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Lena. Only 4-10 percent of the Ob basin is underlain by permafrost (contributing
to low RU/P), in contrast to 78-93 percent of the Lena, which is mostly represented
by continuous permafrost (contributing to high RU/P). The extent of permafrost in
the Yenisey and Mackenzie is between that for the Ob and Lena. In turn, these two
basins have intermediate runoff ratios.
6.5.5
Mean Hydrographs
Long-term annual mean runoff ought to approximately balance P-ET, but this is
not true of means for seasonal and shorter time scales. The most obvious reason is
that for much of the year, the net precipitation is stored in the snowpack. There are
also subsurface storages. The typical annual cycle is as follows: In the cold months,
runoff is small, and can even cease for small rivers. Runoff begins to climb in April
or May when the snowpack begins to melt. It then increases sharply, generally with
a June peak. This is the same general pattern we would see for any region in the
Northern Hemisphere (e.g., the Colorado Rocky Mountains) where there is signifi-
cant seasonal snowpack storage. As assessed over the Arctic drainage as a whole,
approximately 60 percent of the discharge occurs from April to July (Lammers
et al.,
2001
). However, the shapes of the mean hydrographs (plots of runoff or dis-
charge versus time) vary between different basins in response to temperature as
it influences snowmelt and ET, the mean water equivalent of the snowpack, the
seasonality of precipitation, permafrost conditions, soil moisture and groundwater
recharge, runoff storage in wetlands and other factors.
Differences in the mean water-year hydrographs for the Ob, the Yenisei, the
Lena, and the Mackenzie (
Figure 6.17
) capture some of these processes. Although
runoff peaks in June for all basins, the peaks are much sharper in the Yenisey and
Lena as compared with the Ob. In part, this manifests lower mean temperatures in
the Yenisey and Lena from autumn through spring, meaning a longer period of snow
accumulation and delayed spring melt. Once it becomes warm enough in June, the
rapid melt of the snowpack gives rise to sharp runoff peaks. Especially for the Lena,
the large extent of continuous permafrost fosters rapid movement of meltwater to
streams, further contributing to the sharp June runoff peak. Over the 1970-1999
period, 35 percent of total discharge at the mouth of the Lena occurs in June and
a further 20 percent in July (Yang et al.,
2002
). By contrast, especially for the Ob,
a large proportion of precipitation is lost through ET, especially in summer, and is
not available for runoff. Other effects not considered here include snowmelt storage
in lakes and wetlands and aufeis (see
Chapter 2
) that can subdue peak discharge.
Storage in lakes and wetlands can be pronounced for North American rivers such as
the Mackenzie. River and lake ice is a further storage component.
6.5.6
Variability
Precipitation, evaporation, and river discharge are all highly variable on monthly
and shorter time scales.
Figure 6.18
provides one example. This figure shows for
