Geoscience Reference
In-Depth Information
to the south. The rise is caused by melt, but
predominantly from upstream. It is also clear
that for both Norman Wells and the Arctic Red
River stations the highest discharge value of the
year is occurring while the river is still covered
in ice. This creates huge problems for the drain-
age of the area as the water may build up behind
an ice dam. Certainly the water flowing under the
ice will be moving much quicker than the ice and
water mix at the surface. Plate 5 demonstrates the
way that water builds up behind an ice dam,
particularly where there is a constriction on either
side of the river channel.
pillow does not create an obstacle to drifting snow
in its own right. To overcome this, and to lessen
the impact of freezing on the pillow liquid, it is
often buried under a shallow layer of soil or laid flat
on the ground. When connected to a continuous
logging device a snow pillow provides the best
record of snow depth (and water equivalent mass).
Measuring snow depth
The simplest method of measuring snow depth is
the use of a core sampler. This takes a core of snow,
recording its depth. The snow sample can then be
melted to derive the water equivalent depth, the
measurement of most importance in hydrology. The
major drawbacks in using a core sampler to derive
snow depth are that it is a non-continuous reading
(similar to daily rainfall measurement) and the
position of coring may be critical (because of snow
drifting).
A second method of measuring snow depth is
to use a snow pillow . This is a sealed plastic pillow
that is normally filled with some form of anti-
freeze and connected to a pressure transducer (Figure
4.18). When left out over a winter period the
weight of snow on top of the pillow is recorded as
an increasing pressure, which can be recalculated
into a mass of snow. It is important that the snow
Estimating snow cover
The main method of estimating snow cover is using
satellite remote sensing. Techniques exist that give
a reasonably good method of detecting the areal
extent of snow cover, but it is much harder to
translate this into a volume of snow (i.e. by knowing
depth) or water. Optical and thermal infrared data
can be used to estimate snow cover but they rely
heavily on the reflective ability of snow;
unfortunately, other surfaces such as clouds may also
exhibit these properties (Fitzharris and McAlevey,
Layers of snow
Snow
pillow
Pressure
gauge
Figure 4.18 Snow pillow for measuring weight of snow above a point. The snow weight is
recorded as a pressure exerted on the pillow.
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