WATER SUPPLIES FOR IRRIGATION.* 



BY F. C. FINKLE, C. E. 



THE first duty of an irrigation engineer 

 who is intrusted with the work of 

 designing an irrigation system for a tract 

 of land requiring irrigation is the exami- 

 nation of the proposed water supply, if 

 one has already been proposed, or an ex- 

 amination for the purpose of finding an 

 adequate and reliable water supply, if this 

 point still remains unsettled at the time of 

 his taking charge. 



The choosing of a water supply for an 

 irrigation system is a matter which requires 

 the greatest skill and care, as upon it de- ' 

 pends the success or failure of a system, 

 no matter how Well and carefully all other 

 things may be provided for in the con- 

 struction of the plant. Mistakes in the lo- 

 cation of waterways and conduits and 

 errors in designing structures, while they 

 are sometimes serious on a'ccount of the 

 expense necessarily incurred in correcting * 

 them, are not per se a complete cause for 

 the absolute failure of an irrigation sys- 

 tem in which they occur. But a real mis- 

 take in the choice of a supply of water for 

 a system is invariably a sufficient cause for 

 the total failure of the enterprise. There 

 are of course exceptions to this rule in a 

 few cases, where, in the event of the fail- 

 ure partially or totally of the water sup- 

 ply already planned, another supply can 

 be obtained by adding to and extending 

 the works already constructed. But these 

 exceptions are so rare that a mistake in 

 the choice of a water supply can be said 

 to be fatal to the success of an irrigation 

 enterprise, and the greatest care should al- 

 ways be exercised by the person having 

 these things in charge to avoid anything 

 in the line of a water supply that partakes 

 of the doubtful. 



The total quantity of water required is 

 the first thing to be accurately determined. 

 As the amount of water to be carried de- 

 termines the size and character of the 

 conduits and other necessary works, it is 

 the first thing we must have finally settled 

 before beginning the preparation of plans 

 and specifications or the preliminaries of 

 construction. 



The amount of water needed at the head 



*A11 rights reserved by the author. 



4 



of the canal or other conduit is really the 

 point to be kept in sight, as there is gen- 

 erally a considerable loss in carrying the 

 water. After the quantity of water re- 

 quired to be delivered at the land to be 

 irrigated has been settled it is necessary 

 to determine how much will be the loss in 

 transmission from the point where the 

 water is taken into the conduit to the 

 point where it is to be applied to the land. 

 A survey of the line a preliminary or re- 

 connoissance survey is usually sufficient 

 should be made to determine as nearly as 

 possible the character of conduit to be 

 adopted. When this has been done the loss 

 of water by percolation, evaporation or 

 from other causes can be determined and 

 added to the supply which it has been de- 

 cided it will be sufficient to deliver for the 

 proper irrigation of the lands to be served 

 by the proposed system. 



The amount of water required for an 

 acre or other unit of land measure is 

 called the duty of water. The duty of 

 water necessary for the tract to be irrigated 

 must be carefully determined before the 

 needed amount can be stated, and this 

 should be done before the loss in trans- 

 mission is calculated. The duty of water 

 is in itself a very comprehensive subject 

 and, as such, will be discussed in a succeed- 

 ing chapter especially devoted to that sub- 

 ject, and the rules governing the loss of 

 water in transmission will also be carefully 

 discussed hereafter in the chapters dealing 

 with the different kinds of conduits. 



Another question the solution of which 

 requires careful study is the season of the 

 year at which irrigation is necessary. The 

 problem of a water supply must be care- 

 fully studied in connection with this ques- 

 tion. 



The irrigation season in different locali- 

 ties varies as to the time of the year at 

 which it occurs. As a general proposition 

 it may be stated that the irrigation season 

 occurs when crops are growing and matur- 

 ing, but this does not fix any definite time 

 for all localities, as the crop season fol- 

 lows the climate and occurs in different 

 latitudes and longitudes at varying times 

 of the year. The location of a region with 



