Snow Survey
DB: This is Earth and Sky. Melted snow is a primary source of water in much of western United States. Snow accumulates each winter in the mountains – the snowpack acts as a natural reservoir.
JB: Each spring, snow melts into creeks, streams and rivers – and a lot of that meltwater gets diverted to reservoirs and canals. To help predict how much water will be unleashed during the spring and summer thaw, researchers conduct snow surveys. This program is directed by the U.S. Department of Agriculture’s Natural Resources Conservation Service with participation from other organizations like the U.S. Forest Service.
DB: From January through June, they measure the depth and density of the snowpacks. Snowfall that’s wet and heavy yields much more water than light, powdery snow. The density of a snowpack also increases as the snowpack becomes deeper and the lower layers are compressed.
JB: The first snow surveys were done by people on snow shoes. These days, almost half of the sites are fully-automated. In all, manual snow survey data is collected from about 800 sites, and automated snow survey data from more than 600 sites. The data go to the National Water and Climate Center in Portland, Oregon. There, scientists create forecasts of what the water supply will be for various times of the year.
DB: For today, that’s our show. For more on the annual snow survey, visit today’s show on our web site at earthsky.com. We’re Block and Byrd for Earth and Sky.
Our thanks to the following individuals and institutions who assisted in the preparation of this script:
Chris Pacheco
Resource Conservationist
USDA, Natural Resources Conservation Service National Water and Climate Center
Portland, OR
Dr. Kelly Elder
USDA Forest Service
Fort Collins, CO
The following books, articles and web sites were used in preparing this script:
Snow Surveys and Water Supply Forecasting Agriculture Information Bulletin 536 [Contains more detailed information about snow surveys and water forecasting.]
National Water and Climate Center (USDA)
National Snow and Ice Data Center: Ice on the Web
Washington State Cooperative Snow Surveys (USDA)
Idaho Snow Survey Page (USDA)
Idaho Snow Survey Page (USDA)
Snow Survey and Water Supply Forecasting Products & Services (USDA)
Author’s Notes:
Snow as a source of water
by Shireen Gonzaga
In the western United States and Canada, vast areas of land receive just a few inches of rain each year. Yet, despite the meager rainfall, large cities and farming communities are able to flourish. That’s because much of the water supply, as much as 50% to 80%, comes from snow.
Snow begins falling at the higher elevations in autumn, and continues to accumulate in frigid winter conditions until spring. The snowpack begins melting in most mountain areas in April, causing increased water flow in streams that reaches a crescendo in June before falling back to more modest flow levels for the rest of the year.
The amount of meltwater depends on the amount of snow that has accumulated over winter, and the rate of spring snow thawing depends on weather conditions. All this variability needs to be tempered so that a constant water supply is available throughout the year.
Water supply is controlled using reservoirs that release water gradually over the year for use by communities, industry, and agriculture. For instance, the Columbia River drainage covers 258,000 square miles. Within that area, there are 30 dams being utilized for power generation, domestic water supply, and irrigation.
Good water management depends on good data to predict water availability. Decisions need to be made ahead of time about what kinds of crops to plant, how much land to use for agriculture, water use in communities, power generation, fisheries management, and even flood control and drought control.
Predicting water availability from snowpacks can be tricky. Local conditions at the snowpack play a large role in determining the characteristics of a snowpack and the meltwater that it yields. Take the soil that the snow pack rests on, for instance. If the soil is quite dry and porous, some meltwater will seep into the ground instead of flowing into creeks and streams. Other influencing factors include the way the wind redistributes snow drifts, fluctuations in temperatures, and exposure to the sun.
The density of a snowpack tells us how much water it holds. For example, newly-fallen snow that is wet and heavy, the kind that falls in the western Cascade range, with a climate heavily influenced by the Pacific Ocean, yields about 1.5 inches of water for every foot of snow. In the Wasatch mountains of central Utah, the snow is light and powdery, ideal for skiing. This snow contains less water, about 1 inch for every foot of newly-fallen snow. But estimating the amount of water from a snowpack is not that simple; as snow accumulates, the lower layers become more compressed, increasing in density and changing the crystalline structure of the snow. These compressed lower levels of the snowpack not only contain more water compared to the upper layers, but also have different melt rates due to the change in the crystalline structure of the ice crystals.
Snow Survey Program
by Shireen Gonzaga
Since 1935, the Cooperative Snow Survey Program has been conducting snow surveys across the high elevations of the western United States to gather data for predicting the annual water supply. This program is directed by the US Department of Agriculture’s Natural Resources Conservation Service (NRCS) with participation from other federal agencies like the US Forest Service, as well as state and local agencies from Arizona, Colorado, Idaho, Montana, Nevada, Mew Mexico, Oregon, Utah, Washington, Wymoing, and Alaska, as well as southern Canada. (California has its own snow survey program.)
Snow surveys usually start in January and can end as late as June, depending on the location of the survey site. There are several ways to conduct snow surveys. The manual surveys are done at around 800 sites across the high elevations of western United States. The sites, called snow courses, are located in areas shielded from wind, and some are located in remote areas that make for sometimes hazardous winter travel. Teams of usually two people visit these sites once a month, sometimes more often, depending on site conditions, to measure snow depth and density, as well as other weather-related parameters. For inaccessible areas, depth markers placed before the onset of winter allow low-flying aircraft to take measurements of snowpack depth.
An automated snow survey system called SNOTEL (an acronym for SNOwpack TELemetry), run by the NRCS, has been in place since 1977. Each solar-powered SNOTEL site, more than 600 in all, has measuring devices and sensors that record the weight of the snowpack (that indicates how much water is present in a column of snow). There are also sensors to measure rainfall, wind speed and direction, and in some sites, soil temperature. These fully automated sites can operate unattended for as long as a year, and receive annual preventive maintenance and sensor calibrations.
Data collected by SNOTEL sensors are radio-transmitted with the help of meteors. Everyday, several billion grain- sized meteors plunge through the Earth’s atmosphere. As these particles enter the upper atmosphere about 50 to 75 miles above the Earth, they heat up and disintegrate, producing a trail of ionized gases. Radio signals are able to bounce off these ionized trails. Because meteors bombard the Earth so frequently, there are always plenty of ionizing trails to relay the SNOTEL-transmitted radio waves, forwarding them to receiving stations that could be as far away as 1200 miles.
In all, manual snow survey data collected from about 800 snow courses, automated snow survey data from more than 600 SNOTEL sites, water flow volume amounts from several hundred stream gauges, as well as rainfall and other weather-related parameters from many climatological stations, are sent to the NRCS’s National Water and Climate Center in Portland, Oregon. There, at their Centralized Forecasting System, scientists create forecasts of what the water supply will be for various times during the year, as well as maintain a historical archive of past water supply records. States use the water supply prediction data to issue “Basin Outlook Reports,” usually reported monthly from January to June. A more general report is issued by the National Water and Climate Center called the “Water Supply Outlook for the Western United States,” available from January to May. These reports are used by agricultural, industrial, community services, and recreational interests to plan their activities for the year.
