IRRIGATION 



253 



Crop irrigation. Dust mulch, dry farming, and generally 

 fall, winter, and spring irrigation are methods for the con- 

 servation and use of water that under the ordinary conditions 

 of farming would go to waste. Irrigation proper is the appli- 

 cation of water to the growing crop at a time when the 

 moisture is actually needed by the plant. There are three 

 methods of true irrigation: surface, subsurface, and spray. 



After Ii ligation Investigations, U\ S. Dept. Agn. 



PlG. 208. Flood irrigation 



Surface irrigation. There are several methods of surface 

 irrigation, as follows: 



Wild or free flooding. Wild or free flooding (Fig. 208) is 

 one of the pioneer forms of irrigation. It is applicable to 

 grains, alfalfa, and grasses and consists in permitting a free 

 flow of water over the entire surface of the ground. It is 

 necessary to have a surface free from knolls or depressions, 

 and there should be sufficient slope to the land to cause the 

 water to move steadily in one direction. This method is the 

 ordinary one for fall, spring, and winter irrigation and can be 

 used on steep lands. The water supply is carried by ditches 

 along the upper side of the field and is permitted to escape at 

 sufficient intervals to cover the land with a sheet of water 

 of uniform depth at short distances from the supply ditch. 



Border method. The border method of flooding (Fig. 209) 

 is employed in the irrigation of grasses and cereals. Ridges 



