THE IRRIGATION AGE. 



67 



catchment from outcroppings of water-bearing strata; tunneling to 

 intercept such strata when deeply covered; flowing wells; pumping 

 from wells, lakes, or streams, and storage of storm water from surface 

 flow or from drainage systems. 



DIVERSION FROM STREAMS. 



Diversion from perennial streams is the most common method of 

 securing irrigation water, and it is available for either great or small 

 undertakings wherever unappropriated water flows in sufficient 

 amount. In the regions where irrigation is most widely practiced 

 there may no longer be such supplies available, but in the newer parts 

 of the arid region and quite widely in the humid regions, farms are so 

 situated that stream water can be readily secured. 



After assuring himself in the manner described that a nearby 

 stream carries sufficient water for his purpose, the next step for the 







FIG. 3. Diverting a small stream, showing brush dam and headgate. 



farmer is to determine whether the water can be brought to his land 

 at a reasonable expense. This will depend principally upon the length 

 of the ditch which must be constructed. The simplest way to find out 

 how long the ditch must be is to run a line having the necessary grade 

 from the highest point of the land to be irrigated upstream till it 

 strikes the stream. The grade on which the ditch should be built, 

 and consequently npon which this preliminary line should be run, will 

 depend upon the quantity of water to be carried and the nature of the 

 soil over which the ditch will run. In general, the larger the ditch 

 and the lighter the soil the smaller the fall which can be given to the 

 ditch; and the shorter the ditch the smaller the fall. However, the 

 grade should not be too light, for the ditch may have to be made lar- 

 ger to carry the desired supply of water. On the other hand, the 

 grade can not be very heavy or the strong current will wear away the 



