact magnitude of these offsets varies depending upon the
