Snow Surveys and Water Supply Forecasting
Snow Surveys - Manual
Manual surveys require two-person teams to measure snow depth and water
content at designated snow courses (fig. 6). A snow course is a permanent
site that represents snowpack conditions at a given elevation in a given
area. A particular snowpack may have several courses. Generally, the
courses are about 1,000 feet long and are situated in small meadows
protected from the wind.
Figure 6. The surveyors are approaching
a typical snow course marker. This is
one of the nearly 1,600 they or others
will encounter several times each winter.
Measurements generally are taken on or near the first of every month
during the snowpack season. The frequency and timing of these measurements
varies considerably with the locality, the nature of the snowpack,
difficulty of access, and cost. On occasion, special surveys are scheduled
to help evaluate unusual conditions. The manual surveys involve travel and
work in remote areas, often in bad weather, but reliable data are
obtained. Locations that are too hazardous or costly to measure on the
ground can be equipped with depth markers that can be read from aircraft
(fig. 7). Snow depth can be measured in this way with a high degree of
accuracy. Although the amount of water in the snowpack is not measured, it
can be reliably estimated from the observed snow depth.
Figure 7. Another form of manual
surveying is reading depth markers
from aircraft for locations too hazardous
or costly to measure from the ground.
NRCS conducts intensive training in snow sampling techniques, safety,
and mountain survival. On-the-job training and an annual
"west-wide" school develop the needed skills. The school has
become known throughout the Western United States and Canada for its
unique training program offered to NRCS employees and others engaged in
the cooperative surveys. A critical part of the training is the overnight
bivouac in a snow shelter the student constructs (fig. 8). Many graduates
have credited this training with bringing them safely through unforeseen,
Figure 8. Students attenting the "west-wide" school learn
snow shelters such as this trench shelter.
The surveyor makes certain that the tube is clear of all snow and soil
before taking the snow core sample (fig. 9). The team uses a strong,
light-weight, graduated aluminum tube and a weighing scale.
One surveyor measures the snow depth while the other records data (fig.
10). From 5 to 10 measurements are taken at regular intervals along a snow
course. Snow depth is measured by pushing the tube down through the
snowpack to the ground surface and extracting a core.
In taking an accurate snow core sample, the surveyor must verify that
the tube has reached ground level by examining the base of the tube and
finding soil (fig. 11). After clearing out the soil from the tube, the
surveyor determines the amount of water in the snowpack by weighting the
tube with its snow core and subtracting the weight of the empty tube (fig.
12). An average of all samples taken is calculated and used to represent
the snow course.