88 IRRIGATION FARMING. 



Laying Out Reservoirs.— As we have said be- 

 fore, reservoirs should be built on as high ground as 

 possible. Never selec5l a place for a reservoir where 

 the bottom is more than four or five feet below the 

 point of delivery, for all surplus water below this 

 point does no good, and a dam must be built just so 

 much stronger to hold this extra head. The pressure 

 on the dam is no greater where the flowage is large 

 than where it is small. It is the hight of the column 

 of water at the dam that must be figured on. High 

 dams when not properly built are unsafe. Surface 

 is the one thing most desirable in locating a reservoir. 

 Get an idea of the size to be attained before the work 

 is begun, and at the same time make a calculation as 

 to the capacity of the proposed basin when completed. 



The size having been determined, the staking out 

 follows. If the reservoir is to cover a given area, the 

 whole banks will be within the area, and the foot of 

 the outer slope will bound the given area. If the area 

 is to exclude the bank, the foot of the inner slope will 

 bound the area. If the water is to cover a given area, 

 then the high-water line as the point half-way down the 

 bank therefrom will bound the given area, or the area 

 may be bounded by the center line, either of the whole 

 bank or of the top of the bank. These conditions do 

 not of course obtain where the natural sides of a ravine 

 or cation are to form the greater portion of the reser- 

 voir's contour. Usually these considerations will not 

 be of much importance, but in the case of joint owner- 

 ship or of contracting for the constru(5lion they may be 

 important, and should then be clearly understood and 

 carefully specified. 



