Geoscience Reference
In-Depth Information
Table 6.2.
Mean SAT (January and July), number of snow-free days, percent of wetlands
and percent permafrost cover for the four major drainage basins
January
(°C)
July
(°C)
Snow-Free
Days
Wetlands
Cover (%)
% Area Underlain
by Permafrost
Ob
−18.7
18.1
186
6.6
4-10
Yenisey
−26.5
15.2
163
3.7
36-55
Lena
−35.0
14.7
144
0.8
78-93
Mackenzie
−25.0
13.8
167
6.5
32-58
Source
: From Serreze et al. (
2003a
).
(0.47 to 0.54) and the Lena (0.46 to 0.61). Existing estimates for the Mackenzie are
intermediate (0.30 to 0.46). The ranges for each watershed reflect the use of differ-
ent precipitation data sets. The runoff ratios in
Table 6.1
are in accord with previ-
ous estimates. The basins can be similarly contrasted in terms of ET/P. Water-year
precipitation in the Ob is comparable to that for the Yenisey but is about 20 percent
higher than for the Lena and Mackenzie. However, a greater fraction of the precipi-
tation is lost through ET in the Ob.
The contrast between the Ob and the other basins in terms of RU/P and ET/P is
understood from differences in climatological surface air temperature, land cover,
and permafrost extent.
Table 6.2
lists basin averages of mean January and July sur-
face air temperature, the annual mean number of snow-free days and the percent of
the surface represented by wetlands, and the percent of the basin areas covered by
permafrost. The range in permafrost coverage reflects a range of assumptions used
in the calculations (see Serreze et al.,
2003a
). There is a progression from moder-
ately low January temperatures in the Ob (−18.7°C) to very cold conditions in the
Lena (−35.0°C). Low mean temperatures also characterize the Mackenzie basin.
The temperature distribution is in accord with the higher winter ET rates in the Ob.
July temperatures are also highest in the Ob, consistent with the higher summer
ET in this area. As would be expected, the number of snow-free days is also much
greater in the Ob, pointing to earlier and more prolonged warming of the soil. This
will also contribute to higher ET rates. Furthermore, in comparison to the other
Eurasian basins, a larger part of the Ob is characterized as wetlands. ET over these
areas is not limited by available moisture. More than 6 percent of the Mackenzie is
also characterized as wetlands. The net effect of higher ET in the Ob is to reduce
RU/P and increase ET/P relative to the other basins.
The differences in runoff ratios are also related to permafrost.
Figure 2.14
shows
the distribution of continuous, discontinuous and sporadic permafrost across the
Northern Hemisphere. Permafrost acts as an impermeable barrier, such that precipi-
tation can be rapidly channeled into streams and rivers rather than recharging soil
moisture and groundwater. It will also restrict plant growth through the growing
season and limit root depth, reducing water losses through ET. The effect of both is
to increase RU/P. From
Table 6.1
, the largest contrast in RU/P is between the Ob and
