Snow Surveys and Water Supply Forecasting
Snow Surveys and Water Supply Forecasting
The beauty of snow is fascinating, and millions of Americans enjoy the snow-covered landscape as a playground. But beyond its aesthetic and recreational appeal, snow plays a vital role in our lives as a primary source of the water supply in the Western United States.
Increasing and often conflicting demands for water in the West have heightened public awareness of the need for sound management decisions concerning water. Although the West's high mountain ranges hold a vast snowpack that provides 50 to 80 percent of the year's water supply, nature cannot be relied upon to provide an uninterrupted, dependable supply of meltwater to meet all the downstream requirements. To moderate this variability, reservoirs and canals have been built to serve the growing needs of agriculture, industry, and communities. But successful water management begins with an adequate knowledge of the primary source of water in the West: snow.
Obtaining accurate and timely information on the extent and water content of the mountain snowpack requires specially trained people and unique equipment. The Federal, State, and private cooperative snow survey program directed by the U.S. Department of Agriculture's (USDA) Natural Resources Conservation Service (NRCS) has met those needs since the mid-1930's and continues to evolve in response to increasing demands of water users. With a computerized data collection network and forecast system, the program also fills many other requirements for hydrological and climatological data useful in natural resources management and research.
Mountain Snowpack and the Water Supply
To the casual observer, the process by which we get water from the mountain snowpack is simple: the weather cools as winter approaches and precipitation changes from raindrops to snowflakes. Snow accumulates in winter, and with warming of spring and early summer it melts, producing streamflow.
In reality, the relationship between the snowpack and the amount of snowmelt runoff is complex. It depends on many factors, primarily moisture content of the soil, ground water contributions, precipitation patterns, fluctuation in air temperature, use of water by plants, and frequency of storm events. These factors change throughout the year and from year to year. Their relative importance varies depending on location.
The stage is set for the snow-water year even before the first snowflakes fall. The amount of moisture that accumulates in the soil early in winter, before the snowpack develops, will affect runoff the following spring. Dry soils tend to absorb more of the meltwater than wet soils. The amount of moisture that is absorbed depends on soil characteristics as well as precipitation. Wind, air temperature, storm frequency, and the amount of moisture in the atmosphere determine the accumulation of the snowpack. How the snowpack accumulates affects its density (amount of water per unit volume of snow) and texture (crystalline structure). Density increases as the snowpack becomes deeper and the lower layers are compressed. Wetness of the snow also affects density. Compression affects the crystalline structure of the snowpack. Density and crystalline structure affect how fast the snowpack melts and how much water it yields.
Air temperature and availability of atmospheric moisture determine how wet or dry the snow is. Typically, the west slope of the Cascade Range, in response to the Pacific Ocean's strong influence, receives heavy, wet snow. One foot of that snow, newly fallen, can produce up to 1.5 inches of water. In other areas, such as the Wasatch Mountains in central Utah, the snow is much drier. It is light and powdery -- excellent for skiing -- and 1 foot of fresh snowpack might contain only an inch of water.
Winds can redistribute the snow into drifts. Drifts differ from the surrounding snowpack in texture and density because of the weight of additional snow. On unsheltered snowpacks, high winds can evaporate the snow cover at temperatures lower than 32° F -- a process called sublimation. Mountain snowpacks do not melt steadily. Melting varies according to weather, ground temperature, and exposure to the sun's rays. A snowpack begins to melt when its temperature from top to bottom equalizes at 32° F. Before reaching this isothermal state, the snowpack has different temperatures at different depths. Ground temperature, air temperature, and exposure to incoming solar radiation affect how quickly it becomes isothermal. South-facing slopes and open areas receive the most solar radiation and have the highest melt rates.
The Western United States requires a dependable supply of reasonably priced, good-quality water if the economy is to prosper and the quality of life is to remain high. Vast areas that receive just a few inches of annual rainfall produce bountiful crops, but only with irrigation (fig. 3). Decisions on the types of crops to plant, the number of acres, and irrigation scheduling all depend on reliable forecasts of the year's water supply. Much of the power for cities as well as agriculture and industry is generated by hydroelectric energy. Water is truly the life blood of the West.
Wise management of existing water resources in the United States is essential. Water management, however, is complex even under the best of circumstances. Supply, demand, and cost are subject to the climate and to numerous economic and social influences, domestic and international. The decisions made early in the year, based on the best available information, often require significant revision as more data become available.
The Columbia and Colorado rivers are two examples of extremely complex snowmelt-fed river systems. The area draining into the Columbia River comprises about 258,000 square miles, which includes 40,000 square miles in Canada. Along the river, Federal agencies have built 30 major dams for power generation, flood control and irrigation storage. The Columbia and its tributaries support a wealth of fish and wildlife, including several species of fish such as salmon, which live in the sea but spawn in the river's fresh water (fig. 4). Barge traffic on the river is a major link in the area's transportation network for marketing agricultural and other products. Because many communities and industries and millions of acres of agriculture depend directly on this river system for survival, effective and timely management is critical.
Like the Columbia, the Colorado River also begins in high mountain country. It drains about 247,000 square miles. Huge population centers in southern California and Arizona consume enormous quantities of water, as do the expanding agricultural developments, and demands for water of the Colorado are intense. As in the Columbia, numerous storage facilities have been constructed, impounding snowmelt water to produce electricity, irrigate farms, supply water to cities and towns, and prevent floods. Unlike the Columbia, however, the Colorado picks up dissolved salts as it flows through ancient deserts and areas shaped by prehistoric inland seas. Heavy withdrawal of water, evaporation, and irrigation return flows can increase salt concentration downstream and thereby lower the quality of the water. Because multistate agreements and compacts regulate the quality and quantity of streamflow on the Colorado River, accurate management of streamflow and water use is imperative.
Most smaller river basins throughout the West also have management requirements for limited water resources that are just as important for their users. Management decisions are vital every year for big rivers or small, but the years of vast surplus and extreme shortage intensity the demands for management excellence and the importance of snow surveys.
Adapted from Natural Resources Conservation Service -- Snow Surveys and Water Supply Forecasting: Agriculture Information Bulletin 536.