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 deficiencies.

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.