88 



ELEMENTS OF AGRICULTURE 



FIG. 47. Annual rainfall in the United States. Areas receiving less than twenty 

 inches must be farmed with special reference to the conservation of moisture, 

 unless irrigated. 



quickly absorbed, and so that evaporation may be checked 

 as far as is possible. Fair crops of wheat have thus been 

 grown every other year with only twelve inches of rain- 

 fall annually. 



Irrigation 



90. Areas Requiring Irrigation. As stated above, two- 

 fifths of the United States is too dry to produce regular 

 crops without irrigation. If all the water that falls in this 

 area were used for irrigation, only about one-tenth of 

 the land could be irrigated. The total area now irrigated 

 is about 10,000,000 acres. This is a little over one-thou- 

 sandth of the arid area. Evidently we must ever have 

 much arid land that is fit only for grazing, and large tracts 

 will always be too dry for any agricultural use. The im- 

 portance of saving all the water is apparent. 



