IRRIGATION AND DRY-FARMING 



683 



amount. Irrigation is practiced in all parts of the world 

 — in those regions where the rainfall is 50 and 60 inches 

 a year, as well as in those regions where it is only 20 inches 

 or less. (See Fig. 73.) The need of irrigation is de- 

 termined by (1) the time when the rainfall occurs, 

 (2) the way in which it occurs, whether in small or 



Fig. 73. — Diagram showing the extent and distribution of rainfall in an 

 arid region (Yuma, Arizona), and a humid region (Buffalo, New 

 York). 



large quantities, (3) the nature of the soil, (4) the 

 air temperature and wind movement, and (5) the nature 

 and value of the crops grown. Other factors, such 

 as the cost of applying water, methods of tillage, 

 and market facilities, have some influence in deter- 

 mining the practicability of irrigation. Irrigation is 

 usually associated with a low rainfall of 20 or 25 inches 

 a year. Using these figures as a measure of the need 

 of irrigation throughout the world, it appears that about 

 60 per cent of the earth's surface has so low a rainfall that 

 irrigation is necessary in order to secure paying yields of 

 crops. About 25 per cent of the earth's surface receives 

 10 inches or less of rainfall annually. About 30 per cent 

 receives between 10 and 20 inches, and about 10 per cent 



