Physics 69 



dry summer of 1972 while there was an increase in water level towards the 

 end of the summer in 1971 and 1973. As already discussed, there was so 

 much rain in 1973 that the ponds overflowed in late August. In 1972, there 

 was less rain (Table 3-2), but the evaporation rate (Table 3-9) was almost 

 50% higher in August of that year than in 1971 or 1973. 



The depth measurements indicate that the evaporation pan data 

 approximate evaporation. This is best shown for the first month of 1971 

 (Figure 3-8) when some 12 cm of water was lost from the pond and 1 1 cm 

 from the pan (Table 3-9). Of course, the shallow, sloping sides of the pond 

 complicate this argument, which really should be made on the basis of 

 volume. The calculated balance between evaporation and precipitation for 

 July and August (Table 3-10) agrees quite well with the measured water 

 loss for 1972 and 1973 but did not agree in 1971. It is possible that a large 

 rain would saturate the tundra and allow runoff into the pond while a 

 series of small rainfalls, which might add up to a larger total, would not 

 produce runoff into the pond. 



The water budget for Pond B (Table 3-11) indicates a loss of 1 1.7 cm 

 of water in 1972 and a gain of 1.9 cm in 1973. The change we measured in 

 the pond was a loss of 1 1 .0 cm of water in 1972 and a gain of cm in 1973; 

 this is quite good agreement. From these data, we conclude that the 

 evaporation measurements are approximately correct and that there is no 

 additional seepage of water from the soils into the ponds, even when the 

 water levels of the ponds are 5 or 10 cm below the surface of the tundra. 



One strange finding from measurements of the water depth by a series 

 of transects was that the bottom of the pond does not remain constant but 

 apparently rises and falls during the thaw season. Another bit of evidence 

 for this movement comes from litter bags that were suspended 

 immediately above the sediment on 3 July 1972 by strings attached to 

 poles frozen into the permafrost. By 27 August the bags were 9 to 10 cm 

 above the sediment. One explanation for the rise and fall is that as ice 

 crystals grow in the sediment, water moves through the sediment to feed 

 their growth. In this way, the frozen sediment eventually contains much 

 more ice than the original water content of the unfrozen sediment could 

 have produced. In addition, the frost heaving or expansion of the 

 sediments upon freezing is much greater than the 8% increase due to the 

 freezing of water. This ice crystal growth has been shown to cause frost 

 heaving in soils (Taber 1929, 1930). 



In the ponds, the fall or compaction of the sediments changes the 

 water level only slightly. As the sediments thaw, the excess water is 

 excluded from the sediments but this water has 8% less volume than the 

 original ice. Thus, the effect on the pond level is a decrease of 8% of the 

 distance of the sediment compaction. 



When the sediment movement is ignored, we calculated that the 

 volumes of Ponds C and E were reduced by 49%, 43%, 63%, and 41% 

 during the summers of 1970, 1971, 1972, and 1973. These data are based 



