56 S. L, Dingman, et al. 



and water vapor transported upward from the soil condenses in the 

 snowpack. Most of the snow is depleted within the few days of the melt 

 period, but part of the snow is converted to liquid water storage in pud- 

 dles, ponds and lakes. This storage, in turn, is gradually reduced by 

 evaporation through the summer. Persistent snowdrifts in stream valleys 

 may contribute small amounts of stream flow well after the general melt 

 is completed. 



Thermal and Hydrologic Processes During Snowmelt 



As the radiation balance becomes positive around 1 June, the snow- 

 pack warms or ripens and the underlying soil begins to warm. Incoming 

 shortwave solar radiation reaches its maximum values in May, as atmos- 

 pheric transmissivity is high. However, albedo still exceeds 80%, so that 

 most of this radiation is reflected. About 60% of the available radiant 

 energy is used to warm the snowpack and soil (Figure 2-8). Snowmelt 

 begins at the surface when air temperatures rise above 0°C. Heat is trans- 

 ferred downward in the snow by conduction and as sensible and latent 

 heat associated with liquid water movement. It appears that the latter 

 process is responsible for much of the warming of the snowpack, and 

 also contributes to warming of the soil. The percolating meltwater re- 

 freezes in the colder snow, liberating latent heat and forming a complex 

 network of ice glands, lenses and layers. Benson et al. (1975) calculated 

 that about 1.9 MJ m'^ of heat is transported downward for each I cm of 

 ice thickness formed. With the estimated cold content at this time of 2.3 

 MJ m"^ the formation of ice layers totaUng a little over I cm thick would 

 suffice to warm the snow to 0°C. Weller et al. (1972) described the 1971 

 melt in the coastal tundra at Barrow, and reported that melting con- 

 verted an 8 °C temperature gradient across the snowpack to near isother- 

 mal conditions within 2 to 3 days. The soil temperature also rose steeply 

 during this period. 



Initially, the ice layers tend to form at the top of the depth hoar 

 layer, but as the pre-melt season progresses they are found at the base of 

 the snowpack (Benson et al. 1975). The two-layer structure thus breaks 

 down and the density of the pack becomes vertically uniform. Typically, 

 the density of ripe snow is between 0.4 and 0.5 g cm''. The disappearance 

 of the depth hoar layer and the flooding of low areas as melt progresses 

 are major environmental changes for lemmings. They are forced out of 

 the protective snow cover and become subject to environmental extremes 

 and avian predation, with consequent high mortality (Bunnell et al. 

 1975). 



The changes in water content and density of the snowpack during 

 the pre-melt season cause the albedo to decrease from its winter value of 



