Snow Surveys and Water Supply Forecasting
Snow Surveys - SNOTEL
Even though the data from the snow courses provide a valuable body of
information, the typical schedule for manual surveys results in weeks with
no specific insight into the condition of the snowpack. In that time,
intense storms may be adding an abnormally large amount of snow or rain;
perhaps an unseasonable warm spell at high elevation is resulting in a
rapid melt with ensuing flood hazards.
Snow surveyors and water managers realized early in the development of
the program that timely forecasting and management decisions required more
frequent measurements and additional information. They also needed a way
to survey particularly remote and hazardous snowpacks. SNOTEL's automatic
sensing and data transmission were the solution.
Sensing Devices. A typical SNOTEL remote site consists of
measuring devices and sensors, a shelter house for the radio telemetry
equipment, and an antenna that also supports the solar panels used to keep
batteries charged (fig. 13). A standard sensor configuration includes snow
pillows, a storage precipitation gauge, and a temperature sensor. The snow
pillows are envelopes of stainless steel or synthetic rubber, about 4 feet
square, containing an antifreeze solution. As snow accumulates on the
pillows, it exerts pressure on the solution. Automatic measuring devices
in the shelter house covert the weight of the snow into an electrical
reading of the snow's water equivalent -- that is, the actual amount of
water in a given volume of snow.
Figure 13. This drawing depicts a typical remote
SNOTEL site. Pressure pillows are used for measuring
snowfall, a storage precipitation gauge provides
current information about conditions at the site, and
a temperature sensor measures the existing temperature.
The precipitation gauge measures all precipitation in any form that
falls during the year. The temperature sensor determines the minimum,
maximum, and average daily readings.
Additional sensors can be incorporated into a particular site for
measuring wind speed and direction, soil temperature, snow depth, and a
variety of other weather and environmental aspects. The configuration at
each site is tailored to the physical conditions, the climate, and the
specific requirements of the data users.
Telemetry. SNOTEL uses the principle of radio transmission by
meteor burst. Radio signals are aimed skyward where the trails of
meteorites reflect or reradiate the signals back to Earth.
The meteor burst technique allows communications between two locations
as much as 1,200 miles apart. Two master stations -- at Boise, Idaho, and
Ogden, Utah -- cover the 10 Western States, an area of about 1 million
square miles. By cable, the master stations feed the data to SNOTEL's
Centralized Forecasting System in Portland, Oregon. The Alaska Meteor
Burst Communication System (AMBCS) for snow surveys is similar. All remote
SNOTEL sites are interrogated daily on a regular schedule. Additional
interrogations can be conducted on demand, and any special reporting
requirements can be programmed into the site's microprocessors. In the
Alaskan system, hourly interrogations are conducted, and the data are made
immediately available to cooperating agencies.
Quality Control. The sites are designed to operate unattended
for 1 year in severe climates. Each site receives preventative maintenance
and sensor adjustment annually. The reliability of each SNOTEL site is
verified by ground truth measurements taken during regularly scheduled
manual surveys. These readings are compared with telemetered data to check
that values are consistent and compatible. Any values found to be beyond
specified limits are carefully examined and edited to ensure a continuous,
high-quality record. Every year each site's performance is compared
against established performance standards. Sites not meeting rigid
criteria undergo a thorough field evaluation to correct any site
Billions of sand-sized meteorites enter the atmosphere daily. As each
particle heats and burns in the region 50 to 75 miles above the Earth's
surface, its disintegration creates a trail of ionized gases. The trails
diffuse rapidly, usually disappearing within a second, but their short
lifespan is adequate for SNOTEL communications to be completed (fig. 14).
The process has three major steps: (1) master stations request data
from remote sites; (2) sites respond by transmitting their current data;
(3) and finally a master station acknowledges receipt and signals the site
transmitter to stop. This complex exchange, taking place in a fraction of
a second, is possible thanks to microprocessors.
Figure 14. Depicts meteor burst technique.