292 



THE IRRIGATION AGE 



the present volume of production is concerned. So late even as 1880 

 the output is recorded as being 30,000 tons, while the production 

 in 1889 was 282,807 tons. The part played by artificial irrigation 

 it the production of the Hawaiian crop is seen from the following 

 statement: 



Tons. 



Sugar grown by natural rainfall ., 116,382 



Sugar grown by irrigation 166,425 



The area to which water is artifically applied is yearly increasing, 

 and in two years more than two- thirds of the crop, which is also vastly 

 increasing, will be grown by aid of irrigation. 



The richest lands upon the islands are those lying toward and a 

 little above sea level. In most of the districts, however, the rainfall 

 over the low-lying lands, and especially upon the leeward side, is ut- 

 terly insufficient to produce the sugar crop. Until the practice of 

 irrigation was adopted these lowlands were useless, but now they are, 

 beyond comparison, the richest and most productive. 



The primary source of water upon the Hawaiian Islands is rainfall, 

 Two unfavorable conditions attend its precipitation: (1) The maximum 

 quantity falls during the cool season, when the crops are not in a state 

 of,maximum growth and able to make use of it, and (2) the chief pre- 

 cipitation is over the mountain areas, where the water falls, soaks 

 down into the rock strata, and runs largely to the sea, unless arrested 

 and returned to the land. An illustration of the variation of rainfall 

 with altitude is afforded by the following table: 



VARIATION OF RAINFALL WITH ELEVATION. 



The apparently disadvantageous circumstance of heavy precipi- 

 tation at maximum elevations has been turned into a special advantage 

 by engineering means. In certain districts the water is collected by 

 small ditches over the mountain areas, where it falls, and is conducted 

 by main ditches or by the flumes down to the cane-bearing lands be- 

 low, over which it is distributed by gravity. Where the rainfall can 

 not be easily collected over the mountain areas, the water which sinks 

 down into deep substrata is tapped and arrested at or near sea level, 

 where it is found running toward the sea. In places where the lava 

 rock strata run out before reaching the sea the water comes to the 



