Climate, Snow Cover, Microclimate, and Hydrology 61 



tend to increase, as would be expected. 



Based on energy considerations, Weller and Holmgren (1974a) con- 

 cluded that evaporation accounted for only 2% of the total ablation 

 when the snow cover was still complete, but up to 13% when bare patches 

 appeared. They noted also that condensation on the snow surface was 

 likely when air temperature rose above 0°C. Johnson and Kistner's 

 (1967) measured pan evaporation rates of up to 0.47 mm hr"' at midday 

 during snowmelt near Meade River indicate that open-water evaporation 

 begins to become important at this time. 



Post-melt, Summer Hydrology, 

 and Related Processes 



Immediately following snowmelt, the coastal tundra is largely cov- 

 ered with water. However, snowdrifts remain in river channels and most 

 lakes are still ice-covered. The albedo of the surface is 10-15%, causing 

 the net radiation to jump almost an order of magnitude from its pre-melt 

 value, and all energy-balance components are at or near their maxima 

 (Figure 2-8). Over the ice-covered lakes the dramatic reduction of albedo 

 extends over a period of two to three weeks. Once the ice cover disap- 

 pears, the lakes have a somewhat lower albedo than the land surface and 

 absorb more solar radiation. 



During the post-melt season, over 70% of the available energy is 

 utilized in evaporating the extensive surface water. Water balance con- 

 siderations and data on evapotranspiration during the summer season 

 (see below) suggest that an average of 10 mm (= 1 mm day') evaporates 

 during this 10-day period. This value is consistent with the average pan 

 evaporation rates of 2.77 mm day"' and 1 .72 mm day"' measured by Mil- 

 ler et al. (1980) in late June of 1972 and 1973, respectively, if a pan coef- 

 ficient is applied and if less than 100% of the surface is considered to be 

 evaporating. Weller and Holmgren (1974a) estimated considerably high- 

 er rates, 4.2 to 4.6 mm day', on the basis of energy considerations alone. 



Warming of the soil begins during the pre-melt period, but there is 

 no significant thawing until the post-melt season. Thaw is very rapid ini- 

 tially, and can be expressed by an equation of the type applied to a rela- 

 tively dry upland soil by Kelley and Weaver (1969): 



Z = L[\ - e\pi-at)] 



where Z = depth of thaw (cm) 

 / = time in days 



a = empirical constant (1970 value = 0.067 day"'; 1971 value 

 = 0.047 day"; 1972 value = 0.082 day"') 



