216 SUCCESSFUL FARMING 



in irrigating. Second, it overcomes the loss of water by seepage and 

 evaporation which takes place when he has a constant small stream. 



Sources of Water. The chief sources of irrigation water are peren- 

 nial streams, springs and wells. The first named is by far the most 

 important. The first consideration in the development of an irrigation 

 supply from a stream is the volume of water carried at all times during 

 the year; and second, whether or not the water can be brought to the 

 land to be irrigated at a reasonable expense. This will depend prin- 

 cipally upon the length of ditch to be constructed and the character of 

 land that must be traversed by it. In some cases, pipe lines may take 

 the place of ditches without great additional expense and with much less 

 waste of water. 



The larger the ditch and the more porous the soil through which it 

 passes, the smaller should be the fall. If, however, the grade is too 

 small, the ditch must be larger in order to carry the supply of water. In 

 ordinary soils, a grade of one foot in 600 feet may be given. In clay 

 soils, it may be increased to two feet in 600 feet. A slow movement of 

 water in the ditch prevents scouring and encourages the settlement of 

 fine sediment. This ultimately forms an impervious lining and prevents 

 seepage. 



Springs offer an excellent irrigation water supply, and although the 

 volume is much less than that from perennial streams, it is subject to less 

 fluctuation in volume and is consequently more dependable. 



Wells form a considerable source of irrigation water supply in many 

 of the irrigation districts. They are virtually artificial springs secured 

 by boring deep wells provided with iron casings. In some instances, as 

 in case of wells that do not flow, and in elevating water from lakes and 

 streams to land lying above the water level, pumping is resorted to. 



Dams and Reservoirs. Perennial streams are subject to great 

 fluctuation, due to periodic rains and melting snow. Their direct diver- 

 sion for irrigation purposes, therefore, fails to utilize much of the water 

 during high stages. This has led to methods of storing the water to be 

 used as needed, thus increasing the area irrigated. While dams are neces- 

 sary for diverting water from streams into canals, much larger and more 

 expensive ones are required in the building of reservoirs. It is important 

 to select the dam site with a view of securing the largest possible water 

 storage capacity with the minimum expenditure for construction. Such 

 sites are most usually found in the upper courses of a stream where it- 

 passes through a narrows or canyon. Rocky, impervious abutments to 

 which to connect the dam are essential. On large projects the reinforced 

 masonry or concrete dam that will be permanent is advised. The deeper 

 the water in a storage reservoir the less will be the relative loss by 

 evaporation. 



Methods of Transmission. The census of 1910 gave an aggregate 

 of over 125,000 miles of irrigated ditches in the United States. At that 



