THE IRRIGATION OF THE CANE 115 



practices may be attributed to the increased water-holding capacity of soils 

 treated in this way. 



The velocity and flow of the wind are also of importance in determining 

 the evaporation from the soil, and loss in this way may be controlled by 

 planting wind-breaks or belts of trees. 



Another factor of very great importance is the humidity ; Eckart 11 

 has shown that this entirely masks the effect of temperature, so much so 

 that a rise in humidity of 12-5 per cent, decreased the evaporation 50 per 

 cent., although the temperature rose 1-5 Fahrenheit. 



Cost of Irrigation. The cost of irrigation as practised in the Hawaiian 

 Islands is very great, and at the same time very variable with varying local 

 conditions. The cost divides itself naturally into two parts, the cost of 

 furnishing water and the cost of applying it to the field. On those plantations 

 which have irrigation schemes tapping upland supplies the level of the field 

 does not affect the cost, but where the water is pumped the cost rises pro- 

 portionately to the height to which the water has to be elevated. The cost 

 of lifting 1,000,000 U.S. gallons one foot is roughly 0-09 cent, with fuel oil 

 costing 0-8 cent per lb., included herein being interest, depreciation and 

 labour. This amounts to $24 56 for 100 acre-inches lifted to a height of 

 100 feet. The cost of water from ditch systems is considerably less ; one 

 ditch company there supplies water at the rate of $2,500 per year per 1,000,000 

 gallons per day, a figure amounting to $18-79 P er IO acre-inches. A very 

 similar figure is charged in Porto Rico by a Government-owned scheme 

 supplying water in the southern portion of the territory. Here the cost is 

 $2-50 to $3-00 per acre-foot, or $20-82 to $25-00 per 100 acre-inches. 



The actual recorded costs of irrigation in the Hawaiian Islands for the 

 year 1914 are given below. 18 These data refer to plantations entirely 

 dependent on irrigation, each field receiving water on an average probably 

 never less than once in every ten days. The variations in cost are due to 

 differences in level, and to the difference between pumping and gravity 

 supplies. 



IRRIGATION COST PER ACRE AND PER TON OF CANE, CROP OF 1914. 



