Snow Survey in the Last Frontier

Beginning in 2002, the NWS embarked on a ground-breaking survey in this country’s 49th state. Alaska became the newest addition to a long list of states where snow is surveyed by an office of the NWS called NOHRSC (National Operational Hydrologic Remote Sensing Center). The NOHRSC is charged with gathering data on the snow pack in the snow-affected regions of the United States, and then makes the data available to its customers, with the NWS being the primary data recipient. This data is then used largely for river and flood forecasts and water supply control. A majority of the precipitation that falls in mountainous areas falls as snow, and that contributes a great deal to the overall water supply in those areas. The snow pack acts like a natural reservoir, supplying water downstream when demand increases in the spring and summer. Satellite images of Alaska have been used for a long time by NOHRSC to determine where the snow is and where it isn’t. However, the satellites currently in service can’t tell us anything about how much snow is there, and they can’t see under cloud cover-- a major handicap in Alaska due to extensive cloud cover in the winter months. That is why this year in Alaska, NOHRSC introduced its secret weapon in snow research—the Airborne Snow Survey Program. Until now, NOHRSC has only been gathering airborne snow data over the lower 48 states and parts of Canada that affect U.S. hydrologic basins. The Airborne Snow Survey Program has proven its worth in the U.S. through many years of research and survey results, dating back to the birth of the Program in 1978. The success of the Program was enough to convince local hydrologists in Alaska that this method of snow pack measurement was worth a closer look.

Airborne Snow Survey is performed by only a handful of scientists around the world, and the Program at NOHRSC is the only one of its kind in this country. The science is based on the simple principle that background radiation from the earth is attenuated by the amount of water in and upon the top layer of soil. That water can be absorbed in the soil as soil moisture or it can be lying on top of the ground as liquid water, snow, or even ice. NOHRSC has perfected the use of the radiation detection system in NOAA aircraft and uses NOAA pilots to perform the surveys.

A lot of preparation was required for an Airborne Snow Survey to begin in Alaska. The first step was to identify the highest priority areas for the surveys. The areas in need of the most snow data are those with snow pack that affects large populations and that feeds major river branches. Local hydrologists in Alaska outlined these areas and it was up to NOHRSC to find the best position for survey flight lines in each area. In order to provide good coverage in a large area, a large number of measurements must be taken, although NOHRSC relies on mathematic interpolation between the survey lines for complete coverage. Nevertheless, flight lines were identified in each priority area over a diverse sample of terrain, ground cover, and soil type. The land characteristics were collected from a number of data sources including DEMs (Digital Elevation Maps), USGS Topographic Chart DRGs (Digital Raster Graphics) Landsat imagery, and digital maps showing land use and soil type. The terrain of Alaska presented a problem for navigation because the survey equipment must be flown at a very low altitude in order for snow measurements to be made reliably. Therefore, careful attention was paid to the positioning of the flight lines, to ensure that the aircraft could be stabilized at 500 feet above the ground over the entire line. A flight line is approximately 10 miles long, and it was difficult in some cases, to find a 10 mile path in the mountains that would be safe to fly. Finally, over 100 flight lines were identified and digitized on Alaska maps before the pilots departed for Alaska.

The NOHRSC budget only reserved 100 flight hours for the initial Alaska survey, and a significant amount of that was taken up in transit time. From the NOHRSC home base in Minneapolis, MN, it took 15 flight hours and 2 days to get to Fairbanks, AK, where the survey began. Headwinds were expected on the trip up to Alaska, and a route across the states to Seattle and then up to Ketchikan was chosen as the preferred route. A direct line from Minneapolis to Fairbanks wouldn’t give many suitable options for fuel stops and would require a visit with Canadian customs if a landing was made in that country. Instead, the Snow Survey flight crew made stops at only U.S. airports with facilities to approach and land in adverse weather conditions, if necessary.

In any remote and rugged area safety is a major concern when flying aircraft, and this is especially true in Alaska. There are 365 million acres of land in Alaska with a mere 606,000 people and less than 12,000 miles of paved road. Distances between communities and communications and navigations facilities are much greater than in any other state. Because of this, survival equipment was kept onboard the aircraft at all times in the event of a problem. In fact, Alaska law requires certain equipment such as ample food, fishing gear, a firearm, and a mosquito headnet to be on every flight over the state. The aircraft itself can also be an important safety tool. The NOAA TurboCommander is well suited for high altitude transits and can also function at low levels during Snow Survey operations, with adequate power for climbing out of trouble in the mountains. The NOHRSC flight crew, made up of pilots LCDR Dave Savage and LT Gregg Glover, had flown the NOAA twin turboprop aircraft on surveys in the Rocky Mountains in the lower 48, but Alaska would be a new experience. The small number of airports in Alaska and their remote locations and limited services would prove to be a limiting factor in flight planning and survey operations. After sampling a number of airfields across the state, it was discovered that only a handful offered the fuel and hangar space required for the Snow Survey mission. For the majority of the survey, only three major airports were used for an overnight stay, including Fairbanks, Anchorage, and King Salmon.

The initial survey of Alaska was completed this year in August, and is an important first step to future Snow Surveys in the state. The August survey served several purposes, including collecting background radiation data and soil moisture data in preparation for future Snow Surveys. The background data must be collected with the least amount of snow on the ground, and is necessary before reliable Snow Survey data can be collected. The soil moisture data was collected at the same time as NOHRSC ground crews were sampling the soil in order to better classify the data. The initial survey was also critical in order to determine the difficulty of flight operations in the high priority survey areas, and to collect a list of facilities and resources that could be used in the future. The final conclusion is encouraging. Reliable data was collected on almost every one of the planned survey lines, and additional flight lines were identified. The mission demonstrated that Snow Survey would be possible in the Last Frontier, in spite of the challenges Alaska delivers. Of course, the environment that makes Alaska so treacherous and challenging to work in, also makes the job a very rewarding experience. It’s difficult to compete with the awesome sights and scope of Alaska. The real reward for the hard work and preparation going into the survey was the opportunity to experience the natural beauty of the state.