COMMERCIAL OPPORTUNITIES 



31 



in the case of most crops, the weekly amount of precipitation 

 throughout the growing season. Under average conditions, it is 

 safe to say that a drought occurs whenever the rainfall totals less 

 than one inch in any fifteen-day period and crops will usually 

 suffer if they do not receive more than this amount of rain, espe- 

 cially during the spring and early summer months. 



Prof. F. H. King in his book on irrigation and drainage fur- 

 nishes the following data as to the highest probable duty of water 

 per acre for different yields of different crops: 



TABLE XII 

 DUTY OF WATER FOR DIFFERENT CROPS 



Some artificial means of supplying water to the land is there- 

 fore a necessity in the western section of the country, and would 

 be excellent insurance to the central and eastern parts as well. 



Two general methods of supplying this water are now in use: 

 The ordinary gravity flow, such as that of taking water from a 

 reservoir or ditch; and the mechanical lift, such as pumping water 

 from a well, pond, river or lake. Of the two, the development 

 of the mechanical lift has been far more rapid. There are two 

 reasons for this: First, because the land which can be econom- 

 ically irrigated by the gravity method has been practically all 

 taken up; and, second, because the fanner can pump water to 

 almost any elevation, and in this way he is enabled to irrigate 

 land which is above his source of water supply. This is impos- 

 sible when the gravity system is used. 



