158 2HE IRRIGATION AGL. 



STORAGE OF STORM WATER. 



A good supply of water for irrigation can sometimes be secured 

 by collecting and storing the run-off during storms from lands lying 

 higher than those to be watered. Such a source of supply is obviously 

 less trustworthy in an arid than in a humid region, because of the 

 smaller rainfall, the greater evaporation, and the length of time the 

 impounded water must be exposed to loss from that source. Under 

 such conditions reservoirs simply for storing storm waters are often 

 not worth their cost. In regions having heavy summer showers it may 

 be very different, because a comparatively short time may intervene 

 between the falling of the water and the occasion for its use. Storm 

 water is collected by dammiag a ravine or dry run which carries the 

 water running off from the higher lands, and storing the water either 

 in a reservoir formed by the dam or in a reservoir constructed out of 

 the course of the stream to which the water is conducted through a. 

 ditch heading just above the dam. 



The impounding of water by means of a dam across the mouth of 

 a small ravine or canyon is often feasible, and quite a pond may be 

 secured by a few days' work with plows and scrapers; or a swale 

 through which the stream passes may be scooped out into a reservoir. 

 In such work, however, one must know fully the character of the 

 stream and the area of its watershed, and not undertake to restrain a 

 stream of great flood power, though it be but rarely manifested. Such 

 work has led to great injury to lands and improvements below. 



Aside from such dangers, there are at least two objections to cre- 

 ating a reservoir in the bed of a stream. One is the chance of leak- 

 age; another is the rapid filling of the pond by the sediment carried 

 by the flood water, thus decreasing the capacity of the reservoir. 

 Stopping the mouths of ravines is open to both these objections, and 

 is also disappointing, because in most cases much less storage capacity 

 is secured than is expected, unless the dam be raised quite high, and 

 this multiplies cost and danger with great rapidity. It is seldom de- 

 sirable to enter upon such undertakings without competent engineer- 

 ing advice. For these and other reasons the reservoir for storm 

 water, as for the gradual accumulation of a small flow, should in most 

 cases be located out of the course of the stream. 



SmalJ reservoirs in connection with farm irrigation works are de- 

 sirable from many points of view, and in making use of small runs of 

 water are indispensable. A small stream allowed to flow constantly, 

 no matter from what source, is of almost no use for irrigation, because 

 it will not flow any distance when applied to the ground. But by sav- 

 ing the water in a tank or reservoir a strong stream can be made avail- 

 able for a short time, and will spread over a considerable area. The 

 advantage and the cheapness with which such reservoirs can be se- 

 cured need not be enlarged upon in this connection, for full attention 

 has been paid to them in other publications of this series. J 



1 U. S. Dept. Agr., Farmers' Buls. 40 and 116, and "Yearbook for 1896, p. 187. 



To be continued. 



