RIVER FORECAST CENTRE

What is a Snow Survey?

Because snow is subject to drifting, melting and natural compaction, it is notoriously difficult to measure. Any point measurement may be quite different from a reading taken only a few metres away. Thus, to measure the quantity of snow in an area, a systematic method for measuring snow has been developed and this is generally referred to as a "snow survey". Such measurements have been made in British Columbia since 1935. The basic principal is that measurements should be made consistently at the same locations so that previous and subsequent measurements can be compared and related to such measures as water supply and flooding. Individual readings are therefore used as indices as to the quantity of snow in an area.

An ideal snow survey site is located in a relatively sheltered area with as little tree canopy overhead as possible. A forest clearing where the area is at least as wide as the height of the trees is ideal. Sites are chosen at elevations representative of the area and that will have snow for a substantial portion of the winter. The majority of snow courses are located between 1000 and 2000 metres above sea level. Five or ten points within the area are chosen and referenced to trees etc. which will be visible when there is snow on the ground. At each sampling period, measurements are made at all of the points at the site, the "reading" for that site being the average snow depth and water equivalent of the points measured.

Measurements are made by utilizing the "Standard Federal Snow Sampler" which consists of graduated aluminum tubes with a cutter bit affixed to the first section of the tubing. The tubes are driven through the snow to the ground and then carefully withdrawn, extracting a core of snow with them. The tubes and core are then weighed using a scale specially calibrated in centimetres of water. The difference between the empty weight of the tubes deducted from the weight of the tubes and core is the snow water equivalent.

The number of snow courses measured at each sampling period varies through the year. Because the maximum reading at most snow courses occurs near the beginning of April, the greatest number of measurements is made at this date. Also, later in the season, lower elevation snow courses seldom have snow cover, so are not scheduled for sampling.