22 DRY FARMING IN RELATION TO RAINFALL AND EVAPORATION. 



would be required in order to be equivalent to 15 inches of rain- 

 fall along the Canadian boundary. These lines of equivalent rainfall 

 have been established from a consideration of the distribution of the 

 short grass through the Great Plains and from a study of the evapo- 



Fio. 5.— Outline map of the States in the Great Plains region, showing the lines of equal rainfall and 

 lines of equivalent rainfall. The solid lines marked 15 and 20 pass through points of equal rainfall; 

 the corresponding dotted lines pass through points having a rainfall which is equivalent to 15 and 

 20 inches, respectively, on the Canadian boundary. 



ration. Their position should be considered as only approximate, 

 but they serve to emphasize the great influence of the increased 

 evaporation upon the amount of water required for crop production. 



188 



