WATER SUPPLIES FOR IRRIGATION. 



CHAPTER IV. THE DEVELOPMENT OF UNDERFLOWS. 



BY F. C. FINKLE, C. E. 



THE term underflow is often applied to 

 any water below the surface of the 

 ground. In this way it is sometimes em- 

 ployed to designate both artesian and 

 ordinary sub-soil water. Such an appli- 

 cation of the term is decidedly improper 

 and should be discouraged as far as pos- 

 sible. As the term itself expresses, it 

 means water which is both under the sur- 

 face and flowing. It can, therefore, 

 neither mean ordinary sub-soil water, 

 which is standing water merely filling the 

 voids in the sub-soil, nor artesian water, 

 which is confined under pressure in under- . 

 ground reservoirs and channels, and flows 

 only when the impervious layer confining 

 it is perforated by artificial means. 



Since underflow water does not exist on 

 the surface it cannot be taken by simple 

 diversion in the same manner as the sur- 

 face flow of streams. When its utilization 

 is contemplated for irrigation purposes 

 the first step to be taken after its existence 

 has been determined is to bring it to the 

 surface. This is termed developing it. 

 Before the mode of development is fixed 

 upon there are several things which should 

 be carefully determined. The most im- 

 portant of these are: 



(1) Point where the water is to be, or 

 can be used for irrigation. 



(2) Points where development is prac- 

 ticable. 



(3) The probable volume of underflow. 

 If no tract of land requiring irrigation 



is found to exist sufficiently near and be- 

 low the bed of a stream possessing an 

 underflow its development for irrigation 

 purposes will of course be a useless under- 

 taking. While it is a rare occurrence, in- 

 deed, to find such a case in any arid 

 region of the earth, yet it may occur and 

 sometimes does. 



Rivers of slight inclination and high 

 banks are often encountered, and in such 

 cases territory which can be irrigated 

 from them is difficult to reach by means 

 of a gravity system. In doubtful cases 



the only way to determine such questions 

 as the existence or extent of a body of land 

 which can be irrigated from the underflow 

 of a stream, and the cost of conveying the 

 water, is by making surveys. Frequently 

 no surveys are necessary for determining 

 the point where the water can be used, as 

 irrigable land exists in abundance and 

 the fall of the stream and surrounding 

 country is much greater than necessary. 



After a tract of land susceptible of be- 

 ing benefited by the water in a degree 

 which will insure the undertaking to be 

 profitable has been located, a suitable 

 place for the development of the under- 

 flow must be sought. Such a place must, 

 of course, be selected at an elevation suf- 

 ficiently higher than the land to be 

 served to render the conducting of the 

 water to it possible by gravity flow. 



The point for developing an underflow 

 should be as low down on a stream as it is 

 possible to find one, in order to derive 

 benefit from as large an area of watershed 

 as practicable. The narrower the canyon 

 of the stream the more easy it will be to 

 develop the underflow by any of the 

 methods which can be employed. A dam, 

 tunnel or cut will be more cheaply con- 

 structed across a narrow canyon than 

 across a wide one. A place where the 

 depth from the surface down to bed rock 

 or to the impermeable stratum underlying 

 the underflow is shallow is always a desir- 

 able point for making the development. 

 Shallowness to the bottom of the under- 

 flow is even of more importance than a 

 narrow channel. 



But both are of much importance and 

 should be combined in as large a degree 

 as possible in seeking a favorable place 

 for the development of an underflow. 



If the proposed development is to be made 

 by a cut, tunnel or submerged masonry dam, 

 a point on the stream where the grade is 

 rapid should be selected. This is im- 

 portant as a factor in reducing the cost of 

 the proposed works to a minimum. 



