36 J. E. Hobbie 



the onset of runoff. Following spring runoff at Barrow, the tundra is left 

 saturated and covered with numerous flooded areas. 



Precipitation during the summer season (Table 2-2) averaged 6.4 cm 

 over the past decade (range 1.7 to 13.2 cm). Most of this fell as rain but 

 snow can fall during any month. August and September are the wettest 

 and cloudiest months. After the snowmelt period ended, less than 5% of 

 the summer rainfall (3 cm) ran off from a 1.6-km^ watershed during 4 

 years (Brown et al. 1968). There is virtually no water lost by runoff from 

 ponds in polygonal ground during the summer, except in rare summers 

 when the rainfall fills the ponds during late August. Permafrost blocks any 

 downward water movement but some small quantities may move laterally. 



One additional source of moisture during the summer is condensation 

 of fog and dew on the vegetation. As seen in Table 2-1, the average relative 

 humidity is high during this period and the condensation may be equal to 

 23 to 50% of the summer precipitation (L. Dingman, personal 

 communication). Unfortunately, there are no good data on this, but 

 condensation has little effect on the water balance of the ponds. 



As the summer runoff is so small, the most important water loss 

 occurs by evaporation. Brown et al. (1968) measured a pan evaporation of 

 16.0 cm in a "typical" precipitation year. However, it is well known that 

 this method overestimates the true loss from land and vegetation and these 

 authors estimated (from runoff and precipitation) that the actual losses 

 were 6.0 cm. Mather and Thornthwaite (1958) measured about the same 

 amount in large evapotranspirometers at Barrow. Thus, Brown et al. 

 (1968) concluded that the summer precipitation was balanced by 

 evapotranspiration in the watershed they studied. Rates of evaporation 

 from the open water of ponds and lakes are close to the pan-measured 

 rates and likely average 2 to 3 mm day"'. During rainless periods of a 

 month or so, many tundra ponds dry up. 



The evaporation rate for the tundra, as well as the factors that affect 

 it, was described by Weller and Holmgren (1974) (Figures 2-4 and 2-5). It 

 is seen that neither wind speed nor air temperature changed appreciably 

 from pre- to post-melting period; thus, the strong decrease in albedo when 

 the snow melts and the resultant rise in absorbed solar radiation are the 

 main factors in the increased evaporation. 



BIOLOGY 



The following information on the terrestrial biology at the IBP site is 

 covered in greater detail in Bunnell et al. ( 1 975) and Brown et al. (in press). 



