684 SOILS: PBOPJSRTIJES AND MANAGEMENT 



receives between 20 and 30 inches. Every continental 

 area has its arid portion where the rainfall drops below 

 10 inches. (See Fig. 75.) These sections are usually in 

 the interior, but their position depends on the topography 

 of the land and the direction of the moisture-laden winds. 

 Sometimes, as in the western United States, the coastal 

 mountains cause an arid climate in the adjacent interior 

 valleys, some of which extend quite out to the ocean as 

 in southern California. 



JAM FCB MAR /IPR HAY JUNE 



JULY AUG SEPT OCT NOV DEC 



JflN r£B H^iR m MAY JUNE 



JULY AU5 SEPT OCT NOV DEC 



5AN D 



EGO 



BO 



5E 



i 





JUL 



JL 





I 



I 



I 





TUCSON 



DENVER 



Fig. 74.— Four types of rainfall. The diagrams show the distribution 



by months. 



It has been estimated that the total available water 

 supply is suflBicient to irrigate only one-tenth to one-fifth 

 of the proportion of the earth's surface in need of such 

 treatment. 



