IRRIGATION 245 







Most efficient arrangements are being worked out to 

 meet all the conditions of law, of ownership of water 

 rights and lands, of irregular supply of water, and of 

 seasonal needs of crops under large irrigation canals. 

 By combining storage reservoirs with the regular sum- 

 mer water flow, by arrangements for exchange of rights 

 to water at given times, and by other devices, associa- 

 tions of water users, through their officers, can utilize 

 water to the best possible advantage. The building of 

 reservoirs in which to store up flood water has only 

 begun to utilize the vast resources of this class of waters. 



Surveying and mechanical appliances used in irrigation 

 construction are mainly the same as those used in mak- 

 ing drainage systems, as illustrated on previous pages. 



Irrigation canals must have sufficient fall so that they 

 will carry the required amount of water, but should not 

 have so much fall that the water, in rushing through 

 them, will wash or destroy their banks. The aim is to 

 give a velocity that will prevent the deposit of silt in the 

 main canal and not cause serious erosion. Three feet 

 per second is the usual maximum velocity. The grades 

 of western canals and ditches vary from 6 inches 

 to 50 feet per mile. The more nearly level the grade, 

 the larger must be the cross-section of the ditch. The 

 engineer must make the calculation in each individual 

 case and decide upon the plan which will accomplish the 

 desired results in the best manner and at a minimum 

 expense. In case of aqueducts of wood, stone or metal, 

 where the danger of injury from rapidly flowing water is 

 slight, much is gained by having the grade steep so as to 

 have a larger amount of water flow rapidly through a com- 

 paratively small, and therefore less expensive, aqueduct. 



Wood and iron aqueducts. In many places it is neces- 

 sary to carry the water across low areas. In some cases, 

 aqueducts can be made by building a grade of earth or of 

 masonry, as in Figure 136. In other cases the depth is 



