Climate, Snow Cover, Microclimate, and Hydrology 57 



about 85*^0 to about 75% just before the melt season begins (Maykut and 

 Church 1973). 



Virtually every aspect of the surface environment changes dramati- 

 cally during the brief melt season. These changes are rapid because of the 

 relatively high insolation rates during the long days and the operation of 

 positive feedback loops affecting the radiation and energy balances. The 

 energy available at the surface increases by a factor of 3.6, and about 

 60% of this is used in melting (Figure 2-8). 



Once the snowpack is isothermal at 0°C, the further addition of 

 energy produces meltwater that does not refreeze. Initially, this water 

 fills voids in the snowpack and reduces the albedo, typically to values 

 near 50%. Reduction in the albedo increases the absorption of solar radi- 

 ation, which increases the melt rate. 



Snowpacks can hold about 5% of their water equivalent as liquid 

 water (Anderson and Crawford 1964). For the coastal tundra this would 

 amount to about 5 mm of water, which can be produced in a period of a 

 few hours at the rates at which radiant energy enters the snowpack dur- 

 ing melt. Once this capacity is filled, the snowpack is ripe and further 

 melt produces runoff at the snow/ground interface or over ice lenses and 

 layers. As meltwater accumulates in low-lying areas and produces slush, 

 the albedo is further reduced and the melting accelerated. 



Snow-free patches generally appear within a day or so of the onset 

 of melting and initiate the operation of another positive feedback loop. 

 The albedo of the exposed areas is 10 to 15%, so they absorb four to five 

 times as much radiant energy as the snow-covered ground. This addi- 

 tional energy produces local heating of the air and local advection of heat 

 to the surrounding snow, which further accelerates melting (Weller et al. 

 1972, Weller and Holmgren 1974a). 



The upper layer of the soil generally begins to thaw a few days be- 

 fore snowmelt is complete. This layer typically has been desiccated by 

 loss of water to the snowpack during the winter, so that some infiltration 

 of snowmelt water occurs. Data of Guymon (1976) indicated that this in- 

 filtration was most significant in poorly drained areas. However, most of 

 the snowmelt water runs off to streams and lakes after ponds and poly- 

 gon troughs are filled. 



A large fraction of the total annual runoff occurs within a few days 

 (Table 2-6). The spring runoff sequence on the Coastal Plain has been 

 described by Johnson and Kistner (1967), Lewellen (1972), Holmgren et 

 al. (1975) and Hobbie (1980), and for the through-flowing Colville River 

 by Arnborg et al. (1966). In stream channels, the first flow is over ice that 

 is frozen fast to the bottom. The sediment load of this initial runoff is 

 very low. But as the flow increases toward its peak, the channel ice is 

 eroded and melts free from the bottom, generally dislodging large 

 amounts of sediment. In the larger rivers, ice jams frequently occur, 



