THE IRRIGATION OF THE CANE 



109 



Kohala ditch in Hawaii, 14 miles long, 12 feet wide at top, 7I feet wide 

 at bottom, and 4^ feet deep, £83,000. 



Waiahole ditch in Oahu, 14J miles long, with 10 miles of tunnel, 3^ 

 miles of conciete ditch, and i| miles of steel syphon pipe and deUvering 

 80,000,000 gallons daily, /5oo,ooo. 



Two methods of obtaining water are in use : (i) Pumping from sub- 

 terranean sources, and (2) interruption of upland sources and conveyance 

 to the plantations by systems of canals, tunnels, s^'phons and fliunes. Both 

 of these methods are combined \nth systems of reservoirs, whereby an excess 

 flow may be conserved, and where often the night flow from the ditches is 

 also stored. The pmnping plants are located at or near sea level, and it has 

 been found less expensive to elevate the water through long pipe lines than 

 to sink shafts at a high level and to install mining pattern pumps. In igog 



,<it^:z^u=~^ 



it was estimated that the water pumped daily with an average lift of 200 feet 

 was 595,000,000 gallons, of which 360,000,000 gallons were pumped in the 

 Pearl Harbour district in Oahu, 150,000,000 in Central Maui, and the balance 

 mainly in Kauai. Since then the quantity of water pumped has tended to 

 decrease, follo\\-ing on some extension in the ditch systems leading to the 

 mountain areas, which now (1919) deliver some 800,000,000 gallons daily. 

 These ditches, which are mainly concrete-lined so as to prevent seepage, 

 and which in all aggregate several himdred miles in length, have been 

 constructed with great engineering skill ; they have entailed tunnelling^ 

 through mountains and the passage of deep ra\dnes, the system here generally 

 followed being the use of inverted steel syphon pipes reaching to a diameter 

 of eight feet. The largest reservoir built is that at Wahiawa, on the island 

 of Oahu, with a capacity of 2,750,000,000 gallons, the total capacity of all 

 the reserv'oirs approaching 10,000,000,000 gallons. The system of applying 

 water used is always one of furrow irrigation, illustrated in Fig. 20 in plan,. 



