70 IRRIGATION FARMING. 



The Sweetwater dam, built across the mouth of a 

 canon a short distance above Xational City, California, 

 and shown in the full-page photographic reproduction, 

 Figure 18, is one of the boldest pieces of engineering in 

 the world. . The dam is constructed as a crown arch, 

 and is the largest of its character in the world. It is of 

 solid granite and Portland cement, 46 feet thick at the 



and 12 at the top. It is 90 feet high at bed rock, 

 76 feet long at the base and 396 feet at the top. The 

 reservoir covers 700 acres, and has the enormous storage 

 capacity of six billions of gallons. The water is dis- 

 charged from the reservoir by means of a main pipe 36 

 inches in diameter, and then by smaller pipes. Much 

 of the land under this system is high and rolling, but 

 the head is sufficient to carry the water to the highest 

 portions. The dam gathers the rainfall from 186 square 

 miles, and the capacity of the reservoir is sufficient to 

 hold a two years' supply for 10,000 acres. 



Cost and Capacity. In calculating the cost of a 

 reservoir it is necessary to fix the value of a defined vol- 

 ume of water for irrigation purposes. For convenience 

 take a volume of water of one million cubic feet. If this 

 amount is applied without loss in transportation, as 

 through pipes, it will irrigate on an average twenty aciv^ 

 of land for one season when used very carefully. As- 

 suming one-half of this area to allow for unavoidable 

 loss by absorption, evaporation and the loss by the ordi- 

 nary practice of irrigation, the area value of one million 

 cubic feet of water will be considered as ten acres. On 

 1 liis basis of ten acres for one million cubic feet, the cost of 

 a n-st'rvoir built entirely on the surface of nearly U-vel 

 ground from excavation made within the area enclosed, 

 without borrowing or wasting material, has been calcu- 

 lated. The price assumed for earthwork is twenty 

 cents a cubic yard, which for banks twenty feet hii:h, 

 with a long haul, is a fair price. The size calrula! 



