Climate, Snow Cover Microclimate, and Hydrology 51 



showed that average ground temperature has increased 4°C since 1850, 

 so the assumption is not strictly true over several decades. The energy re- 

 quired to warm the snowpack from its typical winter temperature of 

 -30 °C to the melting point is about 7 MJ m"^ 



The net radiation value of 440 MJ m"^ yr"' measured by Maykut and 

 Church (1973) is probably the most reliable mean value, since it repre- 

 sents measurements over five years. However, it should be noted that the 

 data of Weller and Holmgren (1974a) indicate a radiation balance of 

 about 580 MJ m"^ yr'' and Mather and Thornthwaite's (1958) values are 

 even higher. Most of the radiant energy available at the surface is trans- 

 ferred to the atmosphere by conduction and convection (//). During win- 

 ter, this exchange is positive, i.e. the air is warmer than the snow surface 

 and the snow is warmed. Convective exchange becomes negative as the 

 snowpack ripens in late May and early June, with the rate of energy loss 

 increasing to a peak during the summer and declining again during 

 freeze-up. 



During the winter, there is a minor net gain of latent-heat energy due 

 to condensation, and some sublimation occurs prior to and during snow- 

 melt. Open-water evaporation rates are estimated to be about ten times 

 larger than transpiration rates, and the rate of evapotranspiration typi- 

 cally peaks in late June when the tundra is still water-saturated and 

 available radiant energy is at its peak. 



There is a net flow of energy from the ground to the snowpack (G) 

 during the winter, when snow and soil temperatures are higher than sur- 

 face temperatures. As the net radiation increases and surface tempera- 

 tures rise in May, this flow reverses, and most of the available energy is 

 used to warm the snow and soil. After melt, downward heat flow con- 

 tinues as the active layer thaws and warms. Typically over 90% of the 

 thaw occurs by mid- July (Kelley and Weaver 1969). The energy used to 

 warm and thaw the soil in spring must, on the average, be balanced by an 

 upward movement of latent and sensible heat during freeze-up in 

 September. 



HYDROLOGY 



Water Balance 



The water-balance equation at the surface for a specified period of 

 time is expressed as: 



P = E + R + I+AS 



where P = precipitation 



