684 SOILS: PROPERTIES 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 

 in southern California. 



Fig. 74. 



■Four types of rainfall. The diagrams show the distribution 

 by months. 



It has been estimated that the total available water 

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

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

 treatment. 



