A LIMNOLOGICAL RECONNAISSANCE 27 



wind on the same assumption. The areas may be measured 

 with a planimeter, with the following result : 



TABLE 17— CONTRIBUTION OF SUN AND WIND TO DISTRIB- 

 UTING HEAT ON THE ASSUMPTION THAT NO 

 SUN-PLACED HEAT IS LOST 



Depth 



Total 



m.-5 m. 



5 m.-lO m. 



m.-l m. 



1 m.-5 m. 

 Below 1 m. 

 Below 5 m. 



It thus appears that the maximum possible contribution 

 of the sun to the distribution of heat is about 20^. Of this 

 about 95 ^f is in the upper 5 m. of the lake. The sun con- 

 tributed only about 2.6^ to the distribution of heat below 

 five meters, and practically nothing below ten meters. 



The actual contribution of the sun is less than this. If 

 we assume that the percentage of sun-placed heat lost is the 

 same as the average loss, the figure of 20% will be reduced 

 to 7% -8%, since only about one-third of the sun's radia- 

 tion is absorbed by the lake. This is certainly too low an 

 estimate of the sun as heat placed at depths of 2 m. or 3 m., 

 or even at 1 m. would be lost far less easily than that in the 

 upper centimeters of the water. Perhaps no better estimate 

 can be made at present than to assume that the contribu- 

 tions of the sun below 1 m. is a fair amount and that direct 

 insolation is responsible for 10% — 12% of the work of dis- 

 tributing heat and that the wind furnishes 88% — 90%. 



With such estimates we must leave the subject at present. 

 Future studies will be able to furnish greater accuracy, but 

 no result is likely to show that the wind furnishes much 

 below 80% or much above 90% of the work needed in the 

 distribution of the summer heat-income through the water 

 of lake Okoboji. 



Ill— OTHER TEMPERATURE OBSERVATIONS ON 

 OKOBOJI LAKE 



The point has been emphasized that the small volume of 

 the lower water of Okoboji lake gives rise to exceptionally 



