Snow Surveys and Water Supply Forecasting

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 (fig.1).

Figure 1. Melting snow produces streamflow -- a vital
source of water for people living in the West.

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 (fig. 2).

Figure 2. Skiers may prefer light, powdery snow, but
heavy, wet snow contributes more to the water supply.

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.