57 



Comparing the above prices with those for reinforced concrete pipe it is 

 seen that there is no great difference in first cost, but if ultimate cost is 

 considered, the economy is in favor of reinforced concrete pipe. 



However, the use of reinforced concrete pipe is limited to moderate 

 heads, under 100 or 150 feet, and they require careful workmanship and 

 an expensive manufacturing plant. Wooden stave pipes have* the advan- 

 tage that they can be used for higher heads and that they are easily ana 

 quickly put together. 



With concrete pipes it is very necessary to prevent the accumulation o.f 

 air in the pipe to avoid water hammer. This requires that air valves of 

 ample capacity be placed wherever air will collect in the pipe. 



IV.--DUTY OF WATER AND FACTORS INFLUENC- 

 ING THE CORRECT USE OF WATER 

 IN IRRIGATION. 



DUTY OF WATER. 



A knowledge of the quantity of water used in irrigation and of the 

 factors influencing the correct use of water in irrigation is necessary if 

 the best use of the available water supply of British Columbia is to be 

 obtained. The term duty of water is used to express the relation between 

 the area of land served and the quantity of water used. It may be further 

 qualified by using the expressions gross duty, duty measured at heads of 

 laterals, and net duty. 



The gross duty of water represents the relation between the quantity of 

 water diverted from the source of supply and the total area of land irri- 

 gated. It is obtained from measurements of the flow taken at the head of 

 the main canal of the irrigation system and includes besides the quantity 

 applied to the land, all losses and waste in conveyance. 



The duty of water measured at the head of the lateral is higher than 

 the gross duty because the losses obtained from the head of the main canai 

 to the head of the lateral are not included. 



The net duty of water represents the water delivered to the field as 

 obtained by measurements taken at the margin of the field. It includes 

 besides the quantities used by the plants, the losses by evaporation, perco- 

 lation and waste occurring on the field, which can be controlled to a large 

 extent by a skillful irrigator. 



The duty of water is spoken of as high when the area irrigated by a cer- 

 tain volume of water is comparatively large, and as low when the area is 

 comparatively small. The gross duty is lower than the net duty, tne differ- 

 ence depending on the efficiency of the main canal and distributaries. If 

 all losses of conveyance were eliminated, the gross duty and net duty would 

 be equal. A knowledge of the gross duty is necessary for the engineer to 

 plan the irrigation system in order to make allowances for the conveyance 

 loss. It is also of value in determining the efficiency of a system. 



The net duty must be distinguished from the correct amount of water 

 required for maximum production, for it merely represents the volume of 

 water which is used according to the available water and the judgment and 

 skill of the irrigator. In most cases where water is abundant, the quan- 

 tity used is in excess of what is actually needed, while where water is very 

 scarce, it may be less than the correct amount. The correct amount of 

 water to use is that quantity which is necessary to produce maximum yield 



