DRAINAGE, IRRIGATION, AND DRY FARMING 127 



some parts of the world for hundreds of years. In China, wheat 

 has been grown for many centuries without irrigation under a 

 yearly rainfall of less than 20 inches. In the United States, 

 dry farming was first established in Utah, where it has been 

 successfully practiced for the last fifty years. 



154. Dry-farming areas. With a rainfall of less than 30 inches, 

 especially where the seasons are long and evaporation is high, 

 lack of moisture is usually the factor that determines the yield 

 of crops. Nearly one half of the United States receives less 



FIG. 54. Pumping water for irrigating rice in Japan 



than 20 inches of rainfall annually, and one tenth of the United 

 States receives between 20 and 30 inches. There are three 

 distinct dry-farming areas : first, the Pacific-coast region, rep- 

 resented by the states of California, Oregon, and Washington ; 

 second, the Intermountain region, represented by the states of 

 Idaho, Nevada, Montana, Wyoming, Utah, Colorado, Arizona, 

 and New Mexico ; third, the Great Plains region, represented by 

 the states of North Dakota, South Dakota, Nebraska, Kansas, 

 Oklahoma, and Texas. In the Columbia River basin, with a 

 rainfall of from 10 to 12 inches annually, it is estimated that 



