SNOW HYDROLOGY (GEOG 4321): MEASURING SNOWFALL AMOUNT

Instuctor: Mark Williams
Telephone: 492-4794 or 492-8830

Readings:

• Handbook of Snow, Chapter 6
• Avalanche Handbook, pp 241-246

Web Sites for Snow Measurement Programs

• California Cooperative Snow Survey (CCSS)
• National Operational Hydrologic Remote Sensing Center
• NCAR snow measurement experiment near Marshall, Colorado

• Snow melts as it lands or lies on the ground.
• Snow settles as it lies on the ground.
• Snow is easily blown by the wind and redistributed.

1. Snow stakes, permanent or seasonal deployment.
2. Snow probes: essentially rulers that are stuck into the snow.
3. Remote Sensing: not functional. Active microwave (Synthetic Apeture Radar) has potential but not yet functional for measuring snow depth.

1. Gravimetric
   • Melt and weigh snow of a known volume. Cumbersome but effective.
   • Federal or Mt. Rose snow sampler. Long tube of known volume that is stuck into the undisturbed snowpack and retrieves a snow core that hopefully is continuous from the snow surface to the ground surface. The snow core is weighed and converted in SWE. Relatively fast and efficient. The California Cooperative Snow Survey has about 300 snow courses, all of which use this instrument to measure SWE.
   • Density cutter that goes into the sidewall of a snowpit and retrieves snow of a known volume, which is then weighed and converted into SWE.
2. Snow Courses
   • 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.

     

   • 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.
   • 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. Many graduates have credited this training with bringing them safely through unforeseen, hazardous situations.
   • The surveyor makes certain that the tube is clear of all snow and soil before taking the snow core sample. 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. 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. 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. An average of all samples taken is calculated and used to represent the snow course.

     

3. Snow Pillows
   • 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. 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.

     

   • 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.

     

     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.

   • Large mattress filled with anti-freeze.
   • Pressure of overlying snow is measured with a manometer and converted to SWE using a calibration curve that is developed for each snow pillow.
   • SWE amount is telemetered in real time (more or less continuously) to receiving base.
   • Snotel Program: runs about 70 snow pillows in Colorado and several hundred throughout the western US.
   • Advantages:
     • Provides non-destructive measurement of SWE.
     • Automatic operation.
     • Receive results in real time from remote sites.
   • Error sources:
     • Bridging or connection with snowpack outside of snow pillow area, reducing pressure measured and SWE calculated. Ice lenses can exasperate this problem.
     • The presence of the snow pillow changes the temperature and water vapor interactions between the snowpack and soil, potentially changing the amount of snow over the pillow relative to snow outside the pillow.
     • Weird impacts to the snow above the pillow which are hard to evaluate or recognize because of the remote and automatic operation, such as moose walking on the pillows or trees falling on the pillows.
     • Changes in structure of the antifreeze with temperature that can change pressure readings.
     • More details on SNOTEL (adapted from Randy Julander at http://www.civil.utah.edu/~cv5450/.
4. Radioisotope Gauges
   • Nuclear gauges.
     • Place a radioactive source in the ground below the snowpack, such as Cesium-137 or Cobalt-60.
     • Measure the radioactivity without snow using a scintillation counter.
     • Measure the radioactivity above the snowpack using a scintillation counter.
     • Snowpack will attenuate the radioactivity measured as a funtion of the mass of water, eg SWE.
   • NOHRSC Natural gamma radiation
     • The National Operational Hydrologic Remote Sensing Center (NOHRSC) has developed, and currently maintains, an operational Airborne Gamma Radiation Snow Survey Program to make airborne Snow Water Equivalent (SWE) and soil moisture measurements. Airborne SWE measurements are used by NWS Weather Forecast Offices (WFO) and NWS River Forecast Centers (RFC) when issuing river and flood forecasts, water supply forecasts, and spring flood outlooks.

       ---

       Operational Snow Survey Program

       • NOHRSC maintains the Operational Airborne Gamma Radiation Snow Survey Program from its office near Minneapolis, MN.
       • 1900 flight lines
       • covering portions of 25 states and seven Canadian provinces.
       • Airborne SWE data are collected simultaneously by two snow survey aircraft
       • almost continuously from late January through mid-April each year.
       • Over 1,000 real-time airborne SWE measurements are made by both aircraft during a typical snow survey season.
       • Each flight line is typically 16 km long and 300 m wide,
       • covering an area of approximately 5 km².
       • Consequently, each airborne snow water equivalent measurement is a mean areal measure integrated over the 5 km² area of the flight line.

       Airborne SWE Measurement Theory

       The ability to make reliable airborne gamma radiation SWE measurements is based on the fact that natural terrestrial gamma radiation is emitted from the potassium, uranium, and thorium radioisotopes in the upper 8 inches of soil.

       • The radiation is sensed from a low-flying aircraft flying 500 feet above the ground.
       • Water mass in the snow cover attenuates, or blocks, the terrestrial radiation signal.
       • Consequently, the difference between airborne radiation measurements made over bare ground and snow-covered ground can be used to calculate a mean areal SWE value with a root mean square error of less than one-half inch.
       • The technique measures the attenuation of the radiation signal due solely to the intervening water mass.
     • More details on NOHRSC Natural gamma radiation.
5. Precipitation gauges
   • Please read the following paper which was published in the 1997 Proceedings of the Western Snow Conference Overcollection of Solid Precipitation by a Standard Precipitation Gage, Niwot Ridge, Colorado.

Back to Course Content