M7 Seasonal Surface Ice Cycle

Topic

In its simplest form, the seasonal ice cycle comprises the accumulation of snow and ice during cold months and their melting during warm months; this cycle powerfully modulates the entire hydrological cycle of the basin where it occurs. Snow stores solid precipitation for months and subsequently releases it in a concentrated burst during spring—a phenomenon known as snowmelt runoff—thereby generating the characteristic meltwater hydrographs of snow-dominated basins and mountain glaciers.

Snow also possesses a set of thermal properties that make it unique within this system: it has very high reflectivity, capable of reflecting between seven- and nine-tenths of the incoming solar radiation; it has very low thermal conductivity, allowing it to act as an effective insulator for the underlying ground; and, finally, it has a very high latent heat of fusion—approximately 334 kilojoules per kilogram of melted snow.

Snowmelt can be calculated in two distinct ways: using simple yet effective degree-day models, where the melt rate is proportional to the accumulated daily air temperature above zero degrees Celsius; or using energy-balance models—which are more physically rigorous—where the melt rate depends on the total energy available at the snowpack surface (net radiation, sensible heat flux, latent heat flux, and conductive heat flux, minus the change in internal energy stored within the pack itself). This energy must first warm the entire snowpack to zero degrees Celsius before melting can begin. The snowpack's water equivalent, in turn, describes the amount of liquid water the pack would yield if it were to melt completely. Snow interception by trees plays a significant role in boreal forests: up to six-tenths of falling snow can be retained on the branches and evaporate directly—via sublimation—without ever reaching the ground. Furthermore, the ground beneath the snowpack is frozen and nearly impermeable; it is precisely this impermeability that causes the initial spring thaw to generate surface runoff far exceeding normal levels.

The expected behavior of this seasonal ice cycle involves a continuous buildup of the snowpack's water equivalent throughout the winter, a peak in that equivalent when accumulation reaches its maximum, and a rapid decline during the spring melt. Thus, from the initial snow accumulation to the final rapid decline—and encompassing factors such as snow's thermal properties, melt models, tree interception, and frozen-ground impermeability—the seasonal ice cycle ensures that a significant portion of a watershed's water availability ultimately depends on the amount of snow accumulated during the preceding winter.

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