How are Snow Surveys Done?
Several times each winter, Los Angeles Department of Water and Power hydrographers trek far into the Eastern Sierra backcountry to survey the amount of snow that has fallen. The set out on skis, snowshoes or in over-snow-vehicles called "snow cats" to measure the snow depth and amount of water content in the snow at specific mountain snow courses.
The DWP has measured the same 12 courses
located in four major watershed basins since the 
1920’s. The courses are located at
varying elevations between 8,000 and 11,000 feet, and include the Cottonwood Lakes Basin,
Big Pine Canyon, Rock Creek Canyon and the Mammoth Lakes Basin. The sites were selected
because they accurately represent overall snowpack and precipitation conditions at
specific areas and elevations.
Preparations for snow surveys begin well before the snow falls. Big Pine and Cottonwood Canyon surveys require an overnight trip on skis and snowshoes, which means stashing sleeping and eating supplies in the backcountry. In September, DWP hydrographers load provisions onto mules which are used to haul supplies to secluded cabins near the snow courses.
While in the backcountry, the hydrographers also perform maintenance work on the courses and calibrate snow sensor equipment.
DWP hydrographers typically do their first snow surveys at the end of January. Surveys are also done at the end of February and March. For safety reasons and because they often perform difficult work in severe conditions, snow surveyors travel in teams of two or three. All snow surveyors receive intensive training in snow sampling techniques, cross country travel, avalanche safety, first aid and mountain survival.
Snow courses are marked using distinctive
signs. A standard snow course is 1000 ft. long and has 10 measuring points. Hydrographers
take a snow sample at each point and record snow depth and water
content.
These points are
then averaged and an overall water content is determined and recorded for that course.
Snow samples are collected by using a "snow sampling set" consisting of a series of aluminum tubes about 1.5 inches in diameter and 30 inches long that can be screwed together. Hydrographers screw enough tubes together to reach from several feet above the snow surface all the way down through the snowpack to the ground. The bottom tube has a sharpened steel cutter to slice through ice layers in the snowpack.
The hydrographers weigh the long empty tubes
that have been 
attached together to establish their base weight, and then push them down
through the snowpack until they reach the ground surface. Graduations etched on the side
of the tubes indicate the depth of the snow, which the hydrographers record. The tubes are
then raised out of the snow. Slots cut into the tube allow the hydrographers to visually
verify that all of the snow core remained in the tubes as they were being raised. They
then check the bottom of the tubes for signs that ground level was actually reached, such
as dirt and other debris.
The tubes and snow core are then weighed. The hydrographers then determine the snow water content by subtracting the weight of the empty tubes, and record the data. They then dump the snow core out of tubes and move to the next sample point. Depending on weather and snow conditions, up to three courses can normally done each day.