THE PKAIEIES 



205 



The relative rate of evaporation ou tlie opposite sides of the 

 valley is shown in the following summary: 



Piche 

 Evaporimeter 



Cup 

 Evaporimeter 



Date 



Time 



Council 

 Bluffs 



Omaha 



Council 

 Bluffs 



Omaha 



The direction of the wind on the same dates was as follows : 



August 17 Xorth and northeast. 



August 30 South and southeast. 



September 6 Xorth, northeast and northwest. 



September 13 Southeast. 



The greater relative evaporation on the west side on Septem- 

 ber 13th was evidently due to the fact that the station was ex- 

 posed to the southeast wind during the entire day, whereas the 

 Council Bluffs station was sheltered. 



The apparent great discrepancy between the Piche and cup 

 evaporimeters on September 6th was evidently due to the cir- 

 cumstance that at the Omaha station the Piche evaporimeter was 

 exposed to the sun while the cup evaporimeter was in the shade 

 during most of the afternoon. 



The observations already noted which were made at Ute by I\Ir 

 Da^id H. Boot, and those made by the writer in the vicinity of 

 the Lakeside Laboratory- at Lake Okoboji in the summer of 1909. 

 also confirm the general conclusions based on the observations 

 made at Missouri Valley. 



It is now necessary to consider the application of these results 

 to our problem. The prairie areas are uniformlj' so situated that 

 they are fully exposed to the factors which cause rapid evapora- 

 tion, namely the sun and the wind. During much of the year 

 they may present conditions quite favorable to plant growth, but 

 there are seasons and there are portions of the year, especially in 

 mid-summer, when evaporation and consequent dessication be- 

 come so extreme that only those plants which are especially 



