6 



THE IRRIGATION AGE. 



reached by gravity flow unaided; but this 

 is seldom done, as water supplies of the 

 above character are generally valuable on 

 lands lying below their level. 



Pumping plants for irrigation purposes 

 are profitably employed in raising water 

 from wells only, and in other cases are 

 very rare and exceptional, being used only 

 where a small quantity of water is required 

 for land above a regular gravity system, 

 or where hydraulic power is available as a 

 motive power. 



RELATIVE VALUE OF DIFFERENT GRAVITY SYS- 

 TEMS OF WATER SUPPLY. 



No inflexible statement can be made 

 saying that any one of the sources of 

 gravity supply named above is the best, 

 nor is it possible to arrange them in the 

 order in which they are to be preferred 

 without the addition of some qualifying 

 expression. It may be said, however, that 

 where a supply can be obtained by simply 

 diverting the water from a running stream, 

 which contains a sufficient supply of unap- 

 propriated water, this is the wisest thing 

 to be -done rather than to search for a source 

 of supply, which will require considerable 

 money for its development. Still this 

 phase of the question is of importance to 

 the original projectors of an enterprise 

 only, as it does not signify that the water 

 is any better or more valuable to the irri- 

 gators, who will subsequently own and use 

 it, than if it had been originally obtained 

 from a more costly source. The value of 

 a water right depends upon its infallibility 

 quite as much or more than upon its 

 cheapness in first cost, and it is better 

 to have water from an undisputed and un- 

 failing source, which it has cost consider- 

 able to develop, than to overappropriate 

 natural streams and fall heir to the sub- 

 sequent evils of litigations and partial fail- 

 ure of the supply. 



From what has already been adduced in 

 the preceding paragraph we can readily 

 conclude that the flow of a natural stream 

 is a very desirable source of water supply 

 for a system of irrigation works. This is 

 certainly true beyond any question or 

 doubt, providing always that the flow of 

 the stream, where the same is to be 

 diverted, is ample to give the required 

 amount of water. After having investi- 

 gated and finally settled upon the amount 

 of water requisite to make a projected sys- 

 tem adequate for the area to be irrigated, 



it therefore becomes equally necessary to 

 study the stream and determine with the 

 same degree of thoroughness what its flow 

 has been for a number of years past and 

 what we may safely conclude will be its 

 discharge in the future. In order to do 

 so a familiarity with streams and the laws 

 governing them in general must be one of 

 the accomplishments of the irrigation 

 engineer, and in the succeeding paragraphs 

 we will therefore briefly discuss the na- 

 tural philosophy of flowing streams. 



T'HE ORIGIN OF STREAMS. 



Certain conditions of physical geogra- 

 phy are necessary to the existence of flow- 

 ing streams of water in a country. The 

 land must consist of mountains, hills, val- 

 leys, plains, etc, combined in such a man- 

 ner as to cause differences in elevation and 

 inclination to the surface. The extent of 

 any region having a range of elevations 

 from the highest mountain to the lowest 

 plain or valley, all falling in the same 

 direction or in different directions, which 

 ultimately unite into one valley or basin, is 

 one of the most important conditions de- 

 termining the length and size of streams. 

 Other important conditions are of a mete- 

 orological nature and relate to the evapora- 

 tion and condensation of water and the 

 movement and temperature of atmospheric 

 currents. Water readily changes from its 

 liquid to its vapory or gaseous state 

 through subjection to heat. Heat applied 

 to a drop of water whether it is exposed 

 to the rays of the sun or to an artificial 

 heat soon causes it to disappear into the 

 surrounding air. This is due to the ex- 

 pansion of the liquid and its consequent 

 conversion into a vapory substance lighter 

 than air, which readily mingles with the 

 atmosphere, being controlled and moved 

 by it, until by condensation it again be- 

 comes heavier than the air and descends to 

 the earth. The heat of the sun is con- 

 stantly exerted to expand water exposed 

 to its rays, and its power causes evapora- 

 tion from the surfaces of bodies of water 

 and from the water contained in organic 

 and inorganic substances in the form of 

 moisture. 



The vapors thus formed fill the atmos- 

 phere and move with the prevailing winds 

 until they reach higher altitudes, where 

 the colder air condenses them, or until a 

 cold current of air meets a warmer one, 

 which is heavily laden with vapor, and 



