WATER SUPPLY. 37 



pire it into the air in very large quantities. That which 

 is not evaporated from the earth's surface sooner or later, 

 or transpired by plants, is gathered into the rivers ; 

 we call that which ultimately flows out to sea the "run- 

 off" water; and that which is evaporated and which 

 drifts away in the air we call the " fly-off " water. These 

 are two very common, simple terms. In calculating the 

 requirements of modern irrigation, the best authorities 

 hold that the water supply for a given acre should be 

 sufficient to cover it twenty-one inches deep during the 

 course of an irrigating season of 100 days. Some experts 

 place the maximum as high as twenty-four inches, which 

 is an estimate that is certainly liberal enough. 



The Surface Supply. We are safe in claiming 

 four distinctive sources of water supply, which may in 

 turn be divided into two classes. These are the streams, 

 natural lakes and reservoirs, underflow or phreatic waters, 

 and the deep subterranean or artesian basins. Of these 

 the most practicable and available are the living waters of 

 the natural streams. In the older irrigated states, where 

 the legislators have framed laws for the appropriation of 

 running waters, the control thereof is usually placed with 

 an executive officer, generally called the State Engineer, 

 who virtually has under his charge and supervision the 

 control of the running waters. He gauges the streams, 

 keeps a record of their flow, and doles out the canal rights 

 in accordance with the statutes. First come first served, 

 is the rule, and ditch charters which are granted by him 

 are issued in consecutive numerical order, until the full 

 carrying capacity of the stream is allotted, when further 

 issuance of charters ceases. 



In the most successful irrigating water courses taken 

 from the perennial streams, the headworks are almost 

 invariably located well up on the river, to command suffi- 

 cient level in order, if possible, to tap the stream where the 

 water is clear and not laden with silt. By thus locating 



