THE IRR IGA TION AGE. 291 



gone so far that the water is destructive to vegetable life. In most 

 instances the deleterious agent is common salt; in others there is a 

 mixture of common salt with chlorids of magnesium and calcium. The 

 latter are most injurious to plant life and, in lowlands, lying almost 

 level with the sea, where there are no means of getting these salts 

 removed, their impregnation renders the soil useless. 



A considerable portion of the water supyly for irrigatron in the 

 Hawaiian Islands is derived from the underground flow. Ground 

 waters, on account of the considerable proportions of certain highly 

 desirable elements they contain, may be very valuable for application 

 to crops. On the other hand, because of the large amount of sub- 

 stances inimical to plant life held in solution in some cases, they may 

 be quite unfit for irrigation. Numerous instances of the unfitness 'of 

 such waters for plant use are furnished by other countries, and special 

 examples have been found by the writer upon the Hawaiian Islands. 



DUTY OF WATER. 



By the term "duty of water," as used in this bulletin, is under- 

 stood the volume of water that is required to mature a given crop in 

 given conditions of soil and climate. That the duty of water can not 

 be a definite factor, the water being in equal demand and rendering 

 the same service in all locations, has been amply indicated by the facts 

 stated in preceding paragraphs It has been shown that there are 

 locations where the volume of water directly evaporated from the soil 

 is double the amount removed in other locations and under totally 

 different conditions of climatic exposure and action. Further, it was 

 shown that soils themselves vary extremely in their powers to take up 

 and retain moisture, which affords another illustration of the factors 

 that determine the service of applied water in relation to the crop. If 

 a given volume of water is applied to a soil of low absorptive capacity 

 and with a small retentive power, loss occurs by seepage on the one 

 hand and -by extreme evaporation on the other, thus causing a large 

 expenditure by the soil and a minimized service rendered to the crop. 

 Again, crops may vary between very wide extremes in the volume of 

 water they consume per unit of substance formed, and consequently 

 in the volumes necessary to bring them to maturity. 



IRRIGATION PRACTICE ON THE HAWAIIAN ISLANDS. 



The chief crops that are grown by the aid of artificial irrigation in 

 Hawaii are rice and sugar cane. 



The lands used for rice are the lowest flats found at the outlets of 

 valleys and close on the sea. Irrigation is practiced upon all these 

 lands, but no means of determining the volume used per acre have 

 been adopted, and data are not at hand bearing on the question. 



Sugar production is, relatively speaking, a recent matter so far as 



