USE OF THE RAINFALL 235 



yields under irrigation are greatly reduced. As the rain- 

 fall is more carefully conserved, the area that may be 

 served by the available irrigation water will be greatly 

 increased. (Fig. 57.) 



Wheaf Oofs Corn Lucern Potatoes 



FIG. 57. Yield of crops due to rainfall. Shaded areas, yields with irrigation; black 

 areas, yields without irrigation. 



144. Conserving the rainfall. The high crop-produ- 

 cing value of the rainfall in irrigation practice makes it 

 important to understand the best methods whereby the 

 natural precipitation may be conserved. (1) The top 

 soil must be kept in a loose condition, so that the water 

 may enter the soil easily and completely as it falls from 

 the heavens. (2) The soil must be so treated by thorough 

 cultivation that the water which enters the soil will be 

 kept there until needed by the plant. These are the two 

 chief considerations of the irrigation farmer who desires to 

 get the greatest possible returns from the rainfall. 



145. Distribution of rainfall. The methods to be 



