RAINFALL AND EVAPORATION 



that a large portion of the earth's sur- 

 face is arid. This term is commonly 

 meant to imply an annual average of less 

 than 20 inches. The arid region thus de- 

 fined would include, in the United States, 

 most of the country lying west of a line 

 drawn through North Dakota and 

 Texas, extending northwest into Canada 

 and southward into Mexico; while in 

 South Africa it would be found in the 

 Kalahari Desert and in some portions of 

 the Transvaal. The different sections of 

 the United States comprise an Arid re- 

 gion,^ with a rainfall of from zero to 20 

 inches ; a Semi-arid region from 20 to 30 

 inches ; and a Humid region of 30 inches 

 and upward. About two fifths of the 

 United States is more or less arid and 

 must be irrigated or cultivated by dry- 

 farm methods. But as Professor Elwood 

 Mead remarks : "If every drop of water 



^ The driest and warmest State is Arizona. 



91 



