LEVELS AND TEMPERATURE OF LAKE OKOBOJI 



The inflow at the head of the lake, and the outflow over the 

 dam were not gauged, but by inspection they were judged fair- 

 ly to compensate each other. 



The main changes in level were due to evaporation, to precipi- 

 tation and to strong winds. For each continuous period of 

 evaporation without strong wind there was a steady drop in 

 the level of the lake of from .1 to .3 inch per day. A similar 

 effect of evaporation was detected when from the height marked 

 by the gauge the rise due to precipitation was subtracted. The 

 records of evaporation and of precipitation were obtained from 

 a glass battery jar about eight inches in diameter and eight inches 

 high placed over the lake and about a foot above it. 



The rise due to precipitation was very evident, at one time 

 carrying the pointer above the cylinder. (The rise due to pre- 

 cipitation may be seen in the records for July 6, 11, 15, 19, 26 

 and 30. Apparently friction slightly interfered with the free- 

 dom of motion of the pen the first week.) 



The total drainage area of the lakes "West and East Okoboji 

 estimated from the county map of the Iowa Geological Survey 

 is fifty-five square miles; the area of the lake itself eight and 

 four-tenths square miles. One inch of rainfall over the drainage 

 area would raise the level of the lake 6.55 inches if all of the 

 precipitation were to reach the lake. Evidently much of the 

 precipitation would soak into the ground and later be evaporated 

 without reaching the lake at all. Precipitation is generally 

 unevenly distributed over the area in thunder storms, and the 

 immediate effect on the level interfered with by the wind. In 

 one instance precipitation of 1.2 inches at the point of obser- 

 vation was actually accompanied by a fall in the level of the 

 lake at that point. 



Rise and fall due to the wind was not so great as was ex- 

 pected, for the crests of the waves under strong wind pressure 

 present a deceiving appearance. Apparently the differences in 

 level due to the wind are quickly relieved by a general com- 

 pensating movement in the lake. In general a strong wind 

 from the southwest, west and northwest causes a slight fall 

 in the level of the surface of the lake at the laboratory, while 

 a strong northeast, east and southeast wind causes a corre- 

 sponding rise in the surface at the laboratory. Effects of the 

 wind in lowering the level of the lake at the laboratory may 



