FARM DRAINAGE AND IRRIGATION 771 



time, less than four per cent of this mileage was lined or otherwise made 

 impervious to water. A limited amount of irrigation water is conveyed 

 through pipe lines of different types, of which wood, terra-cotta and 

 cement predominate. It is important to construct the irrigation ditch 

 of the proper size to convey the maximum amount of water that will be 

 available or the maximum that can be used by those who irrigate. In 

 this connection it is advised to secure the services of an engineer. It 

 should be understood that the amount of water conveyed depends on the 

 cross section of the canal and the rate of movement of the water. In a 

 small ditch capable of carrying 50 miner's inches, a fall of 2 inches to the 

 rod will give a velocity of 2 feet per second. In a ditch carrying 20 times 

 as much water, a fall of \ inch to a rod will give an equal velocity. Except 

 in hard clay or a mixture of gravel and clay, a velocity greater than 3 

 feet per second is likely to cause serious erosion. A velocity of 2 to 2\ 

 feet is the maximum that should be permitted for ordinary sandy loams 

 or loams. Where the fall of the land is such as to cause a greater velocity 

 of the water, checks in the canals should be provided. These may be 

 wooden dams or obstructions of cobblestones, causing a drop in the water. 



In lined canals erosion is overcome and the velocity of the water 

 may be much greater. Where there is ample fall, such a canal may be 

 much smaller than an ordinary earth canal. The transmission of water 

 through pipes has a still greater advantage in this respect and may be 

 conducted down very steep grades. 



Losses in Transmission. — Much water diverted from streams for 

 irrigation is lost from the ditches by seepage and evaporation, and is 

 still further wasted by over-irrigation and by allowing the water to pene- 

 trate the soil beyond the reach of crops. Water lost in these ways often 

 causes serious damage to the lower lying land in the irrigation district. 

 Numerous water measurements and experiments have led to a conserva- 

 tive estimate that not more than 35 per cent of the water diverted from 

 streams is effective in plant production. 



The efficiency of irrigation water can be greatly increased by the 

 substitution of pipe lines for open ditches and by greater care in the 

 distribution of water in the fields. 



Head Gates. — Head gates are necessary at the point of diversion 

 from a stream into the main irrigation canal, and also at points along 

 the main canal at the juncture of laterals. Such gates are usually con- 

 structed of plank with a gate that slides up and down to control the 

 volume of water. A simple form is shown in the accompanying illustration. 



Preparing Land for Irrigation. — The preparation of the land consists 

 in clearing it of the native vegetation, which in the arid region is usually 

 sage-brush, rabbit-bush, cacti and native grasses. Plowing frequently 

 precedes the clearing operation. This makes easy the gathering and 

 burning of the vegetation. The plowing and clearing should be followed 

 by a thorough harrowing, grading and smoothing of the surface. The 



