Climate, Snow Cover, Microclimate, and Hydrology 65 



pack albedo and then by the absorption of heat by the exposed ground 

 surface. In a few days, the snowpack has disappeared except for larger 

 drifts. Most of the meltwater runs off, and streams are in flood condition 

 as 50% or more of the annual flow volume is discharged in a few days. 

 The ground is covered with extensive areas of shallow surface water. Net 

 radiation is at a maximum during this post-melt period, and most of the 

 energy is used in evaporation. 



The short summer season is subject to relatively small interannual 

 variability in temperature but large variability in precipitation. About 

 two-thirds of the net radiation is used in evapotranspiration, and the rest 

 in heating the air. The depth of thaw in the soil approaches its maximum 

 near the beginning of August. There is a near balance between precipita- 

 tion and evapotranspiration, but significant runoff may occur in wetter 

 years. Only about 10<^o of the evapotranspiration is transpiration. The 

 remainder is evaporation from soil and interception from plant surfaces. 

 Significant evaporation takes place from lakes and ponds, as well. 



Net radiation is still positive but small in September. Air tempera- 

 tures drop consistently below 0°C and the soil begins to freeze from 

 below and above. Precipitation is largely in the form of snow, but inter- 

 mittent melting often occurs. By the end of September, the net radiation 

 becomes negative, the soil active layer may be completely frozen, and the 

 permanent snowpack is becoming established. 



